If you’re over 65 and searching for Medicare Advantage plans with low premiums, you are likely discovering that many of the options you previously had available are now gone. Several health insurance companies have closed Medicare Advantage plans because of changes in government funding and new laws that make the plans unattractive to the insurers.

Cigna, Harvard Pilgrim Health Care, a number of Blue Cross Blue Shield plans as well as others are not renewing hundreds of Medicare Advantage plans. If you are new to the Medicare world, Medicare Advantage plans are simply Medicare policies – aka, senior health plans – that are administered by private insurance companies rather than by the government. Nearly three quarters of a million seniors will be affected by these changes, according to a consulting firm that tracks senior health statistics. For 2011, the Kaiser Family Foundation said there will be a 13% decline in the number of Medicare Advantage plans.

Why are these companies closing down plans that covered hundreds of thousands of seniors? You can thank a piece of legislation from 2008 that made it mandatory for these private plans to offer networks of preferred doctors. The thought behind the law was that with a doctor network, these companies culled offer lower cost plans and, in turn, that they could reduce some of the questionable marketing tactics in which they have been engaged. However, it has had the opposite effect: instead of making the investment in their physicians, insurers simply decided to close down the plans and focus their efforts elsewhere. According to several experts, the recent federal changes to the health care landscape also played a part, since the new laws include $150 billion in Advantage reimbursement costs between now and 2021.

For the next six months, though, consumers who are already enrolled in plans that will not be closed down will actually find that their premiums will stay low and that, in fact, they may have more coverage than ever before because of the government mandated changes to health care.

“It is hard to imagine these cuts to Medicare Advantage and nothing is going to change,” said Michael McCallister, chief executive of Humana. That company shuttered 31 Medicare advantage plans for 2011, but he believes that his company will continue to thrive because the firm is still offering many choices that would appeal to seniors.

Medicare Advantage is expected to bring in more than $51 billion in revenue for major health plans this year, according to an estimate provided by one reputable Wall Street firm. That revenue could fall to as low as $37 billion in the next few years as the cuts kick in,but insurers certainly will find new ways to bring in business, and sales should climb back to $51 billion by 2018.

Humana, where the program accounts for more than half of the company’s sales, says that its continued growth depends on reducing costs 15% below traditional Medicare’s payment rates.