UnitedHealth Group Inc., the nation's largest provider of privately managed Medicare Advantage plans, has dropped thousands of doctors from its networks in recent weeks—spurring protest from lawmakers and physician groups and leaving many elderly patients unsure about whether they need to switch plans to keep seeing their doctors.

Doctors in at least 10 states have received termination letters, some citing "significant changes and pressures in the health-care environment." The notices also tell doctors they can appeal within 30 days. That means many physicians and patients won't know for sure who is in or out of UnitedHealth's Medicare Advantage networks before the open-enrollment period to switch Medicare plans ends on Dec. 7.

UnitedHealth said its provider networks are always changing and that it expects its Medicare Advantage network "to be 85% to 90% of its current size by the end of 2014," although it declined to say how many doctors are being cut in individual states or what criteria it is using.

The company said it is managing its network, in part, to provide more value for members, particularly given Medicare's new five-star rating system that ties bonus payments for insurers to certain measures of cost and quality.

"That's what's driving our actions," said Austin Pittman, president of UnitedHealth's networks. He also said, "It's no secret that we are under substantial funding pressure from the federal government."