GREENVILLE, N.C. (AP) _ U.S. Sen. John East, an academic and ardent advocate of conservative causes who had planned to retire after one term because of illness, committed suicide at home Sunday, authorities said.

East, 55, a Republican and protege of North Carolina's senior senator, Jesse Helms, was found by an aide in the garage of his home shortly after 8 a.m., said a police statement.

Dr. Lawrence Harris, who conducted an autopsy, said East died of asphyxiation by carbon monoxide.

''The manner of death has all the appearance of suicide, based on the autopsy and the scene,'' he said.

''In my opinion, he took his own life,'' Harris said. ''I have no question about it.''

The senator, who was confined to a wheelchair, was found lying on the floor beside the car, said police Detective Clifton Weatherington. He added, ''The car doors were closed. There was no hose attached to the car.''

''A note was found at the scene, the contents of which are considered a personal communication between Senator East and his family,'' said the police statement.

President Reagan, secluded at his mountaintop ranch near Santa Barbara, Calif., hailed his long-time conservative ally as a ''true patriot.''

East ''loved his country and was motivated by a deep sense of duty to his fellow man,'' Reagan said in a statement.

''He was a quiet and effective legislator, who never wavered in his belief in principle and in his determination to keep our country strong,'' Reagan said.

Helms called East's death ''a tragedy of monumental proportions.''

''The Senate has lost a brilliant and effective voice for freedom,'' Helms said in a statement. ''He was a profile in courage.''

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

''He had been making plans to go back to teaching'' and had been reviewing galley proofs of a book, East's press secretary, Jerry Woodruff, told CBS News. ''It seemed to us he was making plans for the future.''

East, who taught political science at East Carolina University in Greenville before his election, last week agreed to rejoin the faculty in January, after his term expired, according to Chancellor John Howell.

Helms said he had recommended to Reagan that East be named a federal judge in Washington, D.C. Helms said East told him he preferred to return to teaching.

Helms said he had last spoken with East on Thursday at dinner in the Capitol.

''He looked well, but one never knows what's inside of another person,'' Helms said. ''I had no inkling whatsoever.''

East was picked by Helms to run against incumbent Democrat Robert Morgan in 1980. Helms' political machine raised the money for television ads that boosted East to an upset victory in that bitter race.

The announcement of his retirement touched off a GOP primary fight in which longtime U.S. Rep. James Broyhill, a moderate, defeated David Funderburk, backed by East and Helms. Broyhill, endorsed by East after the primary, faces former Democratic Gov. Terry Sanford in the November election.

Under the law, Republican Gov. Jim Martin must appoint someone to serve the remainder of East's term, which ends in January. Assuming Martin names another Republican, the GOP will keep its 53-47 advantage in the Senate. The governor on Sunday had not begun considering whom to appoint, said his spokeswoman, Karen H. Rotterman.

East's health had been poor in the months before his announcement. The senator, confined to a wheelchair because of polio when he was 24, was hospitalized early in 1985 for hypothyroidism. He also was hospitalized for treatment of a low white blood cell count and a urinary track blockage, and missed much of the Senate's work last year.

Hypothyroidism, a glandular condition that leads to imbalances in the body's hormones, may have led to East's suicide by causing depression, said Dr. T.K. Gray, a professor of medicine at the University of North Carolina at Chapel Hill.

''Hypothyroidism may have contributed to a depressed state,'' Gray said.

East was born May 5, 1931, in Springfield, Ill. He was a 1953 Phi Beta Kappa graduate of Earlham College, and served as a Marine lieutenant from 1953 to 1955, when he contracted polio.

He earned a law degree in 1959 from the University of Illinois. He received a Ph.D. in political science at the University of Florida in 1964.

As a professor at ECU, he once described himself as ''one of those rare creatures in academe, a conservative political science professor.''

His first political forays were failures. He lost races in 1966 for the U.S. House and in 1968 for North Carolina secretary of state. He served as a GOP national convention delegate in 1968 and 1976 and as a national committeeman in 1976 before being picked by Helms to challenge Morgan, a Democrat with a conservative record.

The money Helms brought in from his political organization, the National Congressional Club, helped pay for a television ad blitz that featured biting attacks on Morgan, portraying him as soft on defense and communism and often showing pictures of him with liberals Edward Kennedy and George McGovern.

In Washington, East rarely cast a vote different from Helms but kept a far lower profile. He and Helms cast the only Senate votes against confirming Caspar Weinberger as defense secretary in 1981, contending Reagan's appointee wasn't hawkish enough for the job.

East supported calls for a restoration of traditional ''family values'' with quotations from philosophers such as Plato, St. Thomas Aquinas and Edmund Burke.

In 1981, he chaired the Judiciary Committee's separation of powers subcommittee at hearings on his bill seeking to overturn the Supreme Court's 1973 abortion decision by having Congress declare that a fetus becomes a human being at the moment of conception.

The bill went nowhere, but East continued to back plans to take away the Supreme Court's authority over such questions as abortion and busing to achieve racial balance in public schools.

In 1982, East and Helms led a week-long filibuster against extending the Voting Rights Act.

In an interview after he had announced his retirement, East said he hoped to be remembered as ''an intelligent, articulate person who was engaged in what I thought ws civil and thoughtful debate on a myriad of issues.''

He said he disagreed with critics who had labeled him a Helms clone, but acknowledged they agreed on most issues. ''I think if I had been in a position to be here longer, that different identities would certainly have emerged,'' East said.

Survivors include East's wife, the former Priscilla Sherk, and two daughters, Kathryn and Martha.

Funeral arrangements were incomplete.