Considering that Americans in Moscow have discovered in the past year that they are being saturated with Soviet microwaves, drinking water contaminated with dangerous parasites and suffering in large numbers from a mysterious blood abnormality, it is understandable that the embassy's doctor should have become a major figure in the community.

But the pronouncements of U.S. Air Force Ltd. Col. Thomas A. Johnson, a genial 36-year-old physician with a background in obstetrics, are regarded with something akin to reverence. He has attained a stature hereabouts that international specialists spend a lifetime trying to achieve - a tribute to his calm assurances and proven reliability as well as to the fact that dismayed Americans feel they have no place else to turn.

Caring for U.S. officials and their families, however, is only a part of Johnson's responsibility. There are, at present, representatives of 90 embassies on the active patient list, plus resident businessmen, students, correspondents, even tourists. Some 20 ambassadors regularly consult him and a senior diplomat at one major Western European embassy relied heavily on him recently during a period of extreme psychological distress.

In many ways his practice resembles that of an old-fashioned country doctor, except that life in Moscow produces a host of special problems in both illness and care that Johnson and his predecessors (all, technically, assistant air attaches at the embassy) have had to deal with.

There is enormous "psychic stress" encountered here, Johnson said in an interview the other day: husbands who work much too hard to meet the demands of what many regard as "the most challenging assignment" of their careers, families who suffer from the ensuing "enforced separation," a general sense of restrictions and constant surveillance, a feeling in some cases of "oppression, of being threatened.

The result, said Johnson, is a high proportion of patients with complaints that reflect those stresses: colitis, inflammation of the large intestine, ulcer diseases, problems with sleep. tremendous anxieties, lack of sexual gratification. There have been four to five nervous breakdowns among just one Western nationality (not Americans) recently, he said, and one official American visitor had to be flown out in a strait-jacket.

"I had to accompany him," Johnson remarked. "He needed medication all the way back. He thought he was going to be done in, if not by the Soviets then by me. We flew on Pan American sitting in the back of a full plane. It was an untenable situation, but there was nothing else I could do."

Because foreigners are generally very reluctant to submit to Soviet medical care unless they must - which Johnson attributes to wariness over what would be unfamiliar surroundings, language problems, a chronic Soviet shortage of medications and often-retold unpleasant experiences - a doctor's concerns here must be logistical as well as medical.

"I have to think about various airline schedules, long flights under pressurized conditions, arrangements on the other end and making sure that you are not placing the patient in significant jeopardy."

In two cases last winter of perforated appendixes involving high-level Americans, Johnson was in a race with time to get them to hospitals in Helsinki. In the 21 months since he arrived in Moscow, hundreds of Johnson's patients have been flown out for treatment.

The alarming disclosure a year ago that high levels of radiation had been detected in the embassy and attributed to Soviet microwave beams exacerbated all the existing difficulties.

"There was a tremendous surge of complaints," said Johnson. "The microwave crisis was a magnificent place for people to express all their pent-up frustration and anxiety.

"From January to June 1 spent a great deal of time counseling, trying to give people a reasonable perspective on the situation, trying to impart a feeling that I was forthright." To told them that if, as a father of four, he felt that his family was endangered he would leave. "That seemed to help."

Yet, despite assurances from Washington that the installation of shields on the embassy building and pressures on the Soviets had substantially reduced the radiation level, plus the fact that new arrivals, had been briefed in Washington and knew what to expect, some uneasiness remains. The effects of sustained exposure to microwave beams are still under study - indeed, Americian diplomats past and present are providing scientists with an ideal sample.

Analysis of blood tests made in the microwave study provided another jolt when it was revealed that a third of these examined had abnormally high white-bloodcell counts, an apparently benign but nonetheless puzzling phenomenon.

No connection has been drawn between that condition and the radiation, but all three children whose blood was sufficiently abnormal to warrant further tests in the United States lived within the embassy building itself, where the microwave beams were detected.

Nonethlss, Johnson feels that Americans have mostly come to terms with the additional hazards of working here. "I haven't seen lately the exaggerated reactions that were commonplace after the initial disclosure," he said.

The presence of glardiasis, a parasite in the water that can make children in particular very sick, is now dealt with calmly, and even the report earlier this month - erroneous, it turned out - that potentially harmful quantities of cyanide and mercury had been found in the water of some buildings where foreigners live barely caused a stir.

But all the fuss over health has put a considerable extra burden on Johnson despite the undoubted satisfaction of knowing his efforts are greatly appreciated. After months of being on call around the clock, handling perhaps 10 telephone consultations a night and about three night-time house calls a week. "I found I was living in a chronic-fatigue state. I had to take some leave."

Moreover, since last September his family has been subjected - presumably by someone who finds his work objectionable - to the daily harassment of strange telephone calls, voices breathing on the line, reading in Russian or music playing. After an unsuccessful attempt to trace the origin, with the help of Soviet operators, the embassy sent a formal note of protest to the Soviet Foreign Ministry. In November, the trunk of Johnson's car was opened and obviously searched while it was parked at Sheremet yevo international airport.

The same weekend the medical kit of the French embassy's physician, one of only two other Western doctors in Moscow, was stolen from his trunk.

Johnson receives no U.S. funds for his dispensary aside from his own salary (around $35,000) and that of the medical technician. Everything else, including medical supplies, pharmaceuticals, the part-time nurses' salaries and so on, must be financed by clinic fees - 10 rubles ($12.50 at official exchange rates) for first visits and five rubles ($6.50) thereafter, plus drug and lab fees. Official Americans and their families are treated free.

That all means a considerable amount of administration and bookkeeping aside from the other aspects of the job. Recently Johnson has had to cut back on non-American patients and he has sought a pledge that a fulltime nurse will be added to the staff. But he is concerned that if the proportion of official American financial support for the dispensary increases, "some bureaucrat in Washington" will decide that only Americans assigned to the embassy may be treated - a move that would leave a massive gap in the health care for thousands of foreigners here.

"That is my great worry," he said.