tndm-424b5.htm

The information in this preliminary prospectus supplement is not complete and may be changed. A registration statement relating to these securities has been declared effective by the Securities and Exchange Commission. This preliminary prospectus supplement and the accompanying prospectus are not an offer to sell these securities and are not soliciting an offer to buy these securities in any jurisdiction where the offer or sale is not permitted.

We are offering shares of our common stock, par value $0.001 per share. Our common stock is listed on the NASDAQ Global Market under the symbol “TNDM.” On August 1, 2018, the last reported sale price of our common stock was $28.57 per share. The actual offering price per share will be as determined between us and the underwriters at the time of pricing, and may be at a discount to the current market price.

We are an “emerging growth company” as defined under the federal securities laws and, as such, may continue to elect to comply with certain reduced public company reporting requirements in future reports.

Investing in our common stock involves a high degree of risk. You should carefully review the risks and uncertainties described under the heading “Risk Factors” beginning on page S-7 of this prospectus supplement, as well as page 28 of our Quarterly Report on Form 10-Q for the quarterly period ended June 30, 2018.

We have granted to the underwriters an option to purchase a maximum of additional shares from us within 30 days following the date of this prospectus supplement (15% of the shares sold) at the price to the public, less the underwriting discount.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus supplement or the accompanying prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

Delivery of the shares of common stock is expected to be made on or about , 2018.

This document is part of a registration statement that was filed with the Securities and Exchange Commission, or the SEC, using a “shelf” registration process and consists of two parts. The first part is this prospectus supplement, including the documents incorporated by reference herein, which describes the specific terms of this offering. The second part, the accompanying prospectus, including the documents incorporated by reference therein, provides more general information about us and our securities. In general, when we refer only to the prospectus, we are referring to both parts of this document combined. Before you invest, you should carefully read this prospectus supplement, the accompanying prospectus, all information incorporated by reference herein and therein, as well as the additional information described under the heading “Where You Can Find More Information.” You are encouraged to carefully consider all of this information when deciding whether to invest in our common stock.

This prospectus supplement may add, update or change information contained in the accompanying prospectus. To the extent there is a conflict between the information contained in this prospectus supplement and the accompanying prospectus, you should rely on information contained in this prospectus supplement, provided that if any statement in, or incorporated by reference into, one of these documents is inconsistent with a statement in another document having a later date, the statement in the document having the later date modifies or supersedes the earlier statement. Any statement so modified will be deemed to constitute a part of this prospectus only as so modified, and any statement so superseded will be deemed not to constitute a part of this prospectus.

You should rely only on the information contained in this prospectus supplement, the accompanying prospectus, any document incorporated by reference herein or therein, or any free writing prospectuses we may provide to you in connection with this offering. Neither we nor the underwriters have authorized anyone to provide you with any different information. We take no responsibility for, and can provide no assurance as to the reliability of, any other information that others may provide to you. The information contained in this prospectus supplement, the accompanying prospectus, any document incorporated by reference herein or therein, and any free writing prospectuses we may provide to you in connection with this offering is accurate only as of the date such information is presented. Our business, financial condition, results of operations and prospects may have changed since those respective dates.

This prospectus supplement and the accompanying prospectus do not constitute an offer to sell or the solicitation of an offer to buy any securities other than the shares of common stock to which it relates, nor do this prospectus supplement and the accompanying prospectus constitute an offer to sell or the solicitation of an offer to buy securities in any jurisdiction to any person to whom it is unlawful to make such offer or solicitation in such jurisdiction.

Unless otherwise indicated, information contained in or incorporated by reference into this prospectus concerning our business and the industry and markets in which we operate, including with respect to the estimated size of our market, our market position and opportunity, the competitive landscape and the regulatory environment, is based on information from our management’s estimates, as well as from industry publications, surveys and studies conducted by third parties. Our management’s estimates are derived from publicly available information, their knowledge of our business and industry, and assumptions based on such information and knowledge, which they believe to be reasonable. In addition, while we believe that information contained in the industry publications, surveys and studies has been obtained from reliable sources, we have not independently verified any of the data contained in these third-party sources, and the accuracy and completeness of the information contained in these sources is not guaranteed.

This prospectus supplement and the accompanying prospectus, and any documents incorporated by reference herein or therein, include statements that are based on various assumptions and estimates that are subject to numerous known and unknown risks and uncertainties. Some of these risks and uncertainties are described under the heading ”Risk Factors” beginning on page S-7 of this prospectus supplement, and beginning on page 28 of our Quarterly Report on Form 10-Q for the quarterly period ended June 30, 2018, or the Q2 Quarterly Report, which is incorporated by reference into the prospectus. These and other important factors could cause our future results to be materially different from the results expected as a result of, or implied by, these assumptions and estimates. You should read the information contained in this prospectus supplement, the accompanying prospectus, and the documents incorporated by reference herein and therein, completely and with the understanding that future results may be materially different from and worse than what we expect. See the information under the heading “Cautionary Note Regarding Forward-Looking Information.”

PROSPECTUS SUPPLEMENT SUMMARY

This prospectus supplement summary discusses the key aspects of the offering and highlights certain information appearing elsewhere in this prospectus supplement and the accompanying prospectus, and in the documents incorporated by reference herein and therein. However, as this is a summary, it does not contain all of the information you should consider before deciding to invest in our common stock. You are encouraged to carefully read the entire prospectus, including the information provided (i) under the heading “Risk Factors” beginning on page S-7 of this prospectus supplement, and beginning on page 28 of our Q2 Quarterly Report, as such may be updated from time to time by other filings we make with the SEC after the date of this prospectus supplement, and (ii) under the heading “Management’s Discussion and Analysis of Financial Condition and Results of Operations”, and our financial statements and the related notes, in our Annual Report on Form 10-K for the fiscal year ended December 31, 2017, the Q2 Quarterly Report, and the other filings we make with the SEC after the date of this prospectus supplement.

Unless otherwise stated in this prospectus supplement and the accompanying prospectus references to “Tandem,” “we,” “us,” or “our” refer to Tandem Diabetes Care, Inc.

Our Company

We are a medical device company with an innovative approach to the design, development and commercialization of products for people with insulin-dependent diabetes. We believe our competitive advantage is rooted in our unique consumer-focused approach, and the incorporation of modern and innovative technology into our product offerings. Our manufacturing and sales activities primarily focus on our flagship product, the t:slim^™ X2 insulin pump, which is based on our proprietary technology platform. The simple-to-use t:slim X2 is the smallest durable insulin pump available, and the only pump currently available in the United States that is capable of remote feature updates, which positions us well to address the evolving needs and preferences of differentiated segments of the insulin-dependent diabetes market. By delivering innovative hardware and software solutions, as well as best-in-class customer support, we aim to improve and simplify the lives of people with diabetes and their healthcare providers. For the past five consecutive years, our company has been ranked #1 by insulin pump users in the United States for customer support in an independent survey by dQ&A, a leading diabetes research firm.

Since the launch of our first product in August 2012 through June 2018, we have shipped nearly 78,000 pumps to customers in the United States, of which over 66,000 pumps have been shipped within the four years ended June 30, 2018. We plan to begin commercialization of t:slim X2 in select geographies outside the United States, including Canada, during the second half of 2018.

We began commercial sales of our first insulin pump, the t:slim^® insulin pump, in August 2012. During 2015, we commenced commercial sales of two additional insulin pumps: t:flex^™ in May 2015 and t:slim G4^™ in September 2015. In October 2016, we commenced commercial sales of t:slim X2 and discontinued new sales of t:slim. In September 2017, we commenced commercial sales of t:slim X2 integrated with the Dexcom G5^® Mobile CGM system and discontinued new sales of t:slim G4. Because the t:slim X2 insulin pump platform has represented nearly 100% of our new pump shipments, we discontinued marketing and new pump sales of t:flex pumps in the third quarter of 2018. We will continue to provide ongoing service and support to existing t:slim, t:slim G4 and t:flex customers.

In June 2018, we received approval by the United States Food and Drug Administration (FDA) for t:slim X2 with Basal-IQ^™technology, our first generation Automated Insulin Delivery (AID) algorithm. We plan to commence commercial sales of this product in the third quarter of 2018. This system uses Dexcom G6^® CGM sensor values to adjust the rate of insulin delivery to help minimize the frequency and/or duration of hypoglycemic events. Recently, the FDA also created a new interoperability designation for integrated continuous glucose monitoring (iCGM) devices. Our t:slim X2 with Basal-IQ technology was the first insulin pump to receive approval for iCGM compatibility, which we expect will streamline the regulatory pathway for integration with future iCGM products approved by the FDA.

Our insulin pumps are compatible with the Tandem Device Updater, a revolutionary new tool that allows pump users to update their pumps’ software quickly and easily from a personal computer. The Tandem Device Updater provides our customers access to new and enhanced features and functionality faster than the industry has been able to in the past. The first use of our Tandem Device Updater was for deployment of the latest t:slim software to in-warranty t:slim pumps purchased before April 2015. In September 2017, we set a new standard of care in our industry by offering all existing t:slim X2 customers integration with the Dexcom G5 Mobile CGM system through a software update using the Tandem Device Updater. In October 2017, we announced that, subject to FDA approval, we intend to make any new features approved by the FDA in 2018 available to all in-warranty users of t:slim X2 at no cost through the Tandem Device Updater. This includes software featuring our new Basal-IQ technology, which was approved by the FDA in June 2018. In the future, this tool has the potential to enable users to add other new features and functionality to their pumps independent of the typical four-year insurance pump reimbursement cycle.

Our innovative approach to product design and development is consumer-focused and based on our extensive market research, as we believe the user is the primary decision maker when purchasing an insulin pump. Our market research consists of interviews, focus groups and online surveys to understand what people with diabetes, their caregivers and healthcare providers are seeking in order to improve diabetes therapy management. We also apply the science of human factors to our design and development process, which seeks to optimize our devices, thereby allowing users to successfully operate them in their intended environment.

We developed our products to provide the specific features that people with insulin-dependent diabetes seek in a next-generation insulin pump. Our proprietary technology platform allows us to design the slimmest and smallest durable insulin pumps on the market, without sacrificing insulin capacity. Our platform features our patented Micro-Delivery technology, and a miniaturized pumping mechanism that draws insulin from a flexible bag within the pump’s cartridge, rather than relying on a syringe and plunger mechanism. It also features an easy-to-navigate software architecture, and a vivid color touchscreen. In addition, the t:slim X2 features an advanced Bluetooth^® radio capable of communicating with multiple compatible devices, such as a CGM sensor, blood glucose meter or mobile device applications. Our platform has a micro-USB connection that supports a rechargeable battery and software updates through the Tandem Device Updater, as well as uploads to t:connect^® Diabetes Management Application, or t:connect. t:connect is our custom cloud-based data management application that provides customers and healthcare providers a fast, easy and visual way to display therapy management data from the pump and supported blood glucose meters. In April 2017, we launched the t:connect HCP Portal, which is designed to streamline healthcare providers’ use of the original t:connect Application and improve office efficiency. Currently, t:connect and the Tandem Device Updater are only available in the United States.

We have rapidly increased sales since our commercial launch by expanding our sales, clinical and marketing infrastructure, by developing, commercializing and marketing multiple differentiated products that utilize our proprietary technology platform and consumer-focused approach, and by providing strong customer support. In our research, approximately 86% of healthcare providers surveyed believe that providing great customer support is the most important attribute in an insulin pump manufacturer. We believe that by demonstrating our product benefits and the shortcomings of existing insulin therapies, more people will choose our insulin pumps for their therapy needs, allowing us to further penetrate and expand the market. We also believe we are positioned well to address consumers’ needs and preferences with our current products and products under development and by offering customers access to our future innovations through the Tandem Device Updater, as they are approved by the FDA. As we continue to develop differentiated products based on our proprietary technology platform, we intend to leverage a single sales, marketing and clinical organization, a shared manufacturing and supply chain infrastructure, and the expertise of our customer support services.

In the second half of 2018, we intend to commence commercial sales of our t:slim X2 with G5 integration in select international geographies. We expect that most of our commercial sales outside the United States will be to independent distributors in select geographies who will perform all sales, customer support and training in their respective territories.

Recent Developments

FDA Approval of t:slim X2 with Basal-IQ

In June 2018, the FDA approved our t:slim X2 Insulin Pump with Basal-IQ technology, a predictive low glucose suspend, or PLGS, feature designed to help reduce the frequency and duration of low glucose events. We plan to launch our new product with Dexcom G6 CGM integration in the third quarter of 2018, which requires no fingersticks for calibration or diabetes treatment decisions, and was the first CGM device to receive the iCGM designation from the FDA earlier this year.

International Expansion

In preparation for a commercial launch outside the United States in the second half of 2018, we have filed for the necessary regulatory approvals for our t:slim X2 pump with Dexcom G5 integration in select geographies. In April 2018, we received CE mark approval. We intend to market our products internationally primarily through third-party distributors. To date, we have entered into distribution agreements with independent distributors for markets in Australia, New Zealand, Italy, Scandinavia, South Africa, Spain and the United Kingdom.

