Humana Inc posted a 21 percent increase in quarterly profit on Monday, helped by growth in its Medicare plans for seniors, and the health insurer raised its full-year forecast.

One of the largest Medicare players, Humana also reported stronger performance in its commercial plans for employers, and its shares rose more than 4 percent.

Health insurers have raised their outlooks after posting higher-than-expected profits during this reporting season, helped by generally lower use of medical services.

But the market's reaction has been tempered by uncertainty over the fallout from the new U.S. healthcare reform law. Humana's second-quarter results included a $147.5 million writedown related to the expected value of its individual business in the wake of the law.

There are many moving pieces here, but this was a solid quarter, and underlying guidance is moving much higher, Citigroup analyst Carl McDonald said in a research note.

Quarterly net income rose to $340.1 million, or $2.00 per share, compared with $281.8 million, or $1.67 per share, a year earlier.

The company wrote down $147.5 million, or 55 cents per share, related to its assessment of its individual policy business in the wake of the new U.S. healthcare reform law, which is expected to bring substantial change to that business.

Humana reported earnings of $2.11 per share excluding the write-down and excluding benefits from releasing claims reserves from prior periods.

Analysts on average expected $1.67 per share, but it was not immediately clear if that figure was directly comparable to the results, according to Thomson Reuters I/B/E/S. Analysts at Sanford Bernstein and Collins Stewart said the results amounted to a significant beat.

Revenue rose 9.5 percent to $8.65 billion. Analysts had looked for about $8.61 billion.

Humana's Medicare Advantage membership grew 17 percent to 1.76 million as of the end of June. Humana is one of the two biggest Medicare players, along with UnitedHealth Group Inc .

Profit in Humana's commercial segment, excluding items, more than tripled to $115.2 million, as the company cited lower use of medical services, administrative cost savings and disciplined pricing for its plans.

Humana projected a full-year profit of $5.65 to $5.75 per share, up from its prior range of $5.55 to $5.65. Analysts have been looking for $5.71.

Excluding the writedown and other charges, Humana's operating earnings forecast is about $6.40 per share, McDonald said.

Humana shares rose 4.5 percent to $49.12 in premarket trading. Through Friday, Humana shares had risen about 7 percent this year, against a roughly 7 percent decline for the S&P Managed Healthcare index <.GSPHMO>.

(Reporting by Lewis Krauskopf; Editing by Derek Caney, Maureen Bavdek, Dave Zimmerman