Thymosin. Not quite a household word yet, but it could represent one of the biggest medical breakthroughs of the century.

Scientists and physicians see use of the thymosins--hormones produced by the thymus gland--as a potential treatment and perhaps cure for a variety of human ills. The list ranges from cancer and the ravages of aging, to arthritis, multiple sclerosis, certain forms of hypertension, to even perhaps hayfever, asthma, hives . . .

Doctors used to believe that the thymus gland, a rather unprepossessing organ under the breastbone, was as vestigial as the appendix. Now they know that the thymus is the master gland of the immune system. (Thymosins are the parent hormones of interferon, also undergoing worldwide clinical trials in immune-system disorders.)

Thymosins were isolated only some 20 years ago, by Dr. Abraham White and Allan L. Goldstein (then postdoctoral fellow to White and now chief of biochemistry at George Washington Medical Center). Since then they have exploded in potential importance--in the laboratory, in the theoretical study of endocrinology and in a few dramatic human trials.

In one study, thymosin extract measurably lengthened the lives of lung-cancer patients. And it has meant the difference between life and death for several children born with rare immune-deficiency ailments that can mean lifetimes in sterilized plastic bubbles.

Thymosins are being used in National Cancer Institute-sponsored cancer trials in five major centers. They are also being used in more than 150 places to treat patients with other disorders now recognized as related to the "T-cell arm" of the immune system, the arm controlled by the thymus, the body's deepest defense against invasion by foreign bodies.

Some diseases are the result of the immune system mistaking the body itself for the enemy and destroying things that it shouldn't. The most widespread of this type (although not necessarily the most serious) is rheumatoid arthritis. It affects about 3 percent of the population, about 6.5 million in the United States alone.

In this area, Dr. Robert P. Jacobs, chief of the rheumatology division of the George Washington Medical Center, is about to start clinical trials with thymosin and some patients with RA. He is recruiting patients with RA who have not responded to traditional medication.

The thymus and its production of thymosins begin to decline just after puberty and continue to decline with age--whether it is cause or effect is unknown--leaving the elderly prone to illnesses against which they formerly had natural protection.

It may be possible to control all of these things to some extent--even the exigencies of age--by the injection of thymosin extract (from the thymus glands of cows). The same may be true of the auto-immune ills, in addition to rheumatoid arthritis, multiple sclerosis, lupus, a wide spectrum of allergies..

"One of the interesting things about thymosin," says Dr. Jacobs, "is that it does not appear to result in a supernormal response. In all the studies to date, what thymosin has been able to do is restore some immunologic abnormalities to normal."

"If there's something that's wrong, if we can correct it to normal, well, how nice. We don't have to worry about whether we are doing too much . . ."

Thymosin's apparent safety is one of the things that makes it particularly attractive to rheumatologists. Researchers agree that rheumatoid arthritis is caused by an overactive immune system, but the medicines available to lessen immune activity leave the patient vulnerable to attack by all sorts of other ills (the result of chemotherapy, for example, with cancer patients).

Rheumatoid arthritis has two aspects. One is inflammation of joints and tissue around the joints; the other, more serious aspect, is destruction of joints, tendons, ligaments and cartilage. (RA is often confused with osteo-arthritis, a "wear-and-tear" disease that tends to be spotty and is considered mechanical rather than systemic.)

Most of the common medicines for rheumatoid arthritis handle the pain and the inflammation, but cannot control the destructive aspects. It is hoped thymosin will work in that area.

"Of course," says Jacobs, "not everybody with rheumatoid arthritis goes on to get a crippling illness. That's awfully important. It is only a small percentage that go on with a disease that doesn't respond to anything, ever, but most patients have a progressive disease that does cause some damage and do need therapy.

"That is really what we are focusing on. We're thinking of this kind of therapy thymosin as a drug that perhaps can stop the disease if we are really approaching some of these basic disease mechanisms."

Rheumatoid arthritis is particularly frustrating--both to patient and therapist--because it rarely behaves the same in two patients, and its course is unpredictable even in the same patient. Victims of RA are peculiarly vulnerable to quack remedies, at least partly because no one remedy exists within established medicine.

As Jacobs has written in a medical journal, "because the disease is a chronic systemic process with enormous destructive potential, the fears and frustrations of patients must be anticipated . . . The expanding array of drugs available to treat rheumatoid arthritis, none curative and all potentially toxic, presents an additional source of confusion."

Although he admits to being "more hopeful than certain," Jacobs hopes to show that thymosin will be not only the safest, but possibly the most effective remedy yet against rheumatoid arthritis.

Patients with diagnosed rheumatoid arthritis that is unresponsive to conventional therapies, and who are interested in participating in the thymosin studies, should phone the George Washington Rheumatology Clinic, 676-3927. Or write Arthritis Thymosin Research Project, George Washington University Medical Center, 2150 Pennsylvania Ave. NW, Washington D.C. 20037.