Helen Thomas knows she is often less than welcome, if not because of who she is then because of what she represents. The White House bureau chief for United Press International can reel off a 10-minute catalogue of the many inventive ways presidents and their friends have tweaked members of her profession, and at a dinner in her honor last night she did.

There was the time Lyndon Johnson went to the hospital for an operation and the mental ward was converted to a press room.

"Johnson asked Bill Moyers, who was his press secretary, 'Where are the patients?' " Thomas remembered last night at the National Press Club. "He said, 'We gave them all press cards.' "

There was the time the press sent Jackie Kennedy a message asking her what she fed her new German shepherd, Clipper.

"She wrote back," said Thomas. " 'Answer A: Reporters.' "

And as for President Reagan, "Well," Thomas said, "it's like one of those silent movies. He thinks we should be seen and not heard."

Thomas' catalogue covers six presidents and 23 years spent at the White House. She was the first woman to be named White House bureau chief of a major wire service, the first woman to be admitted to the Gridiron Club, the first woman to serve as president of the White House Correspondents Association and last night became the first woman and first wire service reporter to receive the National Press Club's Fourth Estate Award.

"You could fill a whole page with what she was first at," said White House press secretary Jim Brady.

And you could fill a whole page, and more, with lists of her admirers. Even the people who have a little trouble with what she does, with the tough and persistent energy and the pugnacious questions, praise her.

"She gives me trouble," said White House deputy press secretary Larry Speakes before the dinner, "but it's all well deserved. You can't fool her. Oh, you try to pull the wool over her eyes, but you can't."

And even Reagan, who has had a few zingers sent his way by Thomas, wrote in a letter read at the dinner, "You have become an important part of the American presidency."

But such pleasant phrases can't take the edge off the terminally feisty Sam Donaldson, who raced in at 10:03 (only three hours late) after being held captive in the Atlanta airport. The ABC correspondent managed to seed his speech with jabs at the constraints the Reagan White House has imposed on reporters.

"There was the day on the South Lawn," he said, "when reporters were allowed on the South Lawn."

It seems that on that day, a cedar of Lebanon had been planted, and after the president left the press corps encouraged Thomas, whose parents were Lebanese, to pick up the shovel and help cover the roots.

"And as she shoveled," Donaldson said, "I heard the ghosts of presidents past and present say, 'Shove her in.' "

Keeping the barbs flying, Speakes described an imagined story by Thomas during his speech: "Diplomatic sources said the president has agreed to a summit . . ." He paused, milked the moment and concluded, "with his son."

Nancy Reagan's press secretary Sheila Tate, NBC White House correspondent Andrea Mitchell, AP White House correspondent Maureen Santini, columnist James J. Kilpatrick, White House deputy press secretary Peter Roussel, Abigail Van Buren (Dear Abby, to the rest of us) and almost 500 other people dripped their way through the rain to attend the dinner at the press club. They all love Thomas.

"Do I love her?" said Richard Strout Christian Science Monitor writer and former Fourth Estate winner. "Passionately! Everybody loves her. She's so capable. She's so much better than most men. How's that?"