By Bhanvi Satija and Sneha S K

(Reuters) -Medical device maker Dexcom beat Wall Street estimates for third-quarter revenue and profit on Thursday, helped by resilient demand for its continuous glucose monitors for diabetes patients and improved sales in international markets.

However, shares tumbled as much as 20% in extended trading after the company reiterated its full-year sales forecast and posted a 2% decline in U.S. sales for the quarter, disappointing some investors.

The stock is down 40% this year, largely due to a slump in July after Dexcom slashed its annual revenue forecast, blaming a restructuring of its sales team, fewer customers, and lower revenue from each customer.

CEO Kevin Sayer said in a post-earnings conference call performance would improve in the fourth-quarter and "get more and more effective over time".

Shares recouped some lost ground and were last down 5%.

Sayer also said a record number of new patients signed up in the third quarter after Dexcom implemented feedback from its distribution channels.

The company also improved access to its devices in markets such as Japan and France to reach more "basal only" patients, and expects to see revenue growth in these markets next year.

Analysts had raised concerns about the pace at which Dexcom was capturing "basal only" diabetes patients who take one insulin injection a day, compared to rival Abbott.

Dexcom in August launched its over-the-counter device Stelo in the U.S. for adults aged 18 and older who do not use insulin, making it the first continuous glucose monitor available for over-the-counter sales.

The company reported third-quarter revenue of $994.2 million, beating analysts' estimates of $990.7 million, according to data compiled by LSEG.

On adjusted basis, it earned a profit of 45 cents per share, slightly above expectations of 43 cents.

(Reporting by Sneha S K and Bhanvi Satija in Bengaluru; Editing by Sriraj Kalluvila)