White House aides are checking out Thailand as a possible side trip for Nancy Reagan when President Reagan attends the economic summit in Tokyo May 4-6. She would fly there as part of her antidrug crusade.

Mrs. Reagan put an international spin on her crusade last year when she invited wives of foreign leaders to confer about drug abuse with her at the White House and again at the United Nations. And in early May, she received papal endorsement for her efforts when she had a private visit with Pope John Paul II in Rome while Reagan attended the economic summit in Bonn.

The first lady's East Wing strategists decided those events were so successful that they have been seeking ways to expand the global parameters of her crusade.

Thailand's role in illegal drugs is certainly global, sources say. The House Select Committee on Narcotics Abuse and Dangerous Drugs says Thailand is a major heroin trafficking center, estimating that about one-fifth of the heroin arriving in the United States comes from there. The State Department says that heroin is produced in the Golden Triangle (Burma, northern Thailand and western Laos).

Last June, the largest seizure of heroin (95 kilos) since 1974 was made by U.S. Customs agents in Seattle. The drug, smuggled inside ice buckets, came directly from Bangkok.

Committee sources say Thailand's master plan to phase out opium production with substitute crops and livestock (since 1972, coffee, ginger, pigs and chickens have seemed to work) hasn't gotten very far because the Thai government can't seem to get its act together. In addition to the opium that the Thais themselves produce, morphine base and finished heroin come into Thailand from Burma.

"The problem is that when diplomats fail to get the Thais out of opium they always say it's because Burma produces it," says a committee spokesman. "But if the Thais would get out of the business altogether, we'd at least know that 30 to 50 tons of the stuff wouldn't be on the world market."

Mrs. Reagan, of course, keeps her distance from the politics of illegal drug production. She advocates prevention through education.

"Her role is working with wives of political leaders to point out the terrible dangers of drug abuse. And we're all for that," the committee spokesman says. "We believe we also have to prevent the availability of it."

Elaine Crispen, Mrs. Reagan's press secretary, had no comment yesterday on the first lady's trip plans, but the White House traditionally does not announce Mrs. Reagan's schedule until after everyone involved has had a chance to assess its role in administration policies.

Getting that big picture for Mrs. Reagan are her departing chief of staff, Lee L. Verstandig, and his announced successor, Jack L. Courtemanche. Looking over the physical arrangements, from the standpoint of the first lady's needs as well as her staff and press corps, is Marty Coyne, her director of scheduling and advance.

Thailand's Ambassador Kasem S. Kasemsri left Washington for Bangkok on Saturday, but no embassy officials could be reached yesterday to say whether his trip was related to that of Mrs. Reagan's advance team.

Guam's Rep. Ben Blaz, a Republican, said yesterday he was "shocked" that Guam's Gov. Ricardo Bordallo, a Democrat, would represent the Guamanians as supporting Philippine President Ferdinand Marcos.

"As the one who represents the territory here in the nation's capital, I am pretty sure that the views of the governor are not reflective of people of Guam," Blaz said.

At a White House dinner for the nation's governors Sunday night, Bordallo was asked what he thought about the possibility of Marcos taking refuge in Guam. Bordallo said he wouldn't oppose it and that Marcos has "quite a lot of followers" in the tiny Pacific territory.

But Blaz, who read the governor's remarks in the newspaper yesterday, said, "Good grief, no, I would bet my very modest congressional pay" that the United States isn't considering offering Guam as a refuge for Marcos.

"He's a man who is extremely controversial, a person [who would be leaving] under very adverse conditions. The last thing we would need is this. The people are very divided on the whole issue," said Blaz, who was elected in 1984.

Blaz said that while some Filipinos on Guam may support Marcos, "there is a lot of sentiment" there for Corazon Aquino. "One must realize that the people who came to Guam over the last 20 years came to America because of their disenchantment with the Marcos regime."

He said Marcos' nesting problems aren't very high on his list of priorities.

"If I had 12 things to worry about, Mr. Marcos' home would be number 13," Blaz said.