The Reagan administration is considering $6 billion more in Medicare cuts for fiscal year 1984, and possibly even larger amounts in subsequent years, according to administration and other sources.

Medicare has already taken some of the largest cuts of any program in the current efforts to reduce the federal budget. Last year Congress voted cuts of $3.6 billion over three years, and changes expected to be approved this year would further reduce outlays by $12.8 billion over three years.

Administration sources emphasized that no decisions have been made on further trims in the program, which at a projected $58 billion for fiscal 1983 is the second most expensive domestic program, trailing only Social Security.

But they said high administration officials had begun a review of possible Medicare changes that could be proposed in President Reagan's fiscal 1984 budget message next February and had set a tentative target of $6 billion in budget savings in fiscal 1984 and even larger amounts in subsequent years.

Edwin L. Dale Jr., spokesman for the Office of Management and Budget, said that while it was entirely possible that some changes in Medicare would be sought in the 1984 fiscal year, "we have nothing at this stage with any such figure" as $6 billion and "there is no such figure in any OMB document even in the planning stage."

However, sources said the figure is being used at the White House level in oral discussions involving several high officials responsible for government policy matters.

Among the reported options for achieving the cuts is charging Medicare patients "co-payments" for hospital days they now get free.

Under the concept being discussed, these payments might be imposed on more well-to-do patients while poorer ones would not have to pay them, thus creating what several sources said could be viewed as a "means test."

Medicare recipients have never been subject to such an income standard, and both proposals would probably meet strong opposition on Capitol Hill and among organizations for the aging.

Also under discussion, according to the sources, are reduced reimbursements to doctors treating Medicare patients.

The search for new Medicare cuts is part of the administration's ongoing, and now intensifying effort to reduce federal budget deficits. White House sources have said that the giant entitlement programs of benefits for the poor and elderly may be a special target.

Medicare, funded largely by an earmarked portion of the Social Security payroll tax, is one of the fastest growing of those programs.

Hospital costs have risen much faster than overall increases in the cost of living, helping to push up program costs by 21.5 percent in 1981. About 29 million elderly and disabled Social Security beneficiaries are entitled to benefits.

The "co-payments" idea would involve making Medicare patients pay for part of the cost of their second to 60th day in a hospital. At present, the Medicare patient pays an initial payment of $260 to cover basic room costs for the first day in the hospital, then Medicare pays the entire basic room cost from the second to 60th day.

Making patients responsible for more of the room costs could save Medicare billions of dollars annually, depending on how much co-payment was required.

Last December, Secretary of Health and Human Services Richard S. Schweiker sent to the White House a co-payment plan suggesting a charge of $26 a day, as part of his proposal to restructure the entire medical reimbursement system and introduce more competitive elements. It was tentatively approved by the OMB but never finally adopted or sent to Congress.