Products Under Development

Our products under development support our strategy of focusing on both consumer and clinical needs, and include AID systems, a next-generation hardware platform, and connected (mobile) health offerings. We intend to leverage our consumer-focused approach and proprietary technology platform to continue to develop products that have the features and functionalities that will allow us to target people in differentiated segments of the insulin-dependent diabetes market:

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t:slim X2 with Control IQ™ technology – Our second generation AID system is expected to integrate the t:slim X2 pump with the treat-to-range technology that we licensed from TypeZero Technologies LLC, as well as Dexcom’s G6 CGM sensor. With TypeZero’s technology, our product is intended to both increase and decrease basal insulin based on a user’s predicted blood glucose levels, as well as deliver automated correction boluses. We are working with Dexcom and TypeZero on the integration of our technologies into the U.S. portion of the Clinical Acceptance of the Artificial Pancreas (DCLP3) portion of the International Diabetes Closed Loop (IDCL) Trial, for which enrollment began in June 2018. This trial will utilize a t:slim X2 integrated with TypeZero’s inControl AID algorithms, which is designed to automatically adjust a person’s insulin based on information from a Dexcom G6 sensor. We intend to use the results from this trial in a PMA submission with the FDA the first module of which we intend to file in September 2018. We have also conducted and anticipate continuing to conduct targeted pediatric studies for a future regulatory submission. Subject to both the timely completion of the DCLP3 Trial with a satisfactory outcome and future FDA approval, our goal is to launch this product in the summer of 2019.

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t:sport Insulin Delivery System – This product is our next generation hardware platform that is expected to be half the size of t:slim and is being designed for people who seek even greater discretion and flexibility with the use of their insulin pump. Subject to FDA approval, our goal is to launch this product in 2020 or 2021.

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Connected (Mobile) Health Offerings – We are currently developing a mobile application that is being designed to utilize the capability of the Bluetooth radio to wirelessly upload pump data to t:connect, receive notification of pump alerts and alarms, integrate other health-related information from third party sources, and support future pump-control capabilities for our products under development. Subject to FDA approval, we intend to launch the first generation of our mobile application in the second half of 2018, with a subset of these features.

Pump Shipments

Since inception, we have derived nearly all of our sales from the shipment of insulin pumps and associated supplies in the United States. We consider the number of units shipped per quarter to be an important metric for managing our business. We have shipped nearly 78,000 insulin pumps since our initial launch in August 2012, of which over 66,000 pumps were shipped within the four year period ended June 30, 2018. Pump shipments are broken down by fiscal quarter as follows:

	Pump Units Shipped for Each of the Three Months Ended in Respective Years⁽¹⁾
	Total
	March 31		June 30		September 30		December 31		Total
2012		-		9		204		844		1,057
2013		852		1,363		1,851		2,406		6,472
2014		1,723		2,235		2,935		3,929		10,822
2015		2,487		3,331		3,431		6,234		15,483
2016		4,042		4,582		3,896		4,418		16,938
2017		2,816		3,427		3,868		6,950		17,061
2018		4,444		5,447	N/A		N/A			9,891

⁽¹⁾	This table does not reflect returns or exchanges of pump products that occur in the ordinary course of business, nor does it reflect approximately 3,300 trade-ins fulfilled in 2016 and 2017 under the Technology Upgrade Program related to our commercial launch of t:slim X2.

Corporate Information

We were incorporated in Colorado in January 2006 and reincorporated in Delaware in January 2008. Our principal executive offices are located at 11075 Roselle Street, San Diego, California 92121. The telephone number of our principal executive office is (858) 366-6900. Our website is www.tandemdiabetes.com. The information contained on or accessed through our website is not incorporated by reference into this prospectus, and you should not consider information contained on our website to be a part of this prospectus or in deciding whether to purchase our common stock. References in this prospectus to our website are to inactive textual references only.

Implications of Being an Emerging Growth Company

We qualify as an “emerging growth company” as defined in the Jumpstart our Business Startups Act of 2012, or the JOBS Act. An emerging growth company may take advantage of specified reduced reporting requirements and is relieved of certain other significant requirements that are otherwise generally applicable to public companies. As an emerging growth company:

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we are exempt from the requirement to obtain an attestation and report from our auditors on the assessment of our internal control over financial reporting pursuant to the Sarbanes-Oxley Act of 2002;

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we are permitted to provide less extensive disclosure about our executive compensation arrangements; and

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we are not required to give our stockholders non-binding advisory votes on executive compensation or golden parachute arrangements.

We may take advantage of these provisions until such time that we no longer qualify as an emerging growth company. We expect to cease being an emerging growth company on December 31, 2018.

We have taken advantage of reduced reporting requirements in this prospectus. Accordingly, the information contained herein may be different from the information you receive from our competitors that are public companies, or other public companies in which you have made an investment.

THE OFFERING


Issuer:		Tandem Diabetes Care, Inc.
Common Stock offered by us:		shares
Option to purchase additional shares:		The underwriters will have an option to purchase additional shares from us. The underwriters can exercise this option at any time within 30 days from the date of this prospectus supplement.
Common Stock to be outstanding immediately after this offering:		shares (or shares if the underwriters fully exercise their option to purchase additional shares)
Use of proceeds:		We estimate that our net proceeds from the sale of shares in this offering will be approximately $ million (or $ million if the underwriters fully exercise their option to purchase additional shares), after deducting the underwriting discount and estimated offering expenses payable by us. We intend to use the net proceeds from this offering to satisfy all or a portion of our indebtedness to Capital Royalty Partners II, L.P. and its affiliated funds, or Capital Royalty Partners, under the Amended and Restated Term Loan Agreement, or the Term Loan Agreement. We may also use any remaining net proceeds for working capital and general corporate purposes. See the information under the heading “Use of Proceeds.”
Risk factors:		Investing in our common stock involves a high degree of risk. You should carefully review the risks and uncertainties described under the heading “Risk Factors” beginning on page S-7 of this prospectus supplement, as well as page 28 of the Q2 Quarterly Report.
NASDAQ Global Market Symbol:		TNDM

The number of shares of our common stock to be outstanding immediately after this offering is based on 53,185,839 shares of our common stock outstanding as of June 30, 2018 and excludes:

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920,988 shares of common stock issuable upon exercise of outstanding warrants as of June 30, 2018, at a weighted average exercise price of $15.25 per share;

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150,051 shares of common stock issuable upon exercise of outstanding options to purchase shares of common stock under our 2006 Stock Incentive Plan, or the 2006 Plan, as of June 30, 2018 at a weighted average exercise price of $24.11 per share (of which options to acquire 150,051 shares of common stock are vested as of June 30, 2018);

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5,408,153 shares of common stock issuable upon exercise of outstanding options to purchase shares of common stock under our 2013 Stock Incentive Plan, or the 2013 Plan, as of June 30, 2018, at a weighted average exercise price of $22.40 per share (of which options to acquire 523,835 shares of common stock are vested as of June 30, 2018) and 1,316,739 shares that are reserved for future issuance under the 2013 Plan as of June 30, 2018; and

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2,101,207 shares of common stock reserved for future grant or issuance under our 2013 Employee Stock Purchase Plan, or the ESPP, as of June 30, 2018.

Unless otherwise indicated, this prospectus supplement reflects and assumes the following:

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no exercise of the outstanding warrants and options described above; and

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no exercise by the underwriters of their option to purchase additional shares of our common stock.

Investing in our common stock involves a high degree of risk. Before making an investment decision, you should carefully consider the risks and uncertainties described below, together with all of the other information included in this prospectus supplement, the accompanying prospectus, and the information incorporated by reference herein and therein, including the risks and uncertainties described under the heading “Risk Factors” beginning on page 28 of the Q2 Quarterly Report, as such may be updated from time to time by other filings we make with the SEC after the date of this prospectus supplement.

If any of the risks described below, or incorporated by reference into this prospectus, actually occur, our business, financial condition, results of operations and prospects could suffer. In that case, the trading price of our common stock may decline and you may lose all or part of your investment. The risks and uncertainties we have described are not the only ones we face. Additional risks and uncertainties not presently known to us or that we currently deem immaterial may also affect our business, financial condition results of operations and prospects.

Certain statements below are forward-looking statements. See the information under the heading “Cautionary Note Regarding Forward-Looking Information” of this prospectus supplement.

If you purchase our common stock in this offering, you will incur immediate and substantial dilution in the book value of your shares.

If you invest in our common stock in this offering, your ownership interest will be immediately diluted to the extent of the difference between the public offering price per share and the pro forma net tangible book value per share upon completion of this offering. As of June 30, 2018, our net tangible book value was $0.46 per share. After giving effect to the issuance and sale of shares of common stock in this offering, and after deducting the underwriting discount and estimated offering expenses payable by us, our pro forma net tangible book value as of June 30, 2018 would have been $ million, or $ per share of our common stock. This represents an immediate increase in net tangible book value of $ per share to our existing stockholders, and an immediate dilution of $ per share to new investors purchasing in this offering. As a result of the dilution to new investors, these investors may receive significantly less than the purchase price paid in this offering in the event of a sale or liquidation of our business. In addition, if the underwriters exercise their option to purchase additional shares, or if our outstanding options or warrants are exercised, you could experience further dilution. See the information under the heading “Dilution.”

The trading price of our common stock has been volatile over the past several years. We believe our stock price has been, and will continue to be, subject to wide fluctuations in response to a variety of factors, including the following:

These and other factors might cause the market price of our common stock to fluctuate substantially. Fluctuations on our stock price may negatively affect the liquidity of our common stock, which could further impact our stock price.

In recent years, the stock market has experienced significant price and volume fluctuations. This volatility has had a significant impact on the market price of securities issued by many companies across many industries. These changes may occur without regard to the financial condition or operating performance of the affected companies. Accordingly, the price of our common stock could fluctuate based upon factors that have little or nothing to do with our company, and these fluctuations could materially reduce the market price of our common stock.

Future sales, or the perception of future sales, of shares of our common stock could materially reduce the market price of our common stock.

Future issuances or sales of our common stock, or the perception in the market that the holders of a large number of our shares intend to sell such shares, could reduce the market price of our common stock, which would reduce the price of our common stock and impair our ability to raise capital through the sale of additional equity securities. A substantial number of the outstanding shares of our common stock are, and the shares of common stock sold in this offering upon issuance will be, freely tradable without restriction or further registration under the Securities Act of 1933, as amended, or the Securities Act. As of June 30, 2018, we had outstanding warrants to purchase 920,988 shares of common stock and outstanding options to purchase 5,558,204 shares of common stock that, if exercised, will result in these additional shares becoming available for sale. Furthermore, as of June 30, 2018, 1,316,739 shares of our common stock were reserved for future issuance under the 2013 Plan and 2,101,207 shares of our common stock were reserved for future issuance under the ESPP.

Upon the completion of this offering, approximately 3,181,872 shares of our outstanding common stock beneficially owned by our executive officers, directors and certain of our other existing stockholders that are affiliated with our directors will be subject to lock-up agreements with the underwriters of this offering that restrict the sale of shares of our common stock by those parties for a period of 90 or 45 days after the date of this prospectus. However, all of the shares sold in this offering and the remaining shares of our common stock outstanding prior to this offering will not be subject to lock-up agreements with the underwriters and, except to the extent such shares are held by our affiliates, will be freely tradable without restriction under the Securities Act. In addition, following the expiration of the applicable lock up period, certain holders of shares of our common stock will have the right, subject to various conditions and limitations, to include their shares in registration statements relating to our securities. In the event registration rights are exercised and additional shares of our common stock are sold in the public market, those sales could reduce the trading price of our common stock.

In the future, we may issue shares of our common stock for a variety of corporate purposes, including capital raising activities, the exercise of outstanding warrants, the exercise of outstanding options, or for other purposes. The number of shares of our common stock that we may issue in the future may be significant as a percentage of our then-outstanding shares.

We have broad discretion in the use of the net proceeds we receive from this offering, and may not use them in ways that improve our results of operations or enhance stockholder value.

Although we currently intend to use the net proceeds from this offering in the manner described under the heading “Use of Proceeds,” our management will have broad discretion in the application of the net proceeds from this offering and could spend the proceeds in ways that do not improve our results of operations, result in us becoming profitable or enhance the value of our common stock. The failure by our management to apply these funds effectively could result in a material adverse effect on our financial condition or operating results, delay the development of or regulatory approval for new products, impede the execution of our strategic plans and cause the market price of our common stock to decline.

We have never declared or paid cash dividends on our capital stock. We currently intend to retain all available funds and any future earnings for use in the operation and expansion of our business and do not anticipate paying any cash dividends in the foreseeable future. In addition, pursuant to our Term Loan Agreement, we are precluded from paying any cash dividends. We intend to satisfy all or a portion of our indebtedness to Capital Royalty Partners under the Term Loan Agreement with the net proceeds of this offering. See "Use of Proceeds." Accordingly, investors may have to sell some or all of their shares of our common stock in order to generate cash flow from their investment.

The expected increase to the number of authorized shares of our common stock could result in further dilution to our existing stockholders and have potential anti-takeover implications.

