HE CALLED himself "Thomas Loder" and over the past month, perpetrated a hoax about a non-existent $20 million diamond on the news media around the world and had more than one respected publication on the verge of printing a story that could have severely damaged, if not defeated, French president Valery Giscard d'Estaing in his upcoming relection bid.

Giscard has been under intense scrutiny by the French press for the last several months for his close friendship with the Central African deposed emperor Jean-Bedel Bokassa.

Bokassa admitted that he had given diamonds to both Giscard and his wife, and the French president has never actually denied that the diamonds were received, only that they were not worth the large amounts claimed by the press.

"Loder," carefully and skillfully building an identify for himself in a series of bogus news stories planted all around the world, has been promising to produce documentation that would prove that Mme. Giscard sold one of the Bokassa diamonds which weighed 170 carats to his mother" in 1977.

The "Loder" rumors have received such wide circulation and have been taken seriously by so many news organizations in recent weeks that Giscard's advisors are known to have become increasingly worried that someone might print them.

But now the Elysee Presidential Palace can relax.

"Thomas Loder," confronted with positive identification last week, has admitted to bing the same young man who pulled a million-dollar prank on two of the world's most prestigious auction houses, Sotheby's and Christie's, in New York in 1977.

The name he used then was "Thomas Shults." Friends who knew him as a freshman at the University of Chicago under that name in 1972 say that they were under the impression that he was the son of a prominent New York City doctor with the same surname.

Last week, after a day of telephone interviews with The Washington Post, "Thomas Loder" confirmed he is the same person who was "using" the name of "Thomas Shults" when he victimized Sotheby's and Christie's four years ago. "

On May 11, 1977, he invited a woman banker whose husband had been a college friend of Shults to accompany him in a Rolls Royce to an invitation-only auction of French impressionist paintings at Sotheby Parke Bernet's Madison Avenue gallery.

Arriving at the door and saying a pair of engraved invitations were waiting for him, Shults conned the gallery into letting him in after the invitations couldn't be found. Then he took a seat in the 15th row and proceded to outbid everyone with a winning bid of $310,000 for Monet's "Young Woman In a Garden".

Five days Later, Shults attended another sale at Christie's on Part Avenue and bid $880,000 for Van Gogh's "End of the Day."

Sotheby's was the first to suspect something was wrong when Shults' credit references with a German bank failed to check out.

The Monet was subsequently re-sold to an underbidder for $300,000.

A spokeman for Christie's said last week that there has been no underbidder for the Van Gogh and that the painting had been returned to the owner unsold.

Shults' mother, reached yesterday by phone in New York, said she had read about a "Thomas Loder" in The New York Post last month in a Eugenia Sheppard column which reported he had sold a pair of 60-carat diamond earrings for $6 million in Switzerland last year. She did not connect "Loder" with her son, she said, although she had noticed that she and "Loder's" mother both had the same first name: Elinor. "I wish [Sir Edmund] Loder were his real father," she said. "I love my husband, but wouldn't it be terrific to be royalty."

Her son hasn't lived at home for five years, she said. She hasn't seen him since July 1980, but talked with him on the phone "about three weeks ago." h

So far as she knows, she said, her son hasn't received psychiatric care since he was 14 or 15.

"He was always considered to be 'very manipulative' and 'unusally intelligent' in the reports we got." Shults' mother said. "The psychiatrist said he should be given every consideration. We sent him to school in Switzerland. He came back after a week. We sent him to a lot of very fancy shcools. I can't remember all of them."

She added:

"He has been very difficult, but this is the first time I have ever heard of him doing anything like contacting the press. Thomas has a problem. I just think he'd like to be somebody he's not. He loves social status."

She said she and his father have always supported him financially and that they were supporting him up until two months ago but wouldn't say what had happened to stop the support.

When she heard about the allegations "Loder" had been making about Giscard, she said: "Oh my heavens, oh my goodness, I said I was never going to be surprised with what he has done, but this is too much.

"We've tried to keep him out of trouble, but he is now 25 and I think it is a little too much. I don't even think it's a fantasy world anymore. Maybe he thinks it's cute."

When asked about the art gallery prank, she said she only knew about the one involving Sotheby's. "I was away with my husband at that particular time for maybe a month," she said. "And I never knew anything about it until some of my husband's patients who had heard it broadcast on CBS asked me about it . . . I asked him how something like that could have happened and he said 'Oh, that was nothing, it was a joke." And I never heard of it again until you mentioned it now."

She said she was having difficulty keeping all the schools straight, but that he had gone to a number of private high schools here and abroad. After high school he went to the University of Chicago and transferred to California Institute of Technology, where he dropped out and re-entered several times before ending up at Princeton University where he was expelled for reasons she would not discuss.

He has never worked, she said. "He doesn't believe in working."

She said her son would frequently see something in the newspaper and identify with it, saying things like "Isn't it terrific that someone could own a 60 carat diamond".

The young man posing as "Thomas Loder" to The Washington Post and other news organizations in four countries over the past month was traced to "Thomas Shults" when a banker friend to whom he was billing hundreds of dollars worth of long distance telephone calls last week was located and guessed the connection between the two identities.

A photograph of "Loder" which The Toronto Star in Canada had taken to run with an 80-inch story about Loder and Giscard that was being prepered for publication this Sunday had been positively identified by two longtime friends.

Shults, in hours of interviews over the phone last week with The Washington Post, said he had perpetrated the hoaxes with Sotheby's and Christie's in 1977 because he wanted to impress a lover "with how rich I was."

There was also an element of "spite" involved, he said, because the lover is connected to one of the galleries.

Shults claims his motive in manipulating the media in his complicated diamond hoax was not personal, although he "got a charge out of seeing my name in print" and having friends who knew him as "Loder" in San Francisco and London and Paris see the stories which were coming out and calling him to say "they didn't know I was that rich."

Shults insists that he was "programmed" by enemies of Giscard in France who hoped to hurt him in the upcoming election by getting enough "facts" into print world-wide that would give an illusion of credibility to the stories Shults was peddling in at least four countries.

Shults told a reporter that he had been approached by a member of a prominent French wine-making family two months ago and coached on details of a story that Shults subsequently was to tell over and over again to some of the world's most respected publications.

Cy Jamison, assistant foreign editor of The Toronto Star, which was on the verge of printing some of Shults' accusations, said last week that "there is no doubt that what he was saying would have damaged Giscard and maybe defeated him if Shults could have gotten a reputable paper to print them."

Jamison pointed out that "Thomas Loder," before he approached The Star, had gotten The Toronto Globe and Mail -- one of Canada's most respected papers -- to print a story on March 16, identifying "Loder" as a "a Canadian citizen who recently sold the world's largest flawless diamond to an anonymous Mideast buyer for a record $20 million."

"Loder, 26, who resides in Switzerland," The Globe reported, "sold the Star of Peace March 5" to someone in Cairo.

The story went on to say that "Loder" believed the purchaser to be either Egyptian president Anwar Sadat or Farah Diba, widow of the shah of Iran.

That story enhanced "Loder's" credibility, Jamison said, but he became suspicious when he spoke French to "Loder" and wasn't understood.

"There's no way that he could have been born in Quebec, as he said he was," Jamison said, "and lived in Switzerland without speaking French."