Biogen Idec Inc., the third-largest US biotechnology company by revenue, reported profit that beat analysts’ estimates as sales increased from its top-selling multiple sclerosis medicine Avonex. The company boosted its full-year forecast.

Second-quarter net income surged 34 percent to $386.8 million, or $1.61 a share, from $288 million, or $1.18, a year earlier, the Weston-based company said Tuesday in a statement. Earnings excluding some items of $1.82 topped by 26 cents the average of 21 analysts’ estimates compiled by Bloomberg.

Biogen said profit this year is expected to be more than $6.20 a share, 5 cents higher than previously forecast. The company has been increasing sales of Avonex and Tysabri while developing new medicines to introduce to the market. The company’s first pill for MS, BG-12, is being weighed for approval by regulators in the US and Europe, and the company expects late-stage data this year on medicines for Lou Gehrig’s disease and hemophilia.

“We view BG-12 as the most promising MS drug in development and anticipate a strong worldwide launch beginning in early 2013,” Eric Schmidt, an analyst with Cowen & Co., wrote in a July 12 research note. “Meanwhile, we believe Biogen Idec’s base business (Avonex, Rituxan, and Tysabri) will continue to outperform expectations and that the profitability and longevity of these franchises are underestimated.”

Biogen fell 2.1 percent to $139.23 Monday, and has gained 27 percent this year.