Seniors will pay less than expected in Medicare Part B premiums next year, the Obama administration announced Thursday.

Monthly premiums for Medicare Part B, which covers doctors’ visits and outpatient procedures, will increase by $3.50, to $99.90, in 2012. Initially, government forecasters had projected a $10.20 premium bump, to $106.60, in seniors’ monthly fees.

The announcement was the third piece of good news about Medicare premiums this year.

The White House said in August that premiums would drop 4 percent in Medicare Advantage, the privately run alternative to the traditional program. Premiums for Medicare’s prescription drug program, Part D, also will drop slightly in 2012.

Administration officials attributed the lower premiums to seniors using less medical treatment than initially expected.

“Today’s announcement confirms a current data trend of much lower utilization,” said Jonathan Blum, deputy administrator at the Center for Medicare and Medicaid Services.

They also cited some of the changes in the nation’s new health-care law as potentially driving down costs.

For most seniors, this will be the first Part B premium increase since 2009. For the past two years, most beneficiaries’ Medicare fees went unchanged because of a “hold harmless” law, which barred the government from increasing them without a cost-of-living adjustment to Social Security.

Last week, the Social Security Administration said seniors will receive a 3.6 percent cost-of-living adjustment in 2012, allowing Part B premiums to increase as well.

AARP applauded the news. “This small increase is welcome news,” said David Lerner, the group’s legislative policy director.

For the quarter of seniors who did not fall under the “hold harmless” protections, generally new enrollees and higher-income seniors, the new Medicare premiums will actually be a decrease in fees. Those seniors, who have seen increases in the past two years, will have premiums decrease, from $115.60 in 2011 to $99.90 in 2012.