Although the availability of the TSH test makes early intervention a real possibility, public health officials haven't yet recommended that it be used as widely as screening tests for hypertension, cervical cancer, elevated cholesterol and the like. Universal test-wing would be expensive and it is not clear what proportion of people whose subclinical hypothyroidism is diagnosed would benefit from early treatment. As a result, thyroid specialists are apt to test people whose medical history, physical examination, or laboratory results indicate that they are more likely than others to develop serious thyroid problems in the future.

A common failing

Some studies indicate that as many as 15% of people over age 60 have subclinical hypothyroidism, defined as an elevated level of TSH coupled with a normal amount of circulating thyroid hormone (measured by T4 or free T4 assays).

The thyroid gland makes its essential, multi-purpose hormones (T3 and T4) with prompting from a pituitary chemical called thyroid stimulating hormaone (TSH) The pituitary can detect even minute changes in the circulating lefvel of hormones. When thyroid hormone levels fall, TSH goes up; when they rise too high, TZSH levels fall.

Almost all spontaneous hypothyroidism is due to Hashimoto's thyroiditis, an autoimmune disorder in which the immune system mistakenly attacks thyroid tissue, setting up an inflammatory process that may progressively destroy the gland. Physicians identify this type of hypothyroidism by using a blood test that measures antithyroid antibodies to thyroid tissue. In people who are predisposed to it, Hashimoto's disease is likely to surface following childbirth or treatment with powerful drugs such as alpha interferon or interleukin.

Hashimoto's is a common cause of goiter, or physical swelling of the gland, among adults. Even in its early stages, experienced physicians can usually feel some changes in the thyroid when they palpate the neck. But what helps nail down the diagnosis is the antibody test.

The remaining cases of hypothyroidism are due not to disease but to prior medical or surgical treatments. Thyroid deficiency often develops after radioactive iodine has been used to destroy all or part of an overactive thyroid gland. Both George and Barbara Bush, for example, underwent this treatment after developing Graves' disease during their time in the White House. Cancer patients whose necks have been irradiated are also at risk. Surgical removal of the gland itself also causes hypothyroidism. Epidemiologists have observed that certain groups of people are more likely to develop hypothyroidism than others. Risk factors include being female, over 50, or having a family history of autoimmune thyroid disorders. Hashimoto's thyroiditis is more common among those with other autoimmune diseases, including insulin- dependent diabetes mellitus, pernicious anemia, vitiligo (loss of skin pigment), Addison's disease (adrenal failure), or myasthenia gravis. There is even some evidence that hypothyroidism is more likely to develop in people who are left-handed or whose hair turns prematurely gray.

Pros and cons

In general, physicians don't hesitate to prescribe thyroid hormone replacement for patients with a TSH level greater than 10mU/L, a low T4 level or symptoms of full-blown hypothyroidism . But the course is less clear when a slight elevation in TSH level (5-lOmU/L) has raised the specter of early thyroid failure.

Before reaching for the prescription pad, physicians will take a number of factors into ac count. One is how the person feels: although the rule of thumb is that symptoms become more pronounced as TSH level soars, some people feel unwell even when their TSH is only slightly increased.

Another consideration is the outcome of the antibody test for Hashimoto's thyroiditis. Numerous studies of asymptomatic people with mild hypothyroidism and positive antithyroid antibodies indicate that 4-8% per year will later develop symptoms or low levels of thyroid hormone.

The hope is that thyroid replacement therapy will relieve symptoms, restore metabolism to normal, and make it easier for major organs to function as they should. Doctors know that treatment makes some people feel better, but beyond that the benefits of early intervention are uncertain.

Although many people benefit from thyroid hormone replacement, not everyone with sub- clinical hypothyroidism needs therapy. Many elderly folks are plagued by diseases that are made worse by thyroid hormone treatment. When TSH is minimally elevated, physicians are often reluctant to prescribe thyroid hormone for older people with severe coronary artery disease because higher levels place more stress on the heart.

Because postmenopausal women who take large doses of thyroid hormone are at risk for accelerated bone loss, it is particularly important that their dosage be carefully selected and monitored. When an appropriate dose is used, there is no association between thyroid replacement therapy and an excess risk for osteoporosis.

Reality therapy

Thyroid replacement therapy should be thought of as lifelong. People who are elderly or who have heart disease are put on a very low dose at first. All newly treated patients are monitored with TSH testing, typically at intervals of 4-12 weeks, to determine the optimum dose and make sure that overtreatment is avoided. Once the proper dose has been established, TSH level is generally measured annually. Meticulous monitoring is especially important for women with thinning bones.

Thyroid replacement is inexpensive as medical treatments go, costing less than $100 per year in most cases. The most commonly prescribed drug is levothyroxine, a synthetic copy of the natural hormone. The drug is sold in both branded and generic forms. Because the on the market are not necessarily identical, it's a good idea to stick with one brand. If an insurer or managed care plan forces a person to switch, TSH testing will be needed to make sure that the dosage of the new drug is right.

Some people whose hypothyroidism was diagnosed many years ago still swear by tablets made from desiccated pork or beef thyroid. Today physicians rarely start newly identified patients on this preparation, however, because the mix and potency of its contents may vary from one batch to the next.

If a dosage that previously relieved symptoms or normalized TSH level is no longer sufficient, several possible explanations must be considered. One is that the degree of thyroid failure may have worsened. Or it may be that another drug is blocking absorption of the thyroid hormone. Iron, for example, interferes with absorption even at doses found in ordinary multivitamins. Other common culprits include the cholesterol-lowering agent cholestyramine and possibly lovastatin, a second antilipid agent. Problems have also been reported in connection with aluminum hydroxide antacids (such as Maalox) and with sucralfate, an ulcer treatment. Thyroid hormone may be broken down faster in the presence of drugs such as dilantin, tegretol, and rifampin.

Finally, there is a widely held misconception that thyroid replacement can transform a fat person into a thin one. As appealing as this may sound, obesity is rarely caused by low thyroid. Even people with clinical hypothyroidism lose only 5-l0 pounds when they first embark on replacement therapy, and often gain them back over the next year or two.