In June 2018, our stockholders approved an amendment to our amended and restated certificate of incorporation to increase the total number of authorized shares of our common stock by 100,000,000 shares, or from 100,000,000 shares to 200,000,000 shares. We expect we will file the amendment to effect the increase in authorized shares with the Secretary of State of the State of Delaware following the completion of this offering. The increase in the number of authorized shares will significantly increase our authorized shares relative to our issued and outstanding shares. This increase will afford us the ability to sell additional shares of our common stock (or securities convertible or exchangeable for our common stock), which would result in further dilution to our stockholders, including stockholders who decide to purchase shares in this offering. In addition, the increase could, under certain circumstances, have anti-takeover implications. For example, the additional shares of common stock that have become available for issuance could be used by us to oppose a hostile takeover attempt or to delay or prevent changes in control or our management.

In the past, securities class action litigation has been instituted against companies following periods of volatility in the overall market and in the price of a company’s securities. We believe this risk may be particularly relevant to us as we have recently and in past years experienced significant stock price volatility. If we face such litigation, it could result in substantial costs and a diversion of management’s attention and resources, which could harm our business, financial condition and results of operations.

If securities or industry analysts do not publish research, or publish inaccurate or unfavorable research, about our business, our stock price and trading volume could decline.

The trading market for our common stock depends, in part, on the research and reports that securities or industry analysts publish about us or our business. If one or more of the analysts who cover us downgrade our stock or publish inaccurate or unfavorable research about our business, our stock price would likely decline. In addition, if our operating results fail to meet the forecasts of analysts, our stock price would likely decline. If one or more of these analysts cease coverage of our company or fail to publish reports on us regularly, demand for our common stock could decrease, which might cause our stock price and trading volume to decline.

This prospectus supplement, the accompanying prospectus, and the documents incorporated by reference herein and therein, contain “forward-looking statements” within the meaning of the federal securities laws, which statements are subject to considerable risks and uncertainties. These forward-looking statements are intended to qualify for the safe harbor from liability established by the Private Securities Litigation Reform Act of 1995. All statements included or incorporated by reference in this prospectus, other than statements of historical fact, are forward-looking statements. You can identify forward-looking statements by the use of words such as “may,” “will,” “could,” “anticipate,” “expect,” “intend,” “believe,” “continue” or the negative of such terms, or other comparable terminology. Forward-looking statements also include the assumptions underlying or relating to such statements. In particular, forward-looking statements included or incorporated by reference in this prospectus relate to, among other things, our future or assumed financial condition, results of operations, liquidity, business forecasts and plans, research and product development plans, manufacturing plans, strategic plans and objectives, capital needs and financing plans, product launches, regulatory approvals, competitive environment and our expected use of the net proceeds from this offering. We caution you that the foregoing list may not include all of the forward-looking statements made in this prospectus.

Our forward-looking statements are based on our management’s current assumptions and expectations about future events and trends, which affect or may affect our business, strategy, operations or financial performance. Although we believe that these forward-looking statements are based upon reasonable assumptions, they are subject to numerous known and unknown risks and uncertainties and are made in light of information currently available to us. Our actual financial condition and results could differ materially from those anticipated in these forward-looking statements as a result of various factors, including those set forth below in the section entitled “Risk Factors” beginning on page S-7 of this prospectus supplement, beginning on page 28 of the Q2 Quarterly Report, as well as in the other reports we file with the SEC. You should read this prospectus with the understanding that our actual future results may be materially different from and worse than what we expect.

Moreover, we operate in an evolving environment. New risk factors and uncertainties emerge from time to time and it is not possible for our management to predict all risk factors and uncertainties, nor can we assess the impact of all factors on our business or the extent to which any factor, or combination of factors, may cause actual results to differ materially from those contained in any forward-looking statements.

Forward-looking statements speak only as of the date they were made, and, except to the extent required by law or the NASDAQ Listing Rules, we undertake no obligation to update or review any forward-looking statement because of new information, future events or other factors.

We qualify all of our forward-looking statements by these cautionary statements.

We estimate that our net proceeds from the sale of shares of our common stock in this offering will be approximately $ million (or $ million if the underwriters fully exercise their option to purchase additional shares), after deducting the underwriting discount and estimated offering expenses payable by us.

We intend to use the net proceeds from this offering to satisfy all or a portion of our indebtedness to Capital Royalty Partners under the Term Loan Agreement. As of June 30, 2018, we had $82.7 million of aggregate borrowings under the Term Loan Agreement. Pursuant to the Term Loan Agreement, interest is payable, at our option, (i) in cash at a rate of 11.5% per annum or (ii) at a rate of 9.5% with 2.0% added to the principal of the loan and subject to accruing interest. Interest-only payments are due quarterly on March 31, June 30, September 30 and December 31 of each year of the interest-only payment period, which ends on December 31, 2019. The principal balance is due in full on the maturity date of the Term Loan Agreement, which is March 31, 2020. For additional information regarding the Term Loan Agreement, see “Management’s Discussion and Analysis of Financial Condition and Results of Operations – Indebtedness – Term Loan Agreement” on page 25 of the Q2 Quarterly Report.

We may also use any remaining net proceeds for working capital and other general corporate purposes.

Our expected use of the net proceeds from this offering is based upon our present plans and business condition. As of the date of this prospectus supplement, we cannot predict with certainty all of the particular uses for the net proceeds to be received upon the completion of this offering or the amounts that we will actually spend on the uses set forth above. The amounts and timing of our actual use of proceeds will vary depending on numerous factors, including the factors described in the section entitled “Risk Factors” beginning on page S-7 of this prospectus supplement, and beginning on page 28 of the Q2 Quarterly Report. In general, management will retain broad discretion over the allocation of the net proceeds from this offering, and investors will be relying on the judgment of our management regarding the application of the net proceeds.

Pending the use of the net proceeds of this offering, we intend to invest the net proceeds in high-quality, short-term interest-bearing obligations, investment-grade instruments, certificates of deposit or direct or guaranteed obligations of the U.S. government.

Our authorized capital stock consists of 100,000,000 shares of our common stock, $0.001 par value per share, and 5,000,000 shares of undesignated preferred stock, $0.001 par value per share. As of June 30, 2018, there were 53,185,839 shares of our common stock outstanding, and no shares of our preferred stock outstanding.

Our common stock is traded on the NASDAQ Global Market under the symbol “TNDM”. On August 1, 2018, the last reported sale price of our common stock on the NASDAQ Global Market was $28.57 per share.

Subject to the updated information set forth above, the material terms of our common stock are described under the heading “Description of Capital Stock” in the accompanying prospectus.

The following table sets forth, for the periods indicated, the high and low intraday sale prices of our common stock as reported by the NASDAQ Global Market.

	Price Range
	High		Low
Year Ended December 31, 2018:
First Quarter	$	5.23	$	2.14
Second Quarter	$	25.50	$	4.75
Third Quarter (through August 1, 2018)	$	28.95	$	20.08
Year Ended December 31, 2017:
First Quarter	$	30.00	$	11.00
Second Quarter	$	13.00	$	7.63
Third Quarter	$	12.20	$	3.90
Fourth Quarter	$	8.88	$	2.15
Year Ended December 31, 2016:
First Quarter	$	118.00	$	65.90
Second Quarter	$	113.00	$	64.80
Third Quarter	$	88.10	$	60.40
Fourth Quarter	$	81.00	$	16.00

As of June 30, 2018, there were approximately 71 holders of record of our common stock. The actual number of common stockholders is greater than the number of record holders, and includes stockholders who are beneficial owners, but whose shares are held in street name by brokers, dealers, banks and other nominees. This number of holders of record also does not include stockholders whose shares may be held in trust by other entities.

On October 9, 2017, we filed an amendment to our amended and restated certificate of incorporation to effect a reverse stock split of our issued and outstanding shares of common stock at a ratio of 1-for-10. The share amounts, exercise prices and other amounts set forth in this prospectus have been adjusted to reflect the impact of the reverse stock split.

We have never declared or paid any cash dividends on our common stock. We have no current plans to declare or pay any dividends and intend to retain all of our future earnings, if any, generated by our operations for the development and growth of our business. Any future decision to pay dividends will be made by our board of directors in its sole discretion and will depend upon our results of operations, financial condition, capital requirements and other factors that our board of directors deems relevant in its informed business judgment. In addition, the terms of our Term Loan Agreement restrict our ability to pay cash dividends. We intend to satisfy all or a portion of our indebtedness to Capital Royalty Partners under the Term Loan Agreement with the net proceeds of this offering. See "Use of Proceeds."

After giving effect to an assumed issuance and sale of 3,000,000 shares of common stock in this offering (at an assumed public offering price of $28.57 per share (the closing price of our common stock on the NASDAQ Global Market on August 1, 2018), and after deducting the underwriting discount and estimated offering expenses payable by us, our pro forma net tangible book value as of June 30, 2018 would have been $105.8 million, or $1.88 per share of our common stock. This represents an immediate increase in net tangible book value of $1.42 per share to our existing stockholders, and an immediate dilution of $26.69 per share to new investors purchasing in this offering.

The table above assumes that the underwriters do not exercise their option to purchase additional shares of our common stock. If the underwriters fully exercise their option and purchase an assumed additional 450,000 shares of common stock (at an assumed public offering price of $28.57 per share (the closing price of our common stock on the NASDAQ Global Market on August 1, 2018), and after deducting the underwriting discount and estimated offering expenses payable by us, our pro forma net tangible book value per share as of June 30, 2018 would have been $2.08 per share of our common stock. This represents an immediate increase in pro forma net tangible book value of $1.62 per share to our existing stockholders, and an immediate dilution of $26.49 per share to new investors purchasing in this offering.

A $0.50 increase or decrease in the assumed public offering price of $28.57 per share, the closing price of our common

stock on the NASDAQ Global Market on August 1, 2018, would increase or decrease our pro forma net tangible book value per share by $0.03 and the dilution per share to new investors in this offering by $0.47 per share, assuming the number of shares of common stock offered by us remains the same. An increase or decrease of 1,000,000 shares in the assumed number of shares of common stock offered by us at the assumed public offering price would increase or decrease our pro forma net tangible book value per share after this offering by $0.44 and $0.46 per share, respectively, and decrease or increase the dilution per share to new investors in this offering by $0.44 and $0.46 per share, respectively. The actual number of shares of common stock offered by us and the offering price per share will be determined between us and the underwriters at the time of pricing, and the offering price per share may be at a discount to the current market price.

The number of shares of our common stock to be outstanding immediately after this offering is based on 53,185,839 shares of our common stock outstanding as of June 30, 2018, and excludes:

In the future, we may issue shares of our common stock for a variety of corporate purposes, including in capital raising activities through future public offerings or private placements, in connection with the exercise of outstanding warrants, in connection with the exercise of outstanding options or other equity awards that may be issued pursuant to our employee benefit plans, as consideration for future acquisitions, collaborations, partnerships or investments, or for other purposes. The number of shares of our common stock that we may issue in the future may be significant as a percentage of our then-outstanding shares. In some cases, the shares we issue may be freely tradable without restriction or further registration under the Securities Act. In other cases, we may grant registration rights covering the shares issued in connection with these issuances, in which case the holders of our common stock will have the right, under certain circumstances, to cause us to register any resale of such shares to the public.

The following is a description of certain U.S. federal income and estate tax considerations related to the purchase, ownership and disposition of our common stock that are applicable to U.S. and non-U.S. holders (defined below), which:

This description constitutes neither tax nor legal advice. Prospective investors are urged to consult their own tax advisors to determine the specific tax consequences and risks to them of purchasing, holding and disposing of our common stock, including the application to their particular situations of any U.S. federal, state, local and non-U.S. tax laws and of any applicable income tax treaty.

For purposes of this discussion, a U.S. holder is a beneficial owner of our common stock that is a “U.S. person” for U.S. federal income tax purposes. A “U.S. person” is any of the following:

If a partnership (or an entity or arrangement treated as a partnership for U.S. federal income tax purposes) owns our common stock, then the U.S. federal income tax treatment of a partner in that partnership, including a partner that is a U.S. person, generally will depend on the status of the partner and the partnership’s activities. Partners and partnerships should consult their own tax advisors with regard to the U.S. federal income tax treatment of an investment in our common stock.

Distributions will constitute dividends for U.S. federal income tax purposes to the extent paid from our current or accumulated earnings and profits, as determined under U.S. federal income tax principles. Distributions made on our common stock that are treated as dividends generally will be included in a U.S. holder’s income as ordinary dividend income. With respect to noncorporate taxpayers, including individuals, such dividends are generally subject to reduced tax rates of U.S. federal income tax provided certain holding period requirements are satisfied.

Amounts not treated as dividends for U.S. federal income tax purposes will constitute a non-taxable return of capital and first be applied against and reduce a U.S. holder’s adjusted tax basis in its common stock, but not below zero. Any excess will be treated as capital gain and will be treated as described below.

Upon the sale, exchange or other taxable disposition of our common stock, a U.S. holder generally will recognize capital gain or loss equal to the difference between (i) the amount of cash and the fair market value of any property received upon the sale or exchange and (ii) the U.S. holder’s adjusted tax basis in the common stock. Such capital gain or loss will be long-term capital gain or loss if the U.S. holder’s holding period in the common stock is more than one year at the time of the sale, exchange or other taxable disposition. Long-term capital gains recognized by certain noncorporate U.S. holders, including individuals, will generally be subject to reduced rates of U.S. federal income tax. The deductibility of capital losses is subject to limitations.

