(Reuters) - Insulet Corp on Thursday raised its annual revenue growth forecast but pressures over unutilised inventory clouded the upbeat outlook, sending shares of the insulin device maker down about 3.5% in extended trading.

The company said it recorded a charge of $13.5 million in the second quarter ended June 30 relating to some unutilised inventory, which hurt gross margin by about 280 basis points.

Its adjusted profit of $0.55 per share for the reported quarter was just shy of analysts' average estimate of $0.56.

The company manufactures and sells insulin delivery devices under the brand Omnipod, which eliminate the need for multiple daily injections using syringes or insulin pens for people with insulin-dependent diabetes.

It did not give further details in its statement over the inventory write-down.

In July, Insulet's larger peer Dexcom had slashed its annual revenue forecast, prompting investor concerns over changing trends in the diabetes market.

However, Massachusetts-based Insulet now expects its annual revenue to grow between 16% and 19%, up from its earlier forecast of 14%-18%, encouraged by the strong sales of its Omnipod devices.

It expects Omnipod revenue growth in the range of 18% to 21% this year, higher than previous forecasts of 15%-19% growth.

The company's total revenue stood at $488.5 million for the quarter, compared with analysts' estimate of $463.09 million.

(Reporting by Sruthi Narasimha Chari in Bengaluru; Editing by Rashmi Aich)