Merck Manual

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Jerome M. Hershman

, MD, MS, David Geffen School of Medicine at UCLA

Last full review/revision Oct 2020| Content last modified Oct 2020
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Hypothyroidism is underactivity of the thyroid gland that leads to inadequate production of thyroid hormones and a slowing of vital body functions.

  • Facial expressions become dull, the voice is hoarse, speech is slow, eyelids droop, and the eyes and face become puffy.

  • Usually only one blood test is needed to confirm the diagnosis.

  • Most people with hypothyroidism need to take thyroid hormone for the rest of their life.

The thyroid gland secretes thyroid hormones, which control the speed at which the body’s chemical functions proceed (metabolic rate). Thyroid hormones affect many vital body functions, such as the heart rate, the rate at which calories are burned, skin maintenance, growth, heat production, fertility, and digestion. There are two thyroid hormones:

  • T4: Thyroxine (also called tetraiodothyronine)

  • T3: Triiodothyronine

The pituitary gland produces thyroid-stimulating hormone (TSH), which stimulates the thyroid gland to produce thyroid hormones. The pituitary gland slows or speeds the release of TSH, depending on whether the levels of thyroid hormones circulating in the blood are getting too high or too low. (See also Overview of the Thyroid Gland.)

Hypothyroidism is common, especially among older people, particularly women. It affects about 10% of older women. It can, however, occur at any age.

Myxedema is the name given to very severe hypothyroidism.


Hypothyroidism may be

  • Primary

  • Secondary

Primary hypothyroidism results from a disorder of the thyroid gland itself. The most common cause is

Other causes of primary hypothyroidism include

  • Thyroid inflammation (thyroiditis)

  • Lack of iodine

  • Radiation to the head and neck

  • Inherited disorders that prevent the thyroid gland from making or secreting enough hormones

Thyroid inflammation can cause temporary hypothyroidism. Subacute thyroiditis is probably caused by an infection with a virus. Autoimmune inflammation that occurs after childbirth (silent lymphocytic thyroiditis) is another cause. The hypothyroidism is usually 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. 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.

Radiation to the head and neck, usually given as radiation therapy to treat cancer, can also cause hypothyroidism.

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 (see also Hypothyroidism in Infants and Children).

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.

  • Facial expressions become dull.

  • The voice is hoarse and speech is slow.

  • Eyelids droop.

  • The eyes and face become puffy.

  • The hair becomes sparse, coarse, and dry.

  • The skin becomes coarse, dry, scaly, and thick.

Many people with hypothyroidism are fatigued, gain weight, become constipated, develop muscle cramps, 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. Women with hypothyroidism may have changes in their menstrual periods.

People with hypothyroidism frequently have high levels of cholesterol in their blood.

Myxedema coma

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.

Spotlight on Aging: Hypothyroidism in Older People

Many older people have some degree of hypothyroidism. About 10% of women and 6% of men are affected.

Typical symptoms, such as weight gain, muscle cramps, tingling of the hands, and the inability to tolerate cold, are less common among older people. When such symptoms do occur among older people, they are less obvious.

Older people may also have less typical symptoms. For example, they may lose weight, become confused, and have a decreased appetite, joint stiffness, joint and muscle pains, weakness, and a tendency to fall.

Because symptoms in older people can be different, are often subtle and vague, and are common among older people who do not have hypothyroidism, doctors may not recognize these symptoms as being caused by hypothyroidism. A screening test, in which blood levels of thyroid-stimulating hormone are measured, is important. The test should be done at age 65 even if people have no symptoms of hypothyroidism.


  • Measurement of thyroid-stimulating hormone levels in blood

Doctors usually suspect hypothyroidism on the basis of the symptoms and findings on physical examination, including a slow pulse.

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.


  • Replacement of thyroid hormone

Treatment involves replacing thyroid hormone using one of several oral preparations. The preferred form of hormone replacement is synthetic T4 (levothyroxine). Another form, desiccated (dried) thyroid, is obtained from the thyroid glands of animals but is no longer used very often. 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.

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