Certain U.S. holders who are individuals, estates or certain trusts must pay a 3.8% tax on the U.S. person’s “net investment income.” Net investment income generally includes, among other things, dividend income and net gains from the disposition of our common stock. A U.S. holder that is an individual, estate or trust should consult its tax advisor regarding the applicability of the Medicare tax to its income and gains in respect of its investment in our common stock.

For purposes of this discussion, a non-U.S. holder is a beneficial owner of our common stock that is (1) not a “U.S. holder” (as defined under the section entitled “U.S. Holder Defined” above) and (2) for an individual, is not present in the United States for 183 days or more in the year of the disposition. An individual who is present in the United States for 183 days or more in a taxable year should consult his or her own tax advisor with respect to the U.S. tax consequences of a disposition of our common stock.

If an entity or arrangement treated as a partnership for U.S. federal income tax purposes owns our common stock, then the U.S. federal income tax treatment of a partner, including a partner that is a non-U.S. person, in that partnership generally will depend on the status of the partner and the partnership’s activities. Partners and partnerships should consult their own tax advisors with regard to the U.S. federal income tax treatment of an investment in our common stock.

Distributions of cash or property, if any, paid to a non-U.S. holder of our common stock will constitute “dividends” for U.S. federal income tax purposes to the extent paid out of our current or accumulated earnings and profits, as determined for U.S. federal income tax purposes. If the amount of a distribution exceeds both our current and accumulated earnings and profits, such excess will first constitute a nontaxable return of capital, which will reduce the holder’s tax basis in our common stock, but not below zero. Any excess will be treated as gain from the sale of our common stock and will be treated as described below.

Subject to the following paragraphs, dividends on our common stock generally will be subject to U.S. federal withholding tax at a 30% gross rate, subject to any exemption or lower rate as may be specified by an applicable income tax treaty. We may withhold up to 30% of either (i) the gross amount of the entire distribution, even if the amount of the distribution is greater than the amount constituting a dividend, as described above or (ii) the amount of the distribution we project will be a dividend, based upon a reasonable estimate of both our current and our accumulated earnings and profits for the taxable year in which the distribution is made. If tax is withheld on the amount of a distribution in excess of the amount constituting a dividend, then a non-U.S. holder may obtain a refund of that excess amount by timely filing a claim for refund with the IRS.

To claim the benefit of a reduced rate of or an exemption from U.S. federal withholding tax under an applicable income tax treaty, a non-U.S. holder will be required (i) to satisfy certain certification requirements, which may be made by providing us or our agent with a properly executed and completed IRS Form W-8BEN or W-8BEN-E (or other applicable form) certifying, under penalty of perjury, that the holder qualifies for treaty benefits and is not a U.S. person or (ii) if our common stock is held through certain non-U.S. intermediaries, to satisfy the relevant certification requirements of the applicable Treasury Regulations. Special certification and other requirements apply to certain non-U.S. holders that are pass-through entities. Non-U.S. holders that do not timely provide us or our paying agent with the required certification, but that qualify for a reduced treaty rate, may obtain a refund of any excess amounts withheld by timely filing an appropriate claim for refund with the IRS. Non-U.S. holders should consult their tax advisors regarding their entitlement to benefits under an applicable income tax treaty.

Dividends that are effectively connected with the conduct of a trade or business by the non-U.S. holder within the United States (and, if required by an applicable income tax treaty, are attributable to a permanent establishment, or a fixed base in the case of an individual non-U.S. holder, that is maintained by the non-U.S. holder in the United States), or effectively connected dividends, are not subject to the U.S. federal withholding tax, provided that the non-U.S. holder certifies, under penalty of perjury, that the dividends paid to such holder are effectively connected dividends on a properly executed and completed IRS Form W-8ECI (or other applicable form). Instead, any such dividends will be subject to U.S. federal income tax on a net income basis in a manner similar to that which would apply if the non-U.S. holder were a U.S. holder.

Corporate non-U.S. holders who receive effectively connected dividends may also be subject to an additional “branch profits tax” at a gross rate of 30% on their earnings and profits for the taxable year that are effectively connected with the holder’s conduct of a trade or business within the United States, subject to any exemption or reduction provided by an applicable income tax treaty.

Any gain realized on the sale, exchange or other taxable disposition of our common stock generally will not be subject to U.S. federal income tax unless:

A non-U.S. holder described in the first bullet point above generally will be subject to U.S. federal income tax on the net gain derived from the sale or other taxable disposition under applicable U.S. federal income tax rates as if the holder were a U.S. holder. If the non-U.S. holder is a corporation, then the gain may also, under certain circumstances, be subject to the “branch profits” tax discussed above.

With respect to the second bullet point, although there can be no assurance, we believe we are not, have not been and will not become a “United States real property holding corporation” for U.S. federal income tax purposes. In the event that we are or become a United States real property holding corporation at any time during the applicable period described in the second bullet point above, any gain recognized on a sale or other taxable disposition of our common stock may be subject to U.S. federal income tax, including any applicable withholding tax, if (i) the non-U.S. holder beneficially owns, or has owned, more than 5% of our common stock at any time during the applicable period or (ii) our common stock ceases to be regularly traded on an “established securities market” within the meaning of the Code. Non-U.S. holders who intend to acquire more than 5% of our common stock are encouraged to consult their tax advisors with respect to the U.S. tax consequences of a disposition of our common stock.

Common stock owned or treated as owned by an individual who is a non-U.S. holder at the time of his or her death generally will be included in the individual’s gross estate for U.S. federal estate tax purposes and may be subject to U.S. federal estate tax unless an applicable estate tax treaty provides otherwise.

Information returns will be filed with the IRS in connection with payments of dividends on our common stock and the proceeds from a sale or other taxable disposition of our common stock. Copies of information returns may be made available to the tax authorities of the country in which a non-U.S. holder resides or is incorporated under the provisions of a specific treaty or agreement.

You may be subject to backup withholding with respect to dividends paid on our common stock or with respect to proceeds received from a disposition of the shares of our common stock. Certain holders (including, among others, corporations and certain tax-exempt organizations) are generally not subject to backup withholding. You will be subject to backup withholding if you are not otherwise exempt and you:

Backup withholding is not an additional tax, but rather is a method of tax collection. You generally will be entitled to credit any amounts withheld under the backup withholding rules against your U.S. federal income tax liability provided that the required information is furnished to the IRS in a timely manner.

A non-U.S. holder may have to comply with certification procedures to establish that it is not a U.S. person in order to avoid information reporting and backup withholding tax requirements. The certification procedures required to claim a reduced rate of withholding under an income tax treaty will satisfy the certification requirements necessary to avoid backup withholding as well. The amount of any backup withholding from a payment to a non-U.S. holder may be allowed as a credit against such holder’s U.S. federal income tax liability and may entitle such non-U.S. holder to a refund, provided that the required information is timely furnished to the IRS.

Under the Foreign Account Tax Compliance Act provisions of the Code and related Treasury guidance, or FATCA, a withholding tax of 30% will be imposed in certain circumstances on payments of (i) dividends on our common stock and (ii) gross proceeds from the sale or other disposition of our common stock after December 31, 2018. In the case of payments made to a “foreign financial institution” (as defined for FATCA purposes), as a beneficial owner or as an intermediary, the tax generally will be imposed, subject to certain exceptions, unless such institution (i) enters into (or is otherwise subject to) and complies with a reporting agreement with the U.S. government, or FATCA Agreement or (ii) complies with applicable foreign law enacted in connection with an intergovernmental agreement between the United States and a foreign jurisdiction in either case to, among other things, collect and provide to the U.S. or other relevant tax authorities certain information regarding U.S. account holders of such institution. In the case of payments made to a foreign entity that is not a financial institution, the tax generally will be imposed, subject to certain exceptions, unless such entity provides the withholding agent with a certification that it does not have any “substantial” U.S. owners (generally,

any specified U.S. person that directly or indirectly owns more than a 10% of such entity) or that identifies its “substantial” U.S. owners. If our common stock is held through a foreign financial institution that enters into (or is otherwise subject to) a FATCA Agreement, such foreign financial institution (or, in certain cases, a person paying amounts to such foreign financial institution) may be required, subject to applicable exceptions, to withhold such tax on payments of dividends and gross proceeds described above made to (i) a person (including an individual) that fails to comply with certain information requests or (ii) a foreign financial institution that has not complied with its obligations under FATCA. Each non-U.S. holder should consult its own tax advisor regarding the application of FATCA to an investment in our common stock.

We have entered into an underwriting agreement with the underwriters named below on , 2018. Oppenheimer & Co. Inc. is acting as the sole book running manager and representative of the underwriters for this offering. The underwriting agreement provides for the purchase of a specific number of shares of common stock by each of the underwriters. The underwriters' obligations are several, which means that each underwriter is required to purchase a specified number of shares, but is not responsible for the commitment of any other underwriter to purchase shares. If an underwriter defaults, the underwriting agreement provides that the purchase commitments of the nondefaulting underwriters may be increased or the underwriting agreement may be terminated.

Subject to the terms and conditions of the underwriting agreement, we have agreed to sell to the underwriters, and each underwriter has severally and not jointly agreed to purchase, at the public offering price less the underwriting discounts and commissions set forth on the cover page of this prospectus, the number of shares of common stock listed next to its name in the following table:

Each underwriter is committed to purchase all the shares of common stock listed next to its name in the above table if it purchases any such securities.

We have granted the underwriters an over-allotment option. This option, which is exercisable for up to 30 days after the date of this prospectus, permits the underwriters to purchase a maximum of additional shares of common stock at a price of $ per share from us to cover over-allotments, if any. If the underwriters exercise all or part of this option, they will purchase shares of common stock covered by the option at the initial public offering price that appears on the cover page of this prospectus, less the underwriting discounts and commissions. If this option is exercised in full, the total price to public will be $ , the total proceeds to us will be $ . The underwriters have severally agreed that, to the extent the over-allotment option is exercised, they will each purchase a number of additional shares of common stock proportionate to the underwriter’s initial amount reflected in the foregoing table.

Each underwriter proposes to offer the shares of common stock directly to the public at the public offering price set forth on the cover page of this prospectus. In addition, the representative may offer some of the shares to other securities dealers at such price less a concession of $ per share. The underwriters may also allow, and such dealers may reallow, a concession not in excess of $ per share to other dealers. After the public offering of the securities, the offering price and other selling terms may be changed by the representative.

The following table shows the underwriting discounts and commissions to be paid by us in connection with this offering, assuming either no exercise and full exercise of the option to purchase additional shares:

We estimate that our expenses associated with the offering, excluding the estimated underwriting discount and commission, will be approximately $244,000. We have also agreed to pay certain reasonable and documented costs and expenses of the underwriters, including the reasonable fees and disbursements of underwriters' counsel, provided that such legal fees may not exceed $120,000.

We have agreed to indemnify the underwriters against certain liabilities, including liabilities under the Securities Act of 1933, or to contribute to payments the underwriters may be required to make in respect of those liabilities.

Subject to certain exceptions set forth in the underwriting agreement or lock-up agreements, as applicable, we, our executive officers and directors, and certain of our other existing stockholders that are affiliated with our directors have agreed not to sell or transfer any common stock or securities convertible into, exchangeable for, exercisable for, or repayable with common stock, for 90 days, with respect to us and our executive officers and directors, and 45 days, with respect to certain of our other existing stockholders that are affiliated with our directors, after the date of this prospectus without first obtaining the written consent of Oppenheimer & Co. Inc.

This lock-up provision applies to common stock and to securities convertible into or exchangeable or exercisable for or repayable with common stock. It also applies to common stock owned now or acquired later by the person executing the agreement or for which the person executing the agreement later acquires the power of disposition. See also "Risk Factors—Risks Related to our Common Stock and this Offering—Future sales, or the perception of future sales, of shares of our common stock could materially reduce the market price of our common stock."

Rules of the SEC may limit the ability of the underwriters to bid for or purchase shares of common stock before the distribution of the shares is completed. However, the underwriters may engage in the following activities in accordance with the rules:

Similar to other purchase transactions, the underwriters’ purchases to cover the syndicate short sales or to stabilize the market price of our common stock may have the effect of raising or maintaining the market price of our common stock or preventing or mitigating a decline in the market price of our common stock. As a result, the price of the shares of our common stock may be higher than the price that might otherwise exist in the open market. The imposition of a penalty bid might also have an effect on the price of the shares if it discourages resales of the shares.

Neither we nor the underwriters make any representation or prediction as to the effect that the transactions described above may have on the price of the shares. These transactions may occur on The NASDAQ Global Market or otherwise. If such transactions are commenced, they may be discontinued without notice at any time.

A prospectus in electronic format may be delivered to potential investors by one or more of the underwriters participating in this offering. The prospectus in electronic format will be identical to the paper version of such preliminary prospectus. Other than the prospectus in electronic format, the information on any underwriter's website and any information contained in any other website maintained by an underwriter is not part of this prospectus, the accompanying prospectus or the registration statement of which this prospectus and the accompanying prospectus forms a part.

The underwriters and their affiliates have provided in the past and may provide from time to time in the future certain commercial banking, financial advisory, investment banking and other services for us and our affiliates in the ordinary course of their business, for which they may receive customary fees and commissions. In addition, from time to time, the underwriters and their affiliates may effect transactions for their own accounts or the accounts of customers, and hold on behalf of themselves or their customers, long or short positions in our debt or equity securities or loans, and may do so in the future.

