When the 73rd annual meeting of the Metropolitan Washington Savings and Loan League ended today at The Homestead, an era also ended. For the past two decades most of the league's conventions have been held at this luxury resort nestled in the mountains of western Virginia. But no more. Next year's meeting is expected to be a one-day affair held in Washington.

Austerity is the key word, according to the league's president-elect, William F. Sinclair. On July 1 he succeeds James L. Harris, president of Washington Federal. During Harris stewardship, the league's membership shrank from 16 to 11 institutions as weaker ones were merged out of existence. And one or two more such moves are expected before this year is over.

Sinclair, 35, who recently came to the presidency of Perpetual American Federal Savings and Loan through a merger of Perpetual with his association, American Federal, described other austerity measures the league plans to take in keeping with hard times the thrift industry is facing.

The budget has been reduced 10-12 percent and may be even more. Board meetings will be cut to one every two months. Travel and seminars will be eliminated so that thrift executives can "stay home and mind the store," he said.

Clinches about things being darkest before the dawn were served up as liberally as bloody Marys at this year's convention. Yet, quipped Sinclair, "To me it looks as though the light at the end of the tunnel is turning out to be a train coming the other way."

Savings and loan associations across the country lost $700 million during the first quarter of this year and stand to lose as much as 20 percent of their net worth for the year as a whole, according to projections by the National Savings and Loan League.

As of the end of last year federally insured S&Ls in the greater Washington area and Baltimore had about three quarters of a billion dollars in net worth. That could mean a loss of $150 million.

Sinclair noted with some pride that his association still expects to make a profit during the first half of this year. One contributing reason is that Perpetual sold off a sizable portion of its low yielding mortgages for tax losses back in 1979. Spreading out tax losses by selling off old mortgages was just one of numerous proposals made by thrift experts during work sessions.

Besides creative accounting and working within the Federal Home Loan Banking system, David deWilde, executive vice president for policy and planning of the Federal National Mortgage Association, urged S&Ls to make greater use of the secondary market by selling the loans they make to investors and recycling the funds.

Although Fannie Mae has not yet announced its terms for the adjustable mortage loans it plans to buy from thrifts, deWilde said it would offer far more flexible terms than those announced last week by the Federal Home Loan Mortgage Corp.

It is believed that Fannie Mae will permit negative amortization -- where the homeowner can increase the outstanding balance of his or her moortgage loan rather than increasing monthly payments if interest rates rise -- as well as more frequent changes in rates (perhaps as often as every 90 days) and the use of several indices.

But thrifts are still looking to the government to help them out of a situation they say is not of their own making. Last week the Reagan administration rejected a bill drafted by the Federal Reserve to make it easier for federal regulators to arrange mergers across state and industry lines. Instead the Treasury Department proposed a less costly plan to offer thrifts whose net worths sink belows 4 percent a temporary line of credit until interest rates decline. In this way supervisory mergers which are normally triggered when net worth sinks that low would be forestalled.

The credit, in the form of demand notes, would be added to an institution's assets to restore public confidence and prevent a rash of withdrawals, particularly of uninsured deposits over $100,000 at weaker S&Ls.

The Treasury is betting that only a very few institutions may actually have to draw on their lines of credit by converting the demand notes into cash. In any case the government's potential liability would be limited to about $2 billion.