(See also Overview of the Thyroid Gland.)
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 may be
Primary hypothyroidism results from a disorder of the thyroid gland itself. The most common cause is
Hashimoto thyroiditis: Hypothyroidism develops as the thyroid is gradually destroyed.
Other causes of primary hypothyroidism include
Thyroid inflammation can cause temporary hypothyroidism. Subacute thyroiditis is probably caused by an infection. Autoimmune inflammation that occurs after childbirth (silent lymphocytic thyroiditis) is another cause. The hypothyroidism is temporary because the thyroid gland is not destroyed.
Treatment of hyperthyroidism or thyroid cancer can cause hypothyroidism because radioactive iodine or drugs used in treatment interfere with the body’s ability to make thyroid hormones (see Drug treatment). 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.
Secondary hypothyroidism occurs when the pituitary gland fails to secrete enough thyroid-stimulating hormone (TSH), which is necessary for normal stimulation of the thyroid. Secondary hypothyroidism is much rarer than primary.
Insufficient thyroid hormones cause body functions to slow. Symptoms are subtle and develop gradually. Some of them may be mistaken for depression, especially among older people.
Many people with hypothyroidism gain weight, become constipated, and are unable to tolerate cold. Some people develop carpal tunnel syndrome, which makes the hands tingle or hurt. 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 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). Myxedema coma is 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. If the thyroid gland is underactive, the level of TSH is high.
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, or tetraiodothyronine). A low level confirms the diagnosis of hypothyroidism.
Many experts suggest that TSH levels should be measured in all people older than 60 whether or not they have symptoms. The test is recommended 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.
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 eventually 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.