The wife of top Soviet defector Arkady Shevchenko has committed suicide in the family's Moscow apartment, her son said yesterday.

Gennady Shevchenko, whose father was undersecretary general of the United Nations secretariat and is an expert on Soviet disarmament policies, refused to give any details of his mother's suicide. But he said by telephone that a verdict of suicide in the death of his mother, Leongina Shevchenko, "has been confirmed by physicians."

Soviet sources with good official connections asserted that Leongina Shevchenko, 48, took an overdose of sleeping pills and hid herself in a large wardrobe in her bedroom either Sunday or Monday.

In New York, Arkady Shevchenko, who quit his $76,000-a-year position after refusing to return home, said his wife's suicide "has been a heavy blow for me." He appealed to the U.S. government to help him bring his 16-year-old daughter Anya, out of the Soviet Union.

Gennady Shevchenko, who is a junior diplomat in the Soviet Foreign Ministry, said his mother's body was discovered Monday afternoon.

The Soviet sources said the family thought Mrs. Shevchenko had gone to visit friends, and when she failed to return, launched a search of the apartment. Living with her were her son, 26, and his family and her daughter.

The son, who is married and has one child, said his mother had been distraught by her husband's refusal to return to the Soviet Union last month and his later decision to seek political asylum in the United States. He said there was "no message of any kind" from his mother describing her state of mind before she died.

He denied an account from Soviet sources that his mother was depressed by Western news reports that her husband had been in contact with Western intelligence agencies for several years and that the had formed a strong liaison with another woman.

The death adds a new and tragic note to the bizarre sequence that has unfloded since early last month, when Shevchenko, known at the United Nations as a doctrinaire Communist, abruptly disappeared while his wife and daughter were preparing to return to the Soviet Union in response to orders to the family from Moscow. The son, Gennady, was then with the Soviet disarmament team in Geneva.

After the women had left, Shevchenko surfaced with a lawyer to say he had refused to return here because of "difference" with his government. The 47-year-old career diplomat, a protege of Soviet Foreign Minister Andrei Gromyko, then tried to bargain for his job and subsequently asked for political asylum in the United States. It is expected to be granted. Shevchenko has intimate knowledge of the Soviet approach to disarmament and the way policy in this crucial area is formulated within the secretive councils of the Politburo.

His defection comes just as the Carter administration is moving slowly toward agreement with the Soviets on a new strategic arms accord.

Mrs. Shevchenko's death casts into this powerful saga of intrigue and shifting allegiances a compelling human ingredient that indicates the shattering strains that can occur within a family.

The first report of the suicide turned up in the London Evening News yesterday in an article filed by Victor Louis. He is a Soviet citizen who frequently writes stories from high official government sources in a country where there are very few accidental leaks or disclosures of any kind.

Gennady Shevchenko at first claimed the Louis article was false and gave Western correspondents who telephoned him for confirmation a detailed story of his mother being stricken by a heart attack and how he took her to a hospital where she died.

Later, in another telephone interview, he said that in fact his mother had committed suicide. He said he had concocted the heart attack story because his maternal grandmother, who did not know the true cause of her daughter's death was with him in the apartment when Western correspondents began calling him about the Louis story.

Gennady Shevchenko said he could not bring himself to confirm it in front of his grandmother.He said she has since been told it was suicide.

The son said he now considers himself the head of the family here and will see to it that his younger sister, Anya, continues her schooling.

Shevchenko was undersecretary general for political and security council affairs, a post that traditionally has gone to a Soviet citizen. Although he claimed he "freely" decided to refuse to return to Moscow, the Soviets accused the United States of "detestable frame-up" and of coercing Shvchenko into his actions. This has been denied by Washington.

In New York, Shevchenko declared through his lawyer that he will "do everything possible" to bring his daughter to New York.

His statement said his wife's suicide "has been a heavy blow for me. I did not even have a chance to talk with her after my decision to break with the Soviet government because - and I am convinced of this - they forced her to return to Moscow. My daughter has been deprived of her mother, but she still has her father. I will do everything possible so that she can come here to with me."

Western diplomatic sources here said there is virtually no chance the Kremlin would respond to any such appeals from Shevchenko or the U.S. government. It has long been Soviet policy to ignore pleas from defectors that their families be allowed to emigrate.

Just two weeks ago, Anya Shevchenko spoke to Western correspondents here of her father's actions: "I don't understand it at all I can't believe he had been coerced into staying in New York.

Leongina Shevchenko herself took a similar view in an interview just a month ago. "He is a marvelous father and husband," she said. "He cannot live without his work and his family. I am one hundred percent sure he will be returning. I know and am certain . . . I have lived with him for 27 years . . ."