The offering is exclusively conducted under applicable private placement exemptions and therefore it has not been and will not be notified to, and this document or any other offering material relating to the securities has not been and will not be approved by, the Belgian Banking, Finance and Insurance Commission ("Commission bancaire, financière et des assurances/Commissie voor het Bank-, Financie- en Assurantiewezen"). Any representation to the contrary is unlawful.

Each underwriter has undertaken not to offer, sell, resell, transfer or deliver directly or indirectly, any securities, or to take any steps relating/ancillary thereto, and not to distribute or publish this document or any other material relating to the securities or to the offering in a manner which would be construed as: (a) a public offering under the Belgian Royal Decree of 7 July 1999 on the public character of financial transactions; or (b) an offering of securities to the public under Directive 2003/71/EC which triggers an obligation to publish a prospectus in Belgium. Any action contrary to these restrictions will cause the recipient and the issuer to be in violation of the Belgian securities laws.

Neither this document nor any other offering material relating to the securities has been submitted to the clearance procedures of the Autorité des marchés financiers in France. The securities have not been offered or sold and will not be offered or sold, directly or indirectly, to the public in France. Neither this document nor any other offering material relating to the securities has been or will

be: (a) released, issued, distributed or caused to be released, issued or distributed to the public in France; or (b) used in connection with any offer for subscription or sale of the securities to the public in France. Such offers, sales and distributions will be made in France only: (i) to qualified investors (investisseurs qualifiés) and/or to a restricted circle of investors (cercle restreint d'investisseurs), in each case investing for their own account, all as defined in and in accordance with Articles L.411-2, D.411-1, D.411-2, D.734-1, D.744-1, D.754-1 and D.764-1 of the French Code monétaire et financier; (ii) to investment services providers authorized to engage in portfolio management on behalf of third parties; or (iii) in a transaction that, in accordance with article L.411-2-II-1° -or-2° -or 3° of the French Code monétaire et financier and article 211-2 of the General Regulations (Règlement Général) of the Autorité des marchés financiers, does not constitute a public offer (appel public à l'épargne). Such securities may be resold only in compliance with Articles L.411-1, L.411-2, L.412-1 and L.621-8 through L.621-8-3 of the French Code monétaire et financier.

In relation to each Member State of the European Economic Area which has implemented the Prospectus Directive (each, a "Relevant Member State") each underwriter has represented, warranted and agreed that with effect from and including the date on which the Prospectus Directive is implemented in that Relevant Member State (the "Relevant Implementation Date") it has not made and will not make an offer of any securities to the public in that Relevant Member State,, except that, with effect from and including the Relevant Implementation Date, an offer to the public in that Relevant Member State of any securities may be made at any time:

provided that no such offer of securities shall result in a requirement for the publication by issuer or any underwriter of a prospectus pursuant to Article 3 of the Prospectus Directive or supplement a prospectus pursuant to Article 16 of the Prospective Directive.

For the purposes of this provision, the expression an "offer to the public" in relation to any securities in any Relevant Member State means the communication in any form and by any means of sufficient information on the terms of the offer and any securities to be offered so as to enable an investor to decide to purchase any securities, as the same may be varied in that Member State by any measure implementing the Prospectus Directive in that Member State and the expression "Prospectus Directive" means Directive 2003/71/EC (as amended including by Directive 2010/73/EU) and includes any relevant implementing measure in each Relevant Member State.

In the State of Israel, the securities offered hereby may not be offered to any person or entity other than the following:

Any offeree of the securities offered hereby in the State of Israel shall be required to submit written confirmation that it falls within the scope of one of the above criteria. This prospectus will not be distributed or directed to investors in the State of Israel who do not fall within one of the above criteria.

The offering of the securities offered hereby in Italy has not been registered with the Commissione Nazionale per la Società e la Borsa ("CONSOB") pursuant to Italian securities legislation and, accordingly, the securities offered hereby cannot be offered, sold or delivered in the Republic of Italy ("Italy") nor may any copy of this document or any other document relating to the securities offered hereby be distributed in Italy other than to professional investors (operatori qualificati) as defined in Article 31, second paragraph, of CONSOB Regulation No. 11522 of 1 July, 1998 as subsequently amended. Any offer, sale or delivery of the securities offered hereby or distribution of copies of this prospectus or any other document relating to the securities offered hereby in Italy must be made:

This prospectus has not been nor will it be registered with or approved by Finansinspektionen (the Swedish Financial Supervisory Authority). Accordingly, this prospectus may not be made available, nor may the securities offered hereunder be marketed and offered for sale in Sweden, other than under circumstances which are deemed not to require a prospectus under the Financial Instruments Trading Act (1991: 980). This offering will be made to no more than 100 persons or entities in Sweden.

The securities offered pursuant to this prospectus will not be offered, directly or indirectly, to the public in Switzerland and this prospectus does not constitute a public offering prospectus as that term is understood pursuant to art. 652a or art. 1156 of the Swiss Federal Code of Obligations. The issuer has not applied for a listing of the securities being offered pursuant to this prospectus on the SWX Swiss Exchange or on any other regulated securities market, and consequently, the information presented in this prospectus does not necessarily comply with the information standards set out in the relevant listing rules. The securities being offered pursuant to this prospectus have not been registered with the Swiss Federal Banking Commission as foreign investment funds, and the investor protection afforded to acquirers of investment fund certificates does not extend to acquirers of securities.

Investors are advised to contact their legal, financial or tax advisers to obtain an independent assessment of the financial and tax consequences of an investment in securities.

This document constitutes an "exempt offering document" as defined in and for the purposes of applicable Canadian securities laws. No prospectus has been filed with any securities commission or similar regulatory authority in Canada in connection with the offer and sale of the securities described herein (the "Securities"). No securities commission or similar regulatory authority in Canada has reviewed or in any way passed upon this document or on the merits of the Securities and any representation to the contrary is an offence.

Canadian investors are advised that this document has been prepared in reliance on section 3A.3 of National Instrument 33-105 Underwriting Conflicts ("NI 33-105"). Pursuant to section 3A.3 of NI 33-105, this document is exempt from the requirement to provide investors with certain conflicts of interest disclosure pertaining to "connected issuer" and/or "related issuer" relationships as would otherwise be required pursuant to subsection 2.1(1) of NI 33-105.

The offer and sale of the securities in Canada is being made on a private placement basis only and is exempt from the requirement to prepare and file a prospectus under applicable Canadian securities laws. Any resale of Securities acquired by a Canadian investor in this offering must be made in accordance with applicable Canadian securities laws, which may vary depending on the relevant jurisdiction, and which may require resales to be made in accordance with Canadian prospectus requirements, a statutory exemption from the prospectus requirements, in a transaction exempt from the prospectus requirements or otherwise under a discretionary exemption from the prospectus requirements granted by the applicable local Canadian securities regulatory authority. These resale restrictions may under certain circumstances apply to resales of the Securities outside of Canada.

Each Canadian investor who purchases the securities will be deemed to have represented to the issuer and to each dealer from whom a purchase confirmation is received, as applicable, that the investor (i) is purchasing as principal, or is deemed to be purchasing as principal in accordance with applicable Canadian securities laws, for investment only and not with a view to resale or redistribution; (ii) is an "accredited investor" as such term is defined in section 1.1 of National Instrument 45-106 Prospectus Exemptions ("NI 45-106") or, in Ontario, as such term is defined in section 73.3(1) of the Securities Act (Ontario); and (iii) is a "permitted client" as such term is defined in section 1.1 of National Instrument 31-103 Registration Requirements, Exemptions and Ongoing Registrant Obligations.

Any discussion of taxation and related matters contained in this document does not purport to be a comprehensive description of all of the tax considerations that may be relevant to a Canadian investor when deciding to purchase the securities and, in particular, does not address any Canadian tax considerations. No representation or warranty is hereby made as to the tax consequences to a resident, or deemed resident, of Canada of an investment in the securities or with respect to the eligibility of the securities for investment by such investor under relevant Canadian federal and provincial legislation and regulations.

Securities legislation in certain of the Canadian jurisdictions provides certain purchasers of securities pursuant to an offering memorandum, including where the distribution involves an "eligible foreign security" as such term is defined in Ontario Securities

Commission Rule 45-501 Ontario Prospectus and Registration Exemptions and in Multilateral Instrument 45-107 Listing Representation and Statutory Rights of Action Disclosure Exemptions, as applicable, with a remedy for damages or rescission, or both, in addition to any other rights they may have at law, where the offering memorandum, or other offering document that constitutes an offering memorandum, and any amendment thereto, contains a "misrepresentation" as defined under applicable Canadian securities laws. These remedies, or notice with respect to these remedies, must be exercised or delivered, as the case may be, by the purchaser within the time limits prescribed under, and are subject to limitations and defences under, applicable Canadian securities legislation. Purchasers should refer to the applicable provisions of the securities legislation of their Canadian jurisdiction for the particulars of these rights or consult with a legal adviser.

Upon receipt of this document, each Canadian investor hereby confirms that it has expressly requested that all documents evidencing or relating in any way to the sale of the Securities described herein (including for greater certainty any purchase confirmation or any notice) be drawn up in the English language only. Par la réception de ce document, chaque investisseur canadien confirme par les présentes qu'il a expressément exigé que tous les documents faisant foi ou se rapportant de quelque manière que ce soit à la vente des valeurs mobilières décrites aux présentes (incluant, pour plus de certitude, toute confirmation d'achat ou tout avis) soient rédigés en anglais seulement.

Certain legal matters relating to this offering will be passed upon for us by Stradling Yocca Carlson & Rauth, P.C., Newport Beach, California. Certain legal matters relating to this offering will be passed upon for the underwriters by Clifford Chance LLP, London, England.

The financial statements of Tandem Diabetes Care, Inc. as of December 31, 2017 and 2016, and for each of the three years in the period then ended appearing in Tandem Diabetes Care, Inc.’s Annual Report on Form 10-K for the year ended December 31, 2017, have been audited by Ernst & Young LLP, independent registered public accounting firm, as set forth in its report thereon, included therein, and incorporated herein by reference. Such financial statements are incorporated herein by reference in reliance upon such report given on the authority of such firm as experts in accounting and auditing.

The SEC allows us to “incorporate” into this prospectus supplement and the accompanying prospectus information that we file with the SEC in other documents. This means that we can disclose important information to you by referring to other documents that contain that information. Any information that we incorporate by reference into this prospectus supplement and the accompanying prospectus is considered part of this prospectus supplement and the accompanying prospectus.

Information contained in this prospectus supplement and the accompanying prospectus, and information that we file with the SEC in the future and incorporate by reference in this prospectus supplement and the accompanying prospectus, automatically modifies and supersedes previously filed information, including information in previously filed documents or reports that have been incorporated by reference in this prospectus supplement and the accompanying prospectus, to the extent the new information differs from or is inconsistent with the old information. Any statement so modified will be deemed to constitute a part of this prospectus supplement and the accompanying prospectus only as so modified, and any statement so superseded will be deemed not to constitute a part of this prospectus supplement or the accompanying prospectus.

We incorporate by reference, as of their respective dates of filing, the documents listed below that we have filed with the SEC and any future documents (excluding information furnished pursuant to Items 2.02 and 7.01 of Form 8-K) that we file with the SEC pursuant to Sections 13(a), 13(c), 14 or 15(d) of the Securities Exchange Act of 1934, as amended, between the date of this prospectus supplement and the termination of the offering of the securities described in this prospectus supplement, other than, in each case, documents or information deemed to have been “furnished” and not “filed” in accordance with SEC rules:

You may request a free copy of any of the documents incorporated by reference into this prospectus. Requests should be made to:

Exhibits to the documents will not be sent, however, unless those exhibits have specifically been incorporated by reference in this prospectus supplement or the accompanying prospectus.

The information contained in this prospectus supplement, the accompanying prospectus, any document incorporated by reference herein or therein, and in any free writing prospectuses we may provide to you in connection with this offering is accurate only as of the date such information is presented. Our business, financial condition, results of operations and prospects may have changed since those respective dates.

We file annual, quarterly and current reports and other information with the SEC. You may read and copy any document we file at the SEC’s Public Reference Room located at 100 F Street, N.E., Washington, D.C. 20549. You may obtain information on the operation of the public reference room by calling the SEC at 1-800-SEC-0330. Our filings with the SEC also are available from the SEC's website at www.sec.gov, which contains reports, proxy and information statements, and other information regarding issuers that file electronically.

This prospectus supplement and the accompanying prospectus are part of a registration statement that we filed with the SEC. As permitted by SEC rules, this prospectus supplement and the accompanying prospectus form a part of the registration statement, but do not contain all of the information that is included in the registration statement. The registration statement contains more information regarding us and our securities, including certain exhibits. You can obtain a copy of the registration statement from the SEC at the address listed above or from the SEC’s website.

We may offer and sell, from time to time in one or more offerings, any combination of common stock, preferred stock, debt securities, warrants or units having an aggregate initial offering price not to exceed $100,000,000.00. The preferred stock may be convertible into shares of our common stock, shares of our preferred stock or debt securities. The debt securities may be convertible into or exchangeable for shares of our common stock, shares of our preferred stock or other debt securities. The warrants may be exercisable for shares of our common stock, shares of our preferred stock or debt securities. The units may consist of any combination of the other types of securities described in this prospectus.

