Sen. John P. East (R-N.C.), a conservative who reluctantly abandoned his reelection plans because of a painful illness, committed suicide in the garage of his Greenville, N.C., home, authorities said yesterday.

East, 55, a protege of Sen. Jesse Helms (R-N.C.), died of carbon monoxide poisoning after starting his car in the closed garage of his stately brick home in a fashionable section of Greenville late Saturday or early yesterday. He left a note for his family mentioning his health problems, according to police. His body was found by an aide.

A little-known political science professor until he was tapped by Helms to seek office, East was confined to a wheelchair after contracting polio at age 24. A solid champion of New Right causes, East was elected to the Senate in the Reagan landslide of 1980. He rarely opposed the president although he voted against the nomination of Caspar W. Weinberger as defense secretary because he doubted Weinberger's commitment to a defense buildup.

President Reagan, in a statement from his ranch in Santa Barbara, hailed East as a "true patriot" who "loved his country and was motivated by a deep sense of duty to his fellow man." East, he said, "put his duty and his constituents ahead of his personal comfort."

Helms, in a statement, called the death "a tragedy of monumental proportions." Describing East's life as "a profile in courage," Helms said, "The Senate has lost a brilliant and effective voice for freedom, and those of us who were privileged to know him and serve with him have lost a dear friend and a constant source of inspiration."

Helms told The Associated Press he had recommended that Reagan nominate East as a federal judge, but that East said he preferred returning to teaching. Helms said he last saw East on Thursday at a dinner in the Capitol. "He looked well, but one never knows what's inside of another person," Helms said.

East was diagnosed last year as suffering severe hypothyroidism, a hormone deficiency that can cause anemia, fatigue, insomnia, intellectual impairment, weight loss and depression. The illness, for which East was hospitalized several times last year, has also been known to cause "hallucinations, disorientation, paranoia and attempted suicide," according to "The Textbook of Medicine."

"There is no doubt at all" that East's death was a suicide, said Greenville Police Sgt. Roger Benton, who was called to the scene. "There was a note he left to his family right beside the body . . . . His health was mentioned in the note." Dr. Lawrence Harris, a state forensic pathologist, conducted an autopsy late yesterday and said the death was a suicide.

"He was not happy about having to decline to run again. He felt the circumstances had really forced him into retiring," said Jerry Woodruff, East's press secretary for six years. "I am not sure 'despondent' or 'depressed' is the right word for John, but he was unhappy about it."

"He felt it was a golden time to be a Republican senator" because the GOP controlled both the Senate and the White House, Woodruff said. "He had felt especially privileged to work in the Senate because he was a lifelong political scientist who got a chance to get on the 'inside' and he felt quite honored to be part of this Republican team."

East's death adds a new dimension to a hotly contested Senate race in which Rep. James T. Broyhill (R-N.C.), a moderate Republican, faces Democrat Terry Sanford, a former governor, in November.

Gov. Jim Martin, a Republican, will appoint a successor to serve until the new Senate convenes in January. Martin's office said the governor would have no statement on the death or the appointment until today, but several state political observers said Broyhill would be the logical choice.

Described by colleagues as a quiet and private man, East missed much of the 1985 legislative session because of hypothyroidism treatments, urinary problems and complications from prostate surgery. Although rumors had circulated that his illness might be terminal and had impaired his intellectual abilities, East's medical condition apparently had improved in recent months as he gained weight and resumed much of his normal Senate routine, Woodruff said.

"He complained that he still did not feel like his old self, but beyond that, he seemed pretty much like the same old John East," Woodruff said, ". . . It is quite a shock to us. We are all numbed."

East's last appointment in Washington was Friday afternoon, when he met at his Capitol Hill office with Judge Antonin Scalia to discuss Scalia's nomination to the Supreme Court. East, a member of the Judiciary Committee, was to have voted on the nomination. East returned to his home in Alexandria Friday afternoon and drove to Greenville Saturday with his special assistant, John Petree, for the Fourth of July congressional recess.

East's wife, Priscilla, was visiting her mother in Hilton Head, S.C., and East apparently was home alone, police said. His body was found by Petree, who had come to see East shortly after 8 a.m., police said.

A political science professor at East Carolina University since 1964, East had never held office until 1980, when he was chosen by Helms' powerful right-wing National Congressional Club, which bankrolled East in his 50-to-49 percent victory over Sen. Robert Morgan (D) in the Reagan landslide.

East focused much of his attention in the Senate on a right-wing agenda of fighting abortion and school busing for integration, promoting school prayer and pushing for increased defense spending. He almost always followed Helms' lead. Affectionate colleagues dubbed him "Helms on wheels."

At least one other U.S. senator had previously committed suicide. Sen. Lester Hunt of Wyoming shot himself in his office in 1954.

East had planned to return to East Carolina University in Greenville to resume teaching after his Senate retirement, and was also completing a book of essays on American conservative political thinkers, using essays East had published in the conservative journal Modern Age, Woodruff said.

University chancellor John M. Howell, a longtime friend, said East was a popular faculty member who was considered a "fascinating lecturer" with a ready wit. Howell, a Democrat, said he had made a $10 campaign donation to East when he first sought office in the 1960s. Howell said East wrote a thank-you note saying, "With enemies like you, who needs friends?"

East's last major political appearance came last month in Greensboro, when he accompanied Reagan and Helms to a fund-raising event for Broyhill. East had strongly supported Broyhill's GOP primary opponent, New Right conservative David Funderburk. But East gave a "very vigorous" speech and endorsement of Broyhill at the event, said Thomas Griscom, executive director of the National Republican Senatorial Committee.

"Everybody knew that Sen. East was confined to a wheelchair, but he got around fairly well, and I think very few of us knew the extent of his problems until he announced he wouldn't seek reelection" last Sept. 17, Griscom said.

"He told me once that being in the wheelchair reminds you how dependent people are on each other," Woodruff recalled. After he was stricken with polio in 1955, East obtained a law degree and a doctorate, Woodruff said. "You would think he would be more equipped to deal with hypothyroidism. He had a great deal of internal fortitude, because he had pulled himself up by the bootstraps after polio."

Staff writer Jonathan Karp contributed to this report.