Hypothyroidism is underactivity of the thyroid gland that leads to inadequate production of thyroid hormones and a slowing of vital body functions.
Hypothyroidism is common, especially among older people, particularly women. It affects about 10% of older women. It can, however, occur at any age. Very severe hypothyroidism is called myxedema.
Hypothyroidism has several causes.
Primary hypothyroidism results from a disorder of the thyroid gland itself. The most common cause is Hashimoto's thyroiditis (see Thyroid Gland Disorders: Hashimoto's Thyroiditis). As the thyroid is gradually destroyed, hypothyroidism develops.
Subacute thyroiditis and silent lymphocytic thyroiditis can both cause transient hypothyroidism. The hypothyroidism is transient because the thyroid is not destroyed.
Hypothyroidism can develop after treatment of hyperthyroidism or thyroid cancer because use of radioactive iodine or drugs that interfere with the body's ability to make thyroid hormones or surgical removal of the thyroid gland leads to a lack of thyroid hormone production.
A chronic lack of iodine in the diet is the most common cause of hypothyroidism in many developing countries. However, iodine deficiency is a rare cause of hypothyroidism in the United States because iodine is added to table salt and is also used to sterilize the udders of dairy cattle and thus is present in dairy products. Rarer causes of hypothyroidism include some inherited disorders in which an abnormality of the enzymes in thyroid cells prevents the gland from making or secreting enough thyroid hormones.
In secondary hypothyroidism, which is much rarer than primary, the pituitary gland fails to secrete enough thyroid-stimulating hormone (TSH), which is necessary for normal stimulation of the thyroid.
Insufficient thyroid hormones cause body functions to slow. Symptoms are subtle and develop gradually. They may be mistaken for depression, especially among older people. Facial expressions become dull, the voice is hoarse and speech is slow, eyelids droop, and the eyes and face become puffy. Many people with hypothyroidism gain weight, become constipated, and are unable to tolerate cold. The hair becomes sparse, coarse, and dry, and the skin becomes coarse, dry, scaly, and thick. Some people develop carpal tunnel syndrome, which makes the hands tingle or hurt (see Hand Disorders: Carpal Tunnel Syndrome). The pulse may slow, the palms and soles may appear slightly orange (carotenemia), and the side parts of the eyebrows slowly fall out. Some people, especially older people, may appear confused, forgetful, or demented—signs that can easily be mistaken for Alzheimer's disease or other forms of dementia.
If untreated, hypothyroidism can eventually cause anemia, a low body temperature, and heart failure. This situation may progress to confusion, stupor, or coma (myxedema coma), a life-threatening complication in which breathing slows, seizures occur, and blood flow to the brain decreases. Myxedema coma can be triggered in a person with hypothyroidism by physical stresses, such as exposure to the cold, as well as by an infection, injury, surgery, and drugs such as sedatives that depress brain function.
Usually hypothyroidism can be diagnosed with one simple blood test: the measurement of TSH. Many experts suggest that the test be done at least every other year in people older than 55 because hypothyroidism is so common among older people yet so difficult, in its mild stages, for doctors to distinguish from other disorders that affect people in this age group.
In those rare cases of hypothyroidism caused by inadequate secretion of TSH, a second blood test is needed. This blood test measures the level of the thyroid hormone T4 (thyroxine) that is not bound by protein (free). A low level confirms the diagnosis of hypothyroidism.
Treatment involves replacing thyroid hormone using one of several oral preparations. The preferred form of hormone replacement is synthetic T4. Another form, desiccated (dried) thyroid, is obtained from the thyroid glands of animals. In general, desiccated thyroid is less satisfactory than synthetic T4 because the content of thyroid hormones in the tablets may vary. In emergencies, such as myxedema coma, doctors may give synthetic T4, T3 (triiodothyronine), or both intravenously.
Treatment begins with small doses of thyroid hormone, because too large a dose can cause serious side effects, although large doses may be necessary. The starting dose and the rate of increase are especially small in older people, who are often most at risk of side effects. The dose is gradually increased until the levels of TSH in the person's blood return to normal. During pregnancy, doses usually need to be increased.
Last full review/revision May 2012 by Jerome M. Hershman, MD