The Depository Institutions Act of 1982, which was approved by Congress late last night, gives new life to the threatened thrift industry and opens the door for a fundamental restructuring in the future. How long the transition takes and what lies on the other side of the threshold remain to be seen.

With a stroke of a pen, the government apparently has solved the immediate problem of keeping the endangered mutual savings banks and savings and loan associations alive. Treasury Secretary Donald T. Regan declared this week, "The adoption of the administration's proposals for short- and long-term assistance and the recent decline in interest rates should relieve the serious problems of the thrift industry."

There had been predictions that as many as a quarter of the thrift institutions would disappear within a couple years. Indeed, 367 savings and loans were merged out of existence during the first eight months of this year compared with 141 during all of 1980 and 328 last year.

Thrift institutions deemed viable by federal regulators will receive short-range, noncash assistance in the form of government promissory notes that will help to raise their net worth and keep them solvent. Nevertheless, there still will be failures and losses. Jonathan Gray, a savings and loan analyst with Sanford C. Bernstein Co. in New York, predicts a return to profitability a year from now if interest rates remain at the current 9 percent level.

The long-range assistance for thrifts includes authorization to offer customers high-yield accounts that would be truly competitive with money market mutual funds. Commercial banks would have the same opportunity. The new act orders the Depository Institutions Deregulation Committee to establish these new accounts within 60 days.

Gray called the new account a "two-edged sword," however, because it brings in new money but raises the cost of funds. Thus, money-center banks, which have few passbook accounts, clearly will benefit, whereas the effect will be negligible for thrifts, which still have a significant number of low-interest accounts that would shift to the higher yields, increasing their costs, Gray added.

Although the growth in money market funds will be slowed, they won't disappear just because banks and thrifts offer insured accounts, said Glen King Parker, chairman of the Institute for Econometric Research in Ft. Lauderdale. King, who rates money market funds on safety, observed that it would take a premium of one-half to three-fourths of a percentage point to get customers to move their dollars out of money market funds, especially the comprehensive-cash-management-type accounts. Should banks and thrifts try to offer premium rates, they would have to invest aggressively and thereby possibly sacrifice safety.

The bankers had to swallow hard to accept legislation permitting thrifts to move into commercial banking. They did it with the promise that Congress would consider legislation next year opening the door for banks to the securities industry. They plan to press for the power to underwrite revenue bonds, insurance and money market funds.

James Christian, chief economist of the U.S. League of Savings Associations, said the new act will have no immediate effect on bringing down the cost of mortgage loans. However, if interest rates do not rise, he said that cheap money from commercial checking accounts could have some positive effect on loan rates by the beginning of next year.

One of the big issues throughout the two-year struggle to pass this legislation has been the future relationship of the thrifts to housing. Jack Carlson, chief economist of the National Association of Realtors, predicted that restructuring the thrifts as nonspecialized institutions would mean one-third fewer dollars for housing from thrifts by 1984.

Michael Sumichrast, chief economist of the National Association of Home Builders, noted that thrifts had provided 56 percent of the funds for all mortgages a decade ago, but currently provide 4 percent "because they are investing to survive." He said the act will enable thrifts to resume lending aggressively.

Real estate brokers, who were upset when the Supreme Court ruled last summer against mortgage assumptions, take heart in the act, which softens that decision. The law will allow assumptions of mortgages made by state-chartered institutions during a period of four or more years. One last-minute change in the law that would affect this area is a provision allowing mortgages sold to, or owned by, the Federal Home Loan Mortgage Corp. to be assumed until April 1 instead of Jan. 1.