A national corporation announced this week it is making up to $1.6 billion available to finance home mortgages for middle-income families throughout the United States.

The Federal National Mortgage Association, known as Fannie Mae, said the loans will enable potential buyers having incomes as low as $19,100 to purchase homes with monthly payments of $446.

Other loans to be offered will carry adjustable interest rates with caps to preclude unlimited payment increases, no down payments in cases where sellers are willing to pledge 5 percent of home values to a special account, options to convert adjustable rate mortgages into fixed-rate loans, options for level payments over a period up to seven years on 15-year mortgages, and deeper buy-downs when builders are willing to cover portions of the interest costs during first one to five years of a mortgage.

The family with the $19,114 income could qualify for the $60,000 mortgage if the builder agreed to "buy-down" the interest costs during the first three years of the loan. The payments would rise from $446 monthly to the first year to $670 a month in the fourth year and for the remainder of the loan.

Adjustable rate mortgages can be converted to fixed rate loans at several points during the life of the mortgage. Under one of the options, a family with an income of $27,686 could qualify for a $60,000 mortgage. Monthly payments could increase yearly from $646 in 1983 to $786 in 1986 as the interest rate rose by one percent per year, starting at 12.625 percent.

A buyer choosing the "early ownership alternative" for a $60,000 mortgage could ask for level payments for the first five years, for example, with annual increases in the sixth and seventh years. The monthly payments would begin at $670 and would be $1,210 for the last eight years. Qualifying income is $28,714.

The $1.6 billion is the "largest single commitment of residential financing of its kind," said Robert J. Mylod, Fannie Mae president and chief executive officer.

Purchasers of new and existing single-family homes can qualify for the loans, which are being made by lenders who have signed up for the program, said Mylod. The program will be in effect until Sept. 30, 1984.

Two lenders in the Washington area, First American Bank in the District of Columbia and Weaver Brothers Inc. in Chevy Chase, are taking part.

The money for the program will be raised through a variety of ways, including debentures, short-term borrowing on the capital markets and income from the corporation's portfolio of more than $74 billion in mortgage loans, said a spokesman.

Mortgage Guaranty Insurance Corp. is working with Fannie Mae to provide insurance coverage of loans made in the program, Mylod said.

Fannie Mae, a congressionally chartered, shareholder-owned corporation, buys mortgages from local lenders, thus making available more money for home loans. The corporation is the largest supplier of residential mortgage money in the country.