This prospectus provides a general description of the securities we may offer. Each time we sell a particular class of securities, we will provide specific terms of the securities offered in a supplement to this prospectus. The prospectus supplement may also add, update or change information in this prospectus. You should read this prospectus and any prospectus supplement, as well as the documents incorporated by reference or deemed to be incorporated by reference herein or therein, carefully before you invest in any of the securities offered pursuant to this prospectus.

This prospectus may not be used to offer or sell our securities unless accompanied by a prospectus supplement relating to the offered securities.

These securities may be sold directly by us, through dealers or agents designated from time to time, to or through underwriters or dealers or through a combination of these methods on a continuous or delayed basis. For additional information on the methods of sale, you should refer to the section entitled “Plan of Distribution” in this prospectus. We will describe the plan of distribution for any particular offering of our securities in a prospectus supplement. If any agents, underwriters or dealers are involved in the sale of any securities with respect to which this prospectus is being delivered, we will set forth in a prospectus supplement the names of such agents or underwriters and any applicable fees, commissions, discounts and over-allotment options. We will also set forth in a prospectus supplement the price to the public of such securities and the net proceeds that we expect to receive from such sale.

Our common stock is listed on the NASDAQ Global Market and traded under the symbol “TNDM.” On April 23, 2018, the last reported sale price for our common stock on the NASDAQ Global Select Market was $7.37 per share.

INVESTING IN OUR SECURITIES INVOLVES A HIGH DEGREE OF RISK. YOU SHOULD REVIEW CAREFULLY THE RISKS AND UNCERTAINTIES DESCRIBED UNDER THE HEADING “RISK FACTORS” BEGINNING ON PAGE 4 OF THIS PROSPECTUS, AS WELL AS THE RISKS AND UNCERTAINTIES DESCRIBED UNDER A SIMILAR HEADING IN ANY APPLICABLE PROSPECTUS SUPPLEMENT AND IN THE DOCUMENTS THAT WE INCORPORATE BY REFERENCE HEREIN OR THEREIN.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or passed upon the accuracy or adequacy of this prospectus. Any representation to the contrary is a criminal offense.

This prospectus is part of a registration statement that we filed with the Securities and Exchange Commission, or the SEC, using a “shelf” registration process. Under this shelf registration process, we may from time to time offer and sell any combination of the securities described in this prospectus in one or more offerings with an aggregate initial offering price not to exceed $100,000,000.00. We have provided to you in this prospectus a general description of the securities we may offer. Each time we offer or sell any of our securities under this prospectus, we will, to the extent required by law, provide a prospectus supplement that will contain specific information about the terms of the offering.

We may add, update or change any of the information contained in this prospectus or in any accompanying prospectus supplement we may authorize to be delivered to you. To the extent there is a conflict between the information contained in this prospectus and any accompanying prospectus supplement, you should rely on the information in the prospectus supplement, provided that if any statement in one of these documents is inconsistent with a statement in another document having a later date-for example, a document incorporated by reference in this prospectus or any prospectus supplement-the statement in the document having the later date modifies or supersedes the earlier statement. Any statement so modified will be deemed to constitute a part of this prospectus only as so modified, and any statement so superseded will be deemed not to constitute a part of this prospectus. This prospectus, together with any accompanying prospectus supplement, includes all material information relating to an offering pursuant to this registration statement.

You should rely only on the information contained in this prospectus, in any accompanying prospectus supplement, or in any document incorporated by reference herein or therein. We have not authorized anyone to provide you with any different information. We take no responsibility for, and can provide no assurance as to the reliability of, any other information that others may provide to you. The information contained in this prospectus, in any applicable prospectus supplement, and in the documents incorporated by reference herein or therein is accurate only as of the date such information is presented. Our business, financial condition, results of operations and future prospects may have changed since those respective dates.

This prospectus and any accompanying prospectus supplement does not constitute an offer to sell or the solicitation of an offer to buy any securities other than the registered securities to which they relate, nor does this prospectus and any accompanying prospectus supplement constitute an offer to sell or the solicitation of an offer to buy securities in any jurisdiction to any person to whom it is unlawful to make such offer or solicitation in such jurisdiction. This prospectus may not be used to offer or sell our securities unless accompanied by a prospectus supplement relating to the offered securities.

The registration statement containing this prospectus, including the exhibits to the registration statement, provides additional information about us and the securities offered pursuant to this prospectus. For a more complete understanding of the offering of the securities, you should refer to the registration statement, including its exhibits. The registration statement can be read on the SEC’s website or at the SEC’s offices mentioned under the heading “Where You Can Find More Information.”

This prospectus, any accompanying prospectus supplement, and the documents incorporated by reference herein and therein, contain forward-looking statements within the meaning of the federal securities laws. These forward-looking statements are intended to qualify for the safe harbor from liability established by the Private Securities Litigation Reform Act of 1995. All statements other than statements of historical fact included in this prospectus, any accompanying prospectus supplement, or the documents incorporated by reference herein or therein, are forward looking statements.

Our forward-looking statements are based on our management’s current assumptions and expectations of future events and trends, which affect or may affect our business, strategy, operations or financial performance. Although we believe that these forward-looking statements are based upon reasonable assumptions, they are subject to numerous known and unknown risks and uncertainties and are made in light of information currently available to us. Many important factors, in addition to the factors described in this prospectus, may materially and adversely affect our results as indicated in our forward-looking statements. You should read this prospectus, any accompanying prospectus supplement, and the documents we incorporate by reference herein and therein, completely and with the understanding that our actual future results may be materially different from and worse than what we expect.

Forward-looking statements speak only as of the date they were made, and, except to the extent required by law or the rules of the NASDAQ Stock Market, we undertake no obligation to update or review any forward-looking statement because of new information, future events or other factors. You should, however, review the risks and uncertainties we describe in the reports we will file from time to time with the SEC, after the date of this prospectus. See the information included under the heading “Where You Can Find More Information.”

Forward-looking statements involve risks and uncertainties and are not guarantees of future performance. As a result of the risks and uncertainties described above, the forward-looking statements discussed in this prospectus might not occur and our future results and performance may differ materially from the information provided in these forward-looking statements due to, but not limited to, the factors mentioned above. Because of these uncertainties, you should not place undue reliance on these forward-looking statements when making an investment decision.

We qualify all of our forward-looking statements by these cautionary statements.

We are a medical device company with an innovative approach to the design, development and commercialization of products for people with insulin-dependent diabetes. We believe our competitive advantage is rooted in our unique consumer-focused approach, and the incorporation of modern and innovative technology into our product offerings. Our manufacturing and sales activities primarily focus on our flagship product, the t:slim X2 Insulin Delivery System, or t:slim X2™, which is based on our proprietary technology platform. The simple-to-use t:slim X2 is the smallest durable insulin pump available, and the only pump currently available in the United States that is capable of remote feature updates, which positions us well to address the evolving needs and preferences of differentiated segments of the insulin-dependent diabetes market. By delivering innovative hardware and software solutions, as well as best-in-class customer support, we aim to improve and simplify the lives of people with diabetes and their healthcare providers.

Since the launch of our first product in August 2012 through March 31, 2018, we have shipped more than 72,000 pumps to customers in the United States, of which over 63,000 pumps have been shipped within the four years ended March 31, 2018. We plan to begin commercialization of t:slim X2 in select geographies outside the United States, including Canada, during 2018.

We began commercial sales of our first insulin pump, the t:slim Insulin Delivery System, or t:slim, in August 2012. During 2015, we commenced commercial sales of two additional insulin pumps: t:flex in May 2015 and t:slim G4 in September 2015. In October 2016, we commenced commercial sales of t:slim X2 and discontinued new sales of t:slim. In September 2017, we commenced commercial sales of t:slim X2 integrated with the Dexcom G5 Mobile CGM system and discontinued new sales of t:slim G4. The t:slim X2 technology platform now represents nearly 100% of our new pump shipments. Accordingly, the Company discontinued marketing and sales of new t:flex pumps in the third quarter of 2018.

Our insulin pump products are generally considered durable medical equipment, and have an expected lifespan of at least four years. In addition to selling insulin pumps, we sell disposable products that are used together with our pumps and replaced every few days, including cartridges for storing and delivering insulin, and infusion sets that connect the insulin pump to a user’s body. In September 2017, we commenced commercial sales of cartridge and infusion set products using t:lock™, which replaces the standard Luer-lok connector that historically joined an infusion set to our cartridge.

Our headquarters and our manufacturing facility are located in San Diego, California and we employed 572 full-time employees as of March 31, 2018.

We were incorporated in Colorado in January 2006 and reincorporated in Delaware in January 2008. Our principal executive offices are located at 11075 Roselle Street, San Diego, California 92121. The telephone number of our principal executive office is (858) 366-6900. Our website is www.tandemdiabetes.com. The information contained on or accessed through our website is not incorporated by reference into this prospectus, and you should not consider information contained on our website to be a part of this prospectus or in deciding whether to purchase our securities. References in this prospectus to our website are to inactive textual references only.

Investing in our securities involves a high degree of risk. Before making an investment decision, you should carefully consider the risks described in the sections entitled “Risk Factors” in our most recent Annual Report on Form 10-K and subsequent Quarterly Reports on Form 10-Q, as filed with the SEC, which are incorporated by reference in this prospectus in their entirety, as well as any amendments or updates to our risk factors reflected in subsequent filings with the SEC, including any applicable prospectus supplement. If any of these risks actually occur, our business, financial condition, results of operations and future prospects could be materially and adversely affected. In that case, the trading price of our securities could decline and you might lose all or part of your investment. For more information, see “Where You Can Find More Information.”

The risks and uncertainties we have described are not the only ones we face. Additional risks and uncertainties not presently known to us or that we currently deem immaterial may also affect our business, financial condition or results of operations.

This prospectus and the documents we incorporate by reference in this prospectus contain forward-looking statements that involve risks and uncertainties. Our actual results could differ materially from those anticipated in these forward-looking statements as a result of certain factors, including the risks and uncertainties mentioned elsewhere in this prospectus. For more information, see “Special Note Regarding Forward-Looking Information.”

Whenever preferred stock and/or debt securities are to be offered and sold pursuant to this prospectus, we will file a prospectus supplement relating to that offer and sale which will, if required at that time, provide a ratio of combined fixed charges and preference dividends to earnings and/or ratio of earnings to fixed charges, respectively.

We intend to use the net proceeds we receive from the sale of our securities, and from the exercise of any warrants issued pursuant hereto, for working capital and other general corporate purposes. We also expect we may use such proceeds to satisfy all or a portion of our indebtedness to Capital Royalty Partners II, L.P. and its affiliated funds, or Capital Royalty Partners, although we have no current plans or arrangements with respect to the payoff of any such indebtedness. Pursuant to our Amended and Restated Term Loan Agreement with Capital Royalty Partners, or the Term Loan Agreement, interest is payable, at our option, (i) in cash at a rate of 11.5% per annum or (ii) at a rate of 9.5% with 2.0% added to the principal of the loan and subject to accruing interest. Interest-only payments are due quarterly on March 31, June 30, September 30 and December 31 of each year of the interest-only payment period, which ends on December 31, 2019. The principal balance is due in full on the maturity date of the Term Loan Agreement, which is March 31, 2020.

We may set forth additional information regarding the anticipated use of proceeds from the sale of securities we offer under this prospectus in a prospectus supplement relating to the specific offering. We have not determined the amount of net proceeds to be used from any specific offering. As a result, our management will have broad discretion in the allocation of the net proceeds.

Pending the use of the net proceeds, we intend to invest the net proceeds in high-quality, short-term interest-bearing obligations, investment-grade instruments, certificates of deposit or direct or guaranteed obligations of the U.S. government.

We may offer and sell, from time to time in one or more offerings, any combination of common stock, preferred stock, debt securities, warrants, and/or units having an aggregate initial offering price not to exceed $100,000,000.00. The preferred stock may be convertible into shares of our common stock, shares of our preferred stock or debt securities. The debt securities may be convertible into or exchangeable for shares of our common stock, shares of our preferred stock or other debt securities. The warrants may be exercisable for shares of our common stock, shares of our preferred stock or debt securities. The units may consist of any combination of the other types of securities described in this prospectus. In this prospectus, we refer to the common stock, preferred stock, debt securities, warrants and units that we may offer collectively as “securities.”

This prospectus provides a general description of the securities we may offer. Each time we sell any of our securities under this prospectus, we will, to the extent required by law, provide a prospectus supplement that will contain specific information about the terms of the offering. The prospectus supplement may also add, update or change information in this prospectus. For more information, see “About this Prospectus.”

The following is a summary of all material characteristics of our capital stock as set forth in our amended and restated certificate of incorporation and amended and restated bylaws. The summary does not purport to be complete and is qualified in its entirety by reference to our amended and restated certificate of incorporation and amended and restated bylaws, copies of which have been filed as exhibits to our SEC filings. For more information, see “Where You Can Find More Information.”

General. We may issue shares of our common stock from time to time. We are authorized to issue 100,000,000 shares of common stock, par value $0.001 per share. As of April 23, 2018, there were 50,067,860 shares of common stock issued and outstanding.

