The Teamsters Union central states pension fund, chief source of retirement security for about 450,000 trucking employees, could run out of money to pay pensions in the next decade unless major management changes are adopted, the fund's director said yesterday.

Daniel J. Shannon, executive director of the troubled fund, said a reduction of the plan's investments in controvsial real-estate projects and a 20 per cent rise in employer pension contributions would be necessary to bring the fund to a "complete funding" status.

"Complete funding" means that a persion plan has sufficient assets to pay full pensions for life to all employees currently contributing to the plan.

Shannon's statement, extracted under questioning at a House Ways and Means Oversight Subcommittee hearing, was the union's first public admission that the huge pension program faces a potential deficit.

The fund, plagued for years by charges of mismanagment and ties to organized crime, has been under investigation for 18 months by the Labor Department and the Internal Revenue Service.

On Sunday, Teamsters President Frank Fitzsimmons and three other men agreed, under pressure from those agencies, to resign as trustees of the fund.

Seven angry rank-and-file Teamsters who testified at yesterday's hearing cheered that development.

Richard Griggs, a truck driver from Charleston, Mo., expressed their general view when he said, "Our pensions will not be safe as long as Frank Fitz and his goons have their hands in the till."

The rank-and-file members expressed gratitude to the Carter administration for its tough attitude toward the pension fund.

The Central States fund serves Teamster members in 33 states. It is the latgest of several Teamsters pension plans and one of the largest multi-employer plans in the nation.

Shannon said yesterday that the fund's assets total between $1.3 billion and $1.5 billion. That is about the same as the assets of a financial institution such as Riggs National Bank, the largest bank in Washington.

Critics have charged that Fitzsimmons and the other trustees have made partof that massive pool of assets available to organized-crime figures for shaky real-estate deals.

Shannon conceded yesterday that about two-thirds of the fund's assets are invested in real-estate. He said most large pension plans have less than one-third of their assets in real-estate, relying for the most part on more conservation investments, such as government securites.

He said the fund has earned an average of 5.41 per cent on its investments over the past four years,

One of the rank-and-file witnesses at the hearing, Tom McGrath of Freemont, Neb., hooted at that figure.

"My 11-year-old son, Kevin, knows a common bank account will pay 5.5 percent interest," he said.

The federal government undertook its investigation of the fund in 1975 because of the fund's investment records and charges of questionable dealings by the trustees. Last June the IRS announced that it would revoke the funds tax-exempt status means that employers cannot deduct contributions to the fund, pensioners owe income tax on benefits and the fund is taxed onn its annual interest income.

Despite that threat, the Labor Department was not able to force the resignation of Fitzsimmons and other trustees last year.

When Labor Secretaty Ray Matshall took office in Januaary, he began intense negotiations with the Teamsters that resulted in Sunday's agreement that Fitzsimmons and the others would br replaced on the board by April 30.

At yesterday's hearing, Rep. Charles B. Rangel (D-N. Y.) asked repeatedly why the trustees agreed to resign despite their insistence that they had done nothing wrong.

"Why didtn't you just tell the government to go to hell and that you'd run your own pension fund?" Rangel asked.

Shannon said the resignations could not be taken as a admission of wrong doing by the trustees.

"Those men decided undividually that only by resigning could they lift this tremendous burden on the fund and its participants," he said.

Subcommittee Chairman Sam M. Gibbons (D-Fla.) said IRS and Labor Department officials had declined to testify at the hearing for fear of impairing negotiations with the fund. He said both agencies agreed to testify in closed session today.

Shannon said the fund's "unfunded liability" - the shortfall it would face if it had to pay all potential benefits as of today - is about $6.5 billion. He said the deficit could be eliminated in three years through management changes and an increase in employer contributions to the fund from the current $31 per week per employee to $37.