Hypothyroidism

Thyroid hormone, made in the thyroid gland located in the neck, is an important regulator of the body's metabolism. Hypothyroidism refers to a lack of thyroid hormone, usually from underproduction. It can result in myriad symptoms, including weight gain or obesity, loss of muscle mass, cold intolerance, constipation, changes in menstruation, depression or slowed thinking, fatigue, congestive heart failure, dry skin, and more. Hypothyroidism is more common in women and in the elderly. Estimates of the prevalence of hypothyroidism vary throughout the world, but it may affect as much as 10 percent of older women in the United States.

Normally, the production of thyroid hormone is very tightly regulated by complex feedback loops. The pituitary gland regulates thyroid gland hormone production by altering levels of thyroid-stimulating hormone (TSH) in response to stress and levels of thyroid hormone in the blood. In turn, the thyroid gland uses iodine to make two different forms of thyroid hormone—T4, a storage form, and T3, the active one—which is released into the bloodstream. An increase in TSH should make the thyroid gland produce and release more of the hormone. However, in most cases of hypothyroidism, the thyroid gland is unable to respond. In developing countries, this is often a result of iodine deficiency; in the developed world, destruction of the gland by autoimmune disease, radiation, trauma, or infiltrative processes is more commonly the culprit. With any of these causes, the TSH level will continue to climb unless thyroid hormone is replaced.

Less commonly, hypothyroidism can be caused by pituitary problems and underproduction of TSH. In these cases, the thyroid gland is normal but simply has not received the signal to make hormone. The TSH may be normal or even low in the face of low T4 and T3 blood levels.

On examination, people with untreated hypothyroidism may appear normal or have coarsened facial features, dry hair and skin, rashes, swelling of the feet, and obesity. The thyroid gland may also be enlarged, resulting in a goiter. The most sensitive test for diagnosing hypothyroidism, except in cases of pituitary disease, is measurement of TSH. Low levels of free T4 confirm the diagnosis. Hypothyroidism, [Page 383]whether from thyroid or pituitary disease, is readily treated by replacement of thyroid hormone in pill form.