Teamsters union President Frank E. Fitzsimmons, who has been hospitalized for three weeks of treatment for lung cancer and is described by friends and associates as suffering significant weight loss and weakness, may be too ill to seek reelection next month at the union's convention.

Speculation about Fitzsimmons' condiiton has prompted what one Teamsters official is calling "unfounded reports" about a major, behind-the-scenes battle to replace Fitzsimmons as leader of the 2.3 million member union, the nation's largest.

"No one is denying that Fitz is sick. But it's substantially not true" that subordinates are trying to force his resignation before the convention to avoid a floor fight over his candiacy, Teamsters spokesman Duke Zeller said yesterday.

Fitzsimmons, 73, has been hospitalized for lung cancer several times in the last year, Zeller said, "and each time he has had treatments, he has been in pretty bad shape" immediately afterward. "But he usually rebounds in time, and we hope he will do the same thing this time," Zeller said.

The Los Angeles Times reported yesterday that a union delegation, headed by Secretary-Treasurer Ray Schoessling, 74, was dispatched to Fitzsimmons' hospital bed at the Scripps Health Clinic in La Jolla, Calif., to persuade him to resign.

Zeller yesterday confirmed that the delegation visited Fitzsimmon's this week but said, "They went to pay their respects to Fitz, to wish him well and to hope that he recovers in time for the convention."

Zeller said the union's 22-member executive board would, "without a doubt," support Fitzsimmons for another five-year term "if Fitz decides that he is well enough to run."

"No one is publicly running for anything without some indication from Fitz as to what he is going to do . . . . It would be inappropriate" to do otherwise, Zeller said.

Prospective candidates for Fitzsimmons' job include Schoessling and vice presidents Roy L. Williams, 66, of Kansas City, Kan.; Joseph W. Morgan, 52, of Miami, and Jackie Presser, 54, of Cleveland.

The cleveland Press yesterday quoted Presser as saying, "I will not be a candidate for president . . . . Roy Williams will run, and I will support him."

A spokesman for Presser said yesterday that he could "neither confirm or deny" the report.