Reverse Stock Split. On October 9, 2017, we filed an amendment to our amended and restated certificate of incorporation to effect a reverse stock split of our issued and outstanding shares of common stock at a ratio of 1-for-10. The share amounts, exercise prices and other amounts set forth in this prospectus have been adjusted to reflect the impact of the reverse stock split.

Dividend Rights. Subject to preferences that may apply to any shares of preferred stock outstanding at the time, the holders of outstanding shares of our common stock are entitled to receive dividends out of funds legally available at the times and in the amounts that our board of directors may determine.

Voting Rights. Holders of our common stock are entitled to one vote per share. We have not provided for cumulative voting for the election of directors in our amended and restated certificate of incorporation. The board of directors is divided into three classes, which are as nearly equal in number as possible. Each director is elected for a three-year term with one class being elected at each year’s annual meeting of stockholders.

No Preemptive or Similar Rights. Our common stock is not entitled to preemptive rights, and is not subject to redemption. There are no sinking fund provisions applicable to our common stock.

Conversion. Our common stock is not convertible into any other shares of our capital stock.

Right to Receive Liquidation Distributions. Upon our liquidation, dissolution, distribution of assets or winding-up, the assets legally available for distribution to our stockholders would be distributable ratably among the holders of our common stock and any participating preferred stock outstanding at that time, if any, after payment of liquidation preferences, if any, on any outstanding shares of preferred stock and payment of claims of creditors.

Fully Paid and Non-Assessable. All of the outstanding shares of our common stock are, and the shares of our common stock issuable pursuant to this prospectus will be, fully paid and non-assessable.

As of April 23, 2018, there were no shares of our preferred stock outstanding. Pursuant to the terms of our amended and restated certificate of incorporation, our board of directors is authorized, subject to limitations prescribed by Delaware law, to issue up to 5,000,000 shares of preferred stock, par value $0.001 per share, in one or more series, to establish from time to time the number of shares to be included in each series, and to fix the designation, powers, preferences and rights of the shares of each series and any of its qualifications, limitations or restrictions, in each case without further action by our stockholders. Our board of directors also can increase or decrease the number of shares of any series of preferred stock, but not below the number of shares of that series then outstanding. Our board of directors may authorize the issuance of preferred stock with voting or conversion rights that could adversely affect the voting power or other rights of the holders of our common stock. The issuance of preferred stock, while providing flexibility in connection with possible acquisitions and other corporate purposes, could, among other things, have the effect of delaying, deferring or preventing a change in our control or the removal of management and could adversely affect the market price of our common stock and the voting and other rights of the holders of our common stock.

Whenever preferred stock is to be offered and sold pursuant to this prospectus, we will file a prospectus supplement relating to that offer and sale which will specify (in each case to the extent applicable):

The DGCL provides that the holders of preferred stock will have the right to vote separately as a class on any proposed fundamental change in the rights of the preferred stock. This right is in addition to any voting rights that may be provided for in the applicable certificate of designation.

All shares of preferred stock offered by this prospectus will, when issued, be fully paid and nonassessable and will not have any preemptive or similar rights.

The provisions of Delaware law, our amended and restated certificate of incorporation and our amended and restated bylaws may have the effect of delaying, deferring or discouraging another person from acquiring control of us by means of a tender offer, a proxy contest or otherwise, or removing incumbent officers and directors. These provisions, summarized below, are expected to discourage certain types of coercive takeover practices and takeover bids that our board of directors may consider inadequate and to encourage any person seeking to acquire control of us to first negotiate with our board of directors.

Delaware Law. We are governed by the provisions of Section 203 of the Delaware General Corporation Law, or the DGCL. In general, Section 203 prohibits a public Delaware corporation from engaging in a “business combination” with an “interested stockholder” for a period of three years after the date such stockholder became an “interested stockholder.” A “business combination” includes mergers, asset sales or other transactions resulting in a financial benefit to the stockholder. An “interested stockholder” is a person who, together with affiliates and associates, owns, or within three years did, prior to the determination of interested stockholder status, own, 15% or more of the corporation’s outstanding voting stock. These provisions may have the effect of delaying, deferring or preventing a change in control of our company not approved in advance by our board of directors.

Certificate of Incorporation and Bylaw Provisions. Our amended and restated certificate of incorporation and our amended and restated bylaws include a number of other provisions that could deter hostile takeovers or delay or prevent changes in control of our management team, including the following:

Our common stock is listed on the NASDAQ Global Market and traded under the symbol “TNDM.” On April 23, 2018, the last reported sale price for our common stock on the NASDAQ Global Market was $7.37 per share.

The transfer agent and registrar for our common stock is American Stock Transfer & Trust Company, LLC. The address of American Stock Transfer & Trust Company is 6201 15th Avenue, Brooklyn, NY 11219 and the telephone number is (718) 921-8200.

We may offer and sell from time to time in one or more series, debt securities that may be issued as senior or subordinated debt securities or as senior or subordinated convertible debt securities. While the terms we have summarized below will apply generally to any debt securities that we may offer under this prospectus, we will describe the particular terms of any debt securities that we may offer in more detail in the applicable prospectus supplement. The terms of any debt securities offered under a prospectus supplement may differ from the terms described below. Unless the context requires otherwise, whenever we refer to the indenture, we also are referring to any supplemental indentures that specify the terms of a particular series of debt securities.

We will issue the debt securities under the indenture that we will enter into with the trustee named in the indenture. The indenture will be qualified under the Trust Indenture Act of 1939, as amended, or the Trust Indenture Act. We have filed the form of indenture as an exhibit to the registration statement of which this prospectus is a part, and supplemental indentures and forms of debt securities containing the terms of the debt securities being offered and sold will be filed as exhibits to the registration statement of which this prospectus is a part and/or will be incorporated by reference from reports that we file with the SEC.

The following summary of material provisions of the debt securities and the indenture is subject to, and qualified in its entirety by reference to, all of the provisions of the indenture applicable to a particular series of debt securities. We urge you to read the applicable prospectus supplements and any related free writing prospectuses related to the debt securities that we may offer and sell under this prospectus, as well as the complete indenture that contains the terms of the debt securities.

The indenture does not limit the amount of debt securities that we may issue. It provides that we may issue debt securities up to the principal amount that we may authorize and may be in any currency or currency unit that we may designate. Except for the limitations on consolidation, merger and sale of all or substantially all of our assets contained in the indenture, the terms of the indenture do not contain any covenants or other provisions designed to give holders of any debt securities protection against changes in our operations, financial condition or transactions.

We may issue the debt securities issued under the indenture as “discount securities,” which means they may be sold at a discount to their stated principal amount. These debt securities, as well as other debt securities that are not issued at a discount, may be issued with “original issue discount,” or OID, for U.S. federal income tax purposes because of interest payment and other characteristics or terms of the debt securities. Material U.S. federal income tax considerations applicable to debt securities issued with OID will be described in more detail in any applicable prospectus supplement.

Whenever debt securities are to be issued and sold pursuant to this prospectus, we will file a prospectus supplement relating to that offer and sale which will specify (in each case to the extent applicable):

We will set forth in the applicable prospectus supplement the terms on which a series of debt securities may be convertible into or exchangeable for our common stock, our preferred stock or our other debt securities. We will include provisions as to settlement upon conversion or exchange and whether conversion or exchange is mandatory, at the option of the holder or at our option. We may include provisions pursuant to which the number of shares of our common stock, our preferred stock or our other debt securities that the holders of the series of debt securities receive would be subject to adjustment.

Unless we provide otherwise in the prospectus supplement applicable to a particular series of debt securities, the indenture will not contain any covenant that restricts our ability to merge or consolidate, or sell, convey, transfer or otherwise dispose of all or substantially all of our assets.

Unless we provide otherwise in the prospectus supplement applicable to a particular series of debt securities, the following are events of default under the indenture with respect to any series of debt securities that we may issue:

If an event of default with respect to debt securities of any series occurs and is continuing, other than an event of default specified in the last bullet point above, the trustee or the holders of at least 25% in aggregate principal amount of the outstanding debt securities of that series, by notice to us in writing, and to the trustee if notice is given by such holders, may declare the unpaid principal of, premium, if any, and accrued interest, if any, due and payable immediately. If an event of default specified in the last bullet point above occurs with respect to us, the principal amount of and accrued interest, if any, of each issue of debt securities then outstanding shall be due and payable without any notice or other action on the part of the trustee or any holder.

The holders of a majority in principal amount of the outstanding debt securities of an affected series may waive any default or event of default with respect to the series and its consequences, except defaults or events of default regarding payment of principal, premium, if any, or interest, unless we have cured the default or event of default in accordance with the indenture. Any waiver shall cure the default or event of default.

Subject to the terms of the indenture, if an event of default under an indenture shall occur and be continuing, the trustee will be under no obligation to exercise any of its rights or powers under such indenture at the request or direction of any of the holders of the applicable series of debt securities, unless such holders have offered the trustee reasonable indemnity. The holders of a majority in principal amount of the outstanding debt securities of any series will have the right to direct the time, method and place of conducting any proceeding for any remedy available to the trustee, or exercising any trust or power conferred on the trustee, with respect to the debt securities of that series, provided that:

We and the trustee may change an indenture without the consent of any holders with respect to specific matters:

In addition, under the indenture, the rights of holders of a series of debt securities may be changed by us and the trustee with the written consent of the holders of at least a majority in aggregate principal amount of the outstanding debt securities of each series that is affected. However, unless we provide otherwise in the prospectus supplement applicable to a particular series of debt securities, we and the trustee may make the following changes only with the consent of each holder of any outstanding debt securities affected:

The indenture provides that we can elect to be discharged from our obligations with respect to one or more series of debt securities, except for specified obligations, including obligations to:

In order to exercise our rights to be discharged, we must deposit with the trustee money or government obligations sufficient to pay all the principal of, any premium, if any, and interest on, the debt securities of the series on the dates payments are due.

We will issue the debt securities of each series only in fully registered form without coupons and, unless we provide otherwise in the applicable prospectus supplement, in denominations of $1,000 and any integral multiple thereof. The indenture provides that we may issue debt securities of a series in temporary or permanent global form and as book-entry securities that will be deposited with, or on behalf of, The Depository Trust Company, or DTC, or another depositary named by us and identified in the applicable prospectus supplement with respect to that series. To the extent the debt securities of a series are issued in global form and as book-entry, a description of terms relating to any book entry securities will be set forth in the applicable prospectus supplement.

At the option of the holder, subject to the terms of the indenture and the limitations applicable to global securities described in the applicable prospectus supplement, the holder of the debt securities of any series can exchange the debt securities for other debt securities of the same series, in any authorized denomination and of like tenor and aggregate principal amount.

Subject to the terms of the indenture and the limitations applicable to global securities set forth in the applicable prospectus supplement, holders of the debt securities may present the debt securities for exchange or for registration of transfer, duly endorsed or with the form of transfer endorsed thereon duly executed if so required by us or the security registrar, at the office of the security registrar or at the office of any transfer agent designated by us for this purpose. Unless otherwise provided in the debt securities that the holder presents for transfer or exchange, we will impose no service charge for any registration of transfer or exchange, but we may require payment of any taxes or other governmental charges.

We will name in the applicable prospectus supplement the security registrar, and any transfer agent in addition to the security registrar, that we initially designate for any debt securities. We may at any time designate additional transfer agents or rescind the designation of any transfer agent or approve a change in the office through which any transfer agent acts, except that we will be required to maintain a transfer agent in each place of payment for the debt securities of each series.

If we elect to redeem the debt securities of any series, we will not be required to:

The trustee, other than during the occurrence and continuance of an event of default under an indenture, undertakes to perform only those duties as are specifically set forth in the applicable indenture. Upon an event of default under an indenture, the trustee must use the same degree of care as a prudent person would exercise or use in the conduct of his or her own affairs. Subject to this provision, the trustee is under no obligation to exercise any of the powers given it by the indenture at the request of any holder of debt securities unless it is offered reasonable security and indemnity against the costs, expenses and liabilities that it might incur.

Unless we otherwise indicate in the applicable prospectus supplement, we will make payment of the interest on any debt securities on any interest payment date to the person in whose name the debt securities, or one or more predecessor securities, are registered at the close of business on the regular record date for the interest.

We will pay principal of and any premium and interest on the debt securities of a particular series at the office of the paying agents designated by us, except that unless we otherwise indicate in the applicable prospectus supplement, we will make interest payments by check that we will mail to the holder or by wire transfer to certain holders. Unless we otherwise indicate in the applicable prospectus supplement, we will designate the corporate trust office of the trustee as our sole paying agent for payments with respect to debt securities of each series. We will name in the applicable prospectus supplement any other paying agents that we initially designate for the debt securities of a particular series. We will maintain a paying agent in each place of payment for the debt securities of a particular series.

All money we pay to a paying agent or the trustee for the payment of the principal of or any premium or interest on any debt securities that remains unclaimed at the end of two years after such principal, premium or interest has become due and payable will be repaid to us, and the holder of the debt security thereafter may look only to us for payment thereof.

The indenture and the debt securities will be governed by and construed in accordance with the internal laws of the State of New York, except to the extent that the Trust Indenture Act of 1939 is applicable.

