President Reagan has sent NATO heads of state letters outlining various new proposals for limiting the number of medium-range missiles deployed in Europe by the United States and the Soviet Union, a senior defense official said yesterday.

The senior official was traveling aboard the aircraft carrying Defense Secretary Caspar W. Weinberger from Madrid back to Washington, D.C.

The official said the letters were sent Wednesday and indicated that their purpose was to obtain reactions from west European allies, not to outline an alternative Reagan has selected.

The president's letter went out as NATO defense ministers were meeting in Portugal to discuss nuclear issues, with ways to reduce the number of medium-range missiles in Europe the main topic.

Reagan's previous proposal to the Soviets was that both sides eliminate all their intermediate-range missiles in Europe, the "zero-zero" plan.

The Soviets have rejected this, and Reagan has come under pressure to propose an alternative. Sources have said he may do so in a speech next Thursday in Los Angeles. A U.S. official at that NATO meeting told reporters that it was "the consensus" of European defense ministers that Reagan should offer the Soviets an alternative to his zero-zero plan.

That plan would require the Soviet Union to retire its 351 SS20 medium-range missiles deployed in eastern Europe along with some older ones. In exchange the United States would forgo the planned deployment of Pershing II and cruise missiles in western Europe, starting this December.

One alternative under consideration would allow the United States and the Soviet Union each to deploy 100 launchers which could carry a total of no more than 300 nuclear warheads. The NATO heads of state were asked to react to this and other options by Monday, according to U.S. officials.

A second alternative to zero-zero plan would allow NATO deployment of Pershing II and cruise missiles to rise to the level of Soviet missiles in the same class. Then both sides would "build down" to a lesser number.

The senior official on the Weinberger plane would not disclose the contents of the president's letter but said that in this stage of deliberations it would be normal practice to solicit the views of allies before making a decision on which alternative to select, if any.

The senior official on the Weinberger plane said that if Reagan settles on an alternative, it could be submitted to the Soviets at any time and would not have to wait for the scheduled start of the next U.S.-Soviet negotiating session in Geneva two months from now.

Weinberger has steadfastly insisted that any alternative to the zero-zero plan now on the table at Geneva would only be a stopping point on the way to that goal of banning all medium-range missiles on both sides of the NATO line. The NATO defense ministers on Wednesday endorsed the zero option as the ultimate objective while hailing U.S. "flexibility" in its willingness to offer alternatives.

Weinberger upon arrival from his European trip, which took him to Vilamoura and then Madrid for talks on NATO defense issues, said on arrival at Andrews Air Force Base yesterday that members of the alliance had achieved an exceptional unity of purpose in their sessions in Portugal.