Unbalancing act.

You're not 20 anymore - what do you expect?" That was what Eve heard from she was feeling tired, losing interest in sex, and gaining weight. Unhappy moving through life in slow motion, and determined at 43 not to accept premature aging, Eve dragged herself to her doctor for a checkup.

"I was lucky," recalls Eve. "When I described my symptoms to my doctor, I expected him to look at me like I was just another hysterical female. Instead, he said he suspected an underactive thyroid."

"Maybe you should take a leave of absence," suggested Sherry's boss, who had been hearing from clients that Sherry's work wasn't up to standard. Recently divorced, the 38-year-old graphic artist was jumpy and irritable, and her hands trembled, making it nearly impossible for her to do her work. Loyal friends reassured her that her ex-husband was probably to blame, but a trip to her doctor proved that Sherry's problem was in fact the opposite of Eve's - an overactive thyroid.

Eve and Sherry are among the ranks of millions who have thyroid dysfunction. The small, butterfly-shaped thyroid gland, located just below the larynx, wields a lot of power, exercising some control over nearly every organ in the body. Thyroid hormones regulate digestion, heart rate, body temperature, sweat glands, the nervous and reproductive systems, and body weight. Thyroid disorder - the result of a gland that is either underactive (hypothyroidism) or overactive (hyperthyroidism) - can creep up gradually, over months or years. Defying even the most rigorous beauty regimen, both conditions can wreak havoc on the skin, hair, fingenails, and facial features. And if left untreated over time, a thyroid disorder can become serious, possibly even fatal.

After diagnosis, treatment and management are relatively simple. Often, however, a diagnosis isn't easy to come by. Unfortunately," says Lawrence C. Wood, thyroidologist at Massachusetts General Hospital in Boston and president and medical director of the Thyroid Foundation of America, "at least 2 million people don't know they have a thyroid disorder." Both patients and doctors, he explains, can be fooled by symptoms that mimic those of stress, menopause, depression, and aging.

Losing steam: hypothyroidism. Chronic fatigue, muscle aches, cold intolerance, and constipation: these are complaints doctors hear from women every day. While they may be an inevitable part of the aging process, they can also signal hypothyroidism. Other symptoms: heavy or frequent menstrual periods, brittle nails, thinning hair, and an elevated cholesterol level. Untreated, the progressive condition causes the face to become puffy, the skin to get dehydrated, and the voice to drop into a deeper and huskier register. Even worse, the brain feels like it's running on empty, affecting memory and concentration.

Between 6 and 7 million people suffer from hypothyroidism, the most common type being Hashimoto's disease, named for the Japanese doctor who first described the process that causes the thyroid to fail. Women, according to Wood, are affected at least four times as often as men and are most commonly diagnosed after the age of 40. "One woman in 10 over the age of 50 has evidence of the earliest stages of hypothyroidism," he adds.

In high gear: hyperthyroidism. When the thyroid gland produces too much hormone, the result can feel like caffeine overload, with the excess hormone revving up metabolism and causing sleep difficulties, lighter and less-frequent menstrual periods, hand tremors, rapid pulse, heat intolerance, and in many cases, significant personality changes.

Distended eyes in the case of Barbara Bush and an unexplained weight loss in that of George Bush were tip-offs to their doctors that both Bushes had Graves' disease, the most common type of hyperthyroidism.

"Stressful events, such as the death of a spouse, a divorce, the loss of a job, an auto accident, or even a severe loss in the stock market, may trigger the thyroid to produce too much hormone," explains David E. Aftergood, an internist and endocrinologist at Cedars Sinai Medical Center in Los Angeles. Unfortunately," he adds, "even when life gets better, your thyroid problem won't, unless it's treated."

Treatment. The determinant for thyroid dysfunction is a simple blood test. "When women come to me with symptoms of depression, such as loss of appetite, feeling sad for more than two weeks, trouble sleeping, and lack of interest in normal activities, I usually screen them for thyroid problems with a thyroid blood test," says Irwin Ruben, a Beverly Hills psychiatrist and assistant clinical professor of psychiatry at the University of California School of Medicine in Los Angeles.

The TSH test, a component of the thyroid blood test, measures thyroid-stimulating hormone levels in the blood and detects any abnormalities. "Sometimes what seems like a psychiatric problem can be related to a thyroid disorder," says Ruben. "By adjusting the thyroid hormone level, symptoms of depression can be alleviated."

Treatment for hypothyroidism is easy to handle: a thyroid hormone tablet once a day (usually a lifetime prescription) should produce a fairly rapid recovery. In addition, some patients may notice an unexpected bonus: weight loss.

"Thyroid hormone was given years ago by doctors to overweight patients," explains Jerome M. Hershman, former president of the American Thyroid Association and a professor at the University of California School of Medicine in Los Angeles. "But excessive amounts were found to cause not only nervousness but also numerous deleterious effects on the heart and bones." Now, he says, it's regarded as "inappropriate and even quackery" to give thyroid-hormone therapy to induce weight loss. "I always caution patients to take the exact amount of hormone prescribed," he adds, "and not to take any extra to try to lose weight."

An overactive thyroid is a little more complicated to treat. In his book Your Thyroid.- A Home Reference, cowritten with Drs. David Cooper and E. Chester Ridgway, Massachusetts General's Lawrence Wood describes three ways to lower the overproduction of thyroid hormone.

First, a patient can take antithyroid pills to block hormone production, but allergies to the drug are common, and only 15 percent of those who use antithyroid pills find them effective in the long run. Most adults who develop hyperthyroidism are treated instead with radioactive iodine.

Swallowing radioactive iodine in capsule or liquid form, the second choice, destroys some of the thyroid tissue, thus reducing the amount of hormone produced. This is not an option for pregnant women because the iodine can cross the placenta and affect the baby's thyroid as well. Surgery, the third alternative, removes most of the thyroid gland. Sooner or later, says Wood, virtually everyone treated either with radioactive iodine or with surgery ends up with a hypothyroid condition; in fact, Wood believes that thyroid dysfunction progresses naturally over time from overactive to underactive. Fortunately, a daily supplement of thyroid hormone brings the level back to normal.

Periodic checkups to monitor, adjust, and fine-tune the thyroid hormone level are essential. An excess of thyroid hormone may upset the body's digestive, nervous, and reproductive systems, and can also induce heart problems. In addition to these concerns, Jerome Hershman says, some studies indicate that excessive doses of thyroid-replacement hormone over a long period of time can increase the risk of osteoporosis, although, he cautions, there's still some controversy surrounding this subject.

"If you suspect a thyroid problem or if you have a family history of thyroid disease, be a detective and ask your doctor to perform a TSH test," suggests Cedars Sinai's David Aftergood. "If thyroid's the problem, treatment is easy and recovery is possible in a matter of months."

For more information, contact the Thyroid Foundation of America, Inc., Massachusetts General Hospital, Ruth Sleeper Hall, Room 350, Boston, Massachusetts 02114; 800-832-8321.