We may offer and sell, from time to time, warrants for the purchase of shares of common stock, shares of preferred stock and/or debt securities. We may issue warrants independently or together with common stock, preferred stock and/or debt securities, and the warrants may be attached to or separate from those securities. If we issue warrants, they will be evidenced by warrant agreements or warrant certificates issued under one or more warrant agreements, which will be contracts between us and the holders of the warrants or an agent for the holders of the warrants. The forms of warrant agreements or warrant certificates, as applicable, relating to the warrants will be filed as exhibits to the registration statement of which this prospectus is a part and/or will be incorporated by reference from reports that we file with the SEC.

The following summary of material provisions of the warrants and warrant agreements are subject to, and qualified in their entirety by reference to, all of the provisions of the warrant agreement and warrant certificate applicable to a particular series of warrants. We urge you to read the applicable prospectus supplement and any related free writing prospectus, as well as the complete warrant agreements and warrant certificates that contain the terms of the warrants.

Whenever warrants are to be issued and sold pursuant to this prospectus, we will file a prospectus supplement relating to that offer and sale which will specify (in each case as applicable):

Each warrant will entitle its holder to purchase the number of shares of common stock or preferred stock, or the principal amount of debt securities, at the exercise price set forth in (or calculable as set forth in) the applicable prospectus supplement. Unless we otherwise specify in the applicable prospectus supplement, holders of the warrants may exercise the warrants at any time up to the specified time on the expiration date that we set forth in the applicable prospectus supplement. After the close of business on the expiration date, unexercised warrants will become void.

A holder of warrant certificates may exchange them for new warrant certificates of different denominations, present them for registration of transfer, and exercise them as indicated in the applicable prospectus supplement. Until any warrants to purchase common stock or preferred stock are exercised, the holders of the warrants will not have any rights of holders of the underlying common stock or preferred stock, including any voting rights or any rights to receive dividends or payments upon any liquidation, dissolution or winding up on the common stock or preferred stock, if any. Until any warrants to purchase debt securities are exercised, the holder of the warrants will not have any rights of holders of the debt securities that can be purchased upon exercise, including any rights to receive payments of principal, premium or interest on the underlying debt securities or to enforce covenants in the applicable indenture.

We may offer and sell, from time to time, units comprised of one or more of the other securities described in this prospectus in any combination. Each unit will be issued so that the holder of the unit is also the holder of each security included in the unit. Thus, the holder of a unit will have the rights and obligations of a holder of each included security. If we issue units, they will be evidenced by unit agreements or unit certificates issued under one or more unit agreements, which will be contracts between us and the holders of the units or an agent for the holders of the units. The unit agreement under which a unit is issued may provide that the securities included in the unit may not be held or transferred separately, at any time or at any time before a specified date. The forms of unit agreements or unit certificates, as applicable, relating to the units will be filed as exhibits to the registration statement of which this prospectus is part of and/or will be incorporated by reference from reports that we file with the SEC.

The following summary of material provisions of the units and unit agreements are subject to, and qualified in their entirety by reference to, all of the provisions of the unit agreements applicable to the units. We urge you to read the applicable prospectus supplement and any related free writing prospectus, as well as the complete unit agreements that contain the terms of the units.

Whenever units are to be issued and sold pursuant to this prospectus, we will file a prospectus supplement relating to that offer and sale which will specify (in each case as applicable):

We may sell our securities from time to time in any manner permitted by the Securities Act, including any one or more of the following ways:

The securities may be sold at a fixed price or prices, which may be changed, at market prices prevailing at the time of sale, at prices relating to the prevailing market prices or at negotiated prices.

Offers to purchase offered securities may be solicited by agents designated by us from time to time. Any agent involved in the offer or sale of the offered securities in respect of which this prospectus is delivered will be named, and any commissions payable by us will be set forth, in the applicable prospectus supplement. Unless otherwise set forth in the applicable prospectus supplement, any agent will be acting on a reasonable best efforts basis for the period of its appointment. Any agent may be deemed to be an underwriter, as that term is defined in the Securities Act, of the offered securities so offered and sold.

We will set forth in a prospectus supplement the terms of the offering of our securities, including:

If offered securities are sold to the public by means of an underwritten offering, either through underwriting syndicates represented by managing underwriters or directly by the managing underwriters, we will execute an underwriting agreement with an underwriter or underwriters, and the names of the specific managing underwriter or underwriters, as well as any other underwriters, will be set forth in the applicable prospectus supplement. In addition, the terms of the transaction, including commissions, discounts and any other compensation of the underwriters and dealers, if any, will be set forth in the applicable prospectus supplement, which prospectus supplement will be used by the underwriters to make resales of the offered securities. If underwriters are utilized in

the sale of the offered securities, the offered securities will be acquired by the underwriters for their own account and may be resold from time to time in one or more transactions, including:

We may grant to the underwriters options to purchase additional offered securities to cover over-allotments, if any, at the public offering price with additional underwriting discounts or commissions, as may be set forth in the applicable prospectus supplement. If we grant any over-allotment option, the terms of the over-allotment option will be set forth in the applicable prospectus supplement.

We may authorize agents or underwriters to solicit offers by certain types of institutional investors to purchase securities from us at the public offering price set forth in the prospectus supplement pursuant to delayed delivery contracts providing for payment and delivery on a specified date in the future. We will describe the conditions to these contracts and the commissions we must pay for solicitation of these contracts in the prospectus supplement.

We may indemnify agents, underwriters and dealers against specified liabilities, including liabilities incurred under the Securities Act, or to contribution by us to payments they may be required to make in respect of such liabilities. Agents, underwriters or dealers, or their respective affiliates, may be customers of, engage in transactions with or perform services for us or our respective affiliates, in the ordinary course of business.

Unless otherwise specified in the applicable prospectus supplement, each class or series of securities will be a new issue with no established trading market, other than our common stock, which is traded on the NASDAQ Global Market. We may elect to list any other class or series of securities on any exchange and, in the case of our common stock, on any additional exchange. However, unless otherwise specified in the applicable prospectus supplement, we will not be obligated to do so. It is possible that one or more underwriters may make a market in a class or series of securities, but the underwriters will not be obligated to do so and may discontinue any market making at any time without notice. We cannot give any assurance as to the liquidity of the trading market for any of the offered securities.

Any underwriter may engage in over-allotment, stabilizing transactions, short-covering transactions and penalty bids in accordance with Regulation M under the Securities Exchange Act of 1934, as amended, or the Exchange Act. Over-allotment involves sales in excess of the offering size, which create a short position. Stabilizing transactions permit bids to purchase the underlying security so long as the stabilizing bids do not exceed a specified maximum price. Syndicate-covering or other short-covering transactions involve purchases of the securities, either through exercise of the over-allotment option or in the open market after the distribution is completed, to cover short positions. Penalty bids permit the underwriters to reclaim a selling concession from a dealer when the securities originally sold by the dealer are purchased in a stabilizing or covering transaction to cover short positions. Those activities may cause the price of the securities to be higher than it would otherwise be. If commenced, the underwriters may discontinue any of the activities at any time.

To comply with the securities laws of certain states, if applicable, the securities offered by this prospectus will be offered and sold in those states only through registered or licensed brokers or dealers.

In compliance with guidelines of the Financial Industry Regulatory Authority, or FINRA, the maximum consideration or discount to be received by any FINRA member or independent broker dealer may not exceed 8% of the aggregate amount of the securities offered pursuant to this prospectus and any applicable prospectus supplement.

Certain legal matters, including the validity of the issuance of the securities offered by this prospectus, will be passed upon for us by Stradling Yocca Carlson & Rauth, P.C., Newport Beach, California.

The financial statements of Tandem Diabetes Care, Inc. as of December 31, 2017 and 2016, and for each of the three years in the period ended December 31, 2017, appearing in Tandem Diabetes Care, Inc.’s Annual Report on Form 10-K for the year ended December 31, 2017, have been audited by Ernst & Young LLP, independent registered public accounting firm, as set forth in their report thereon, included therein, and incorporated herein by reference. Such financial statements are incorporated herein by reference in reliance upon such report given on the authority of such firm as experts in accounting and auditing.

The SEC allows us to “incorporate” into this prospectus information that we file with the SEC in other documents. This means that we can disclose important information to you by referring to other documents that contain that information. Any information that we incorporate by reference into this prospectus is considered part of this prospectus.

Information contained in this prospectus and information that we file with the SEC in the future and incorporate by reference in this prospectus automatically modifies and supersedes previously filed information, including information in previously filed documents or reports that have been incorporated by reference in this prospectus, to the extent the new information differs from or is inconsistent with the old information. Any statement so modified will be deemed to constitute a part of this prospectus only as so modified, and any statement so superseded will be deemed not to constitute a part of this prospectus. For more information, see “About this Prospectus.”

We incorporate by reference, as of their respective dates of filing, the documents listed below that we have filed with the SEC and any future documents that we file with the SEC pursuant to Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act, including any documents filed after the date on which the registration statement of which this prospectus is a part is initially filed until the offering of the securities covered by this prospectus has been completed, other than, in each case, documents or information deemed to have been “furnished” and not “filed” in accordance with SEC rules:

These filings have not been included in or delivered with this prospectus. We will provide to each person, including any beneficial owner to whom this prospectus is delivered, a copy of any document that is incorporated by reference in this prospectus. You may obtain a copy of these documents, at no cost, from our website (www.tandemdiabetes.com) or by contacting us using the following information:

Exhibits to the documents will not be sent, however, unless those exhibits have specifically been incorporated by reference in this prospectus.

We file annual, quarterly and current reports and other information with the SEC. You may read and copy any document we file at the SEC’s Public Reference Room located at 100 F Street, N.E., Washington, D.C. 20549. You may obtain information on the operation of the public reference room by calling the SEC at 1-800-SEC-0330. Our filings with the SEC also are available from the SEC’s website at www.sec.gov, which contains reports, proxy and information statements, and other information regarding issuers that file electronically.

This prospectus is part of a registration statement that we filed with the SEC. As permitted by SEC rules, this prospectus and any accompanying prospectus supplement that we may file, which form a part of the registration statement, do not contain all of the information that is included in the registration statement. The registration statement contains more information regarding us and our securities, including certain exhibits. You can obtain a copy of the registration statement from the SEC at the address listed above or from the SEC’s website.

	Per Share	Total
Public offering price	$	$
Underwriting discount⁽¹⁾	$	$
Proceeds, before expenses, to us	$	$

PROSPECTUS SUPPLEMENT
ABOUT THIS PROSPECTUS SUPPLEMENT	S-1
PROSPECTUS SUPPLEMENT SUMMARY	S-2
RISK FACTORS	S-7
CAUTIONARY NOTE REGARDING FORWARD-LOOKING STATEMENTS	S-10
USE OF PROCEEDS	S-11
DESCRIPTION OF COMMON STOCK	S-12
DIVIDEND POLICY	S-13
DILUTION	S-14
CERTAIN U.S. FEDERAL TAX CONSIDERATIONS APPLICABLE TO HOLDERS OF COMMON STOCK	S-16
UNDERWRITING	S-21
LEGAL MATTERS	S-28
EXPERTS	S-28
INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE	S-29
WHERE YOU CAN FIND MORE INFORMATION	S-30
ACCOMPANYING PROSPECTUS
ABOUT THIS PROSPECTUS	1
SPECIAL NOTE REGARDING FORWARD-LOOKING INFORMATION	2
ABOUT THE COMPANY	3
RISK FACTORS	4
RATIO OF EARNINGS TO FIXED CHARGES	5
USE OF PROCEEDS	6
THE SECURITIES WE MAY OFFER	7
DESCRIPTION OF CAPITAL STOCK	8
DESCRIPTION OF DEBT SECURITIES	11
DESCRIPTION OF WARRANTS	18
DESCRIPTION OF UNITS	19
PLAN OF DISTRIBUTION	20
LEGAL MATTERS	22
EXPERTS	22
INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE	23
WHERE YOU CAN FIND MORE INFORMATION	24

	Per Share	Total Without Exercise of Option	Total With Full Exercise of Option
Public offering price	$	$	$
Underwriting discount and commission	$	$	$
Proceeds, before expenses, to us	$	$	$

	Page
ABOUT THIS PROSPECTUS	1
SPECIAL NOTE REGARDING FORWARD-LOOKING INFORMATION	2
ABOUT THE COMPANY.	3
RISK FACTORS	4
RATIO OF EARNINGS TO FIXED CHARGES.	5
USE OF PROCEEDS	6
THE SECURITIES WE MAY OFFER	7
DESCRIPTION OF CAPITAL STOCK	8
DESCRIPTION OF DEBT SECURITIES.	11
DESCRIPTION OF WARRANTS	18
DESCRIPTION OF UNITS	19
PLAN OF DISTRIBUTION	20
LEGAL MATTERS	22
EXPERTS	22
INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE	23
WHERE YOU CAN FIND MORE INFORMATION	24

Assumed public offering price per share			$	28.57
Historical net tangible book value per share as of June 30, 2018	$	0.46
Increase in pro forma net tangible book value per share attributable to investors purchasing in this offering	$	1.42
Pro forma net tangible book value per share after this offering			$	1.88
Dilution per share to investors purchasing in this offering			$	26.69