MSD Manual

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Glenn D. Braunstein

, MD, Cedars-Sinai Medical Center

Reviewed/Revised Sep 2022
Topic Resources

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 Thyroid

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 Overview of the Thyroid Gland The thyroid is a small gland, measuring about 2 inches (5 centimeters) across, that lies just under the skin below the Adam’s apple in the neck. The two halves (lobes) of the gland are connected... read more .)

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.

Causes of Hypothyroidism

Hypothyroidism may be

  • Primary

  • Secondary

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

Other causes of primary hypothyroidism include

A chronic lack of iodine in the diet is the most common cause of hypothyroidism in many developing countries. However, iodine deficiency Iodine Deficiency Iodine deficiency, which is common worldwide, can lead to enlargement of the thyroid gland. (See also Overview of Minerals.) Iodine occurs in seawater. A small amount of iodine from seawater... read more 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 Hypothyroidism in Infants and Children Hypothyroidism is decreased production of thyroid hormone. Hypothyroidism in children usually occurs when there is a structural problem with the thyroid gland or the thyroid gland is inflamed... read more ).

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.

Symptoms of Hypothyroidism

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 Carpal Tunnel Syndrome Carpal tunnel syndrome is a painful compression (pinching) of the median nerve as it passes through the carpal tunnel in the wrist. The cause of most cases of carpal tunnel syndrome is unknown... read more 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 Alzheimer Disease Alzheimer disease is a progressive loss of mental function, characterized by degeneration of brain tissue, including loss of nerve cells, the accumulation of an abnormal protein called beta-amyloid... read more or other forms of dementia. Women with hypothyroidism may have changes in their menstrual periods.

Myxedema coma

If untreated, hypothyroidism can eventually cause anemia Overview of Anemia Anemia is a condition in which the number of red blood cells is low. Red blood cells contain hemoglobin, a protein that enables them to carry oxygen from the lungs and deliver it to all parts... read more , a low body temperature, and heart failure Heart Failure (HF) Heart failure is a disorder in which the heart is unable to keep up with the demands of the body, leading to reduced blood flow, back-up (congestion) of blood in the veins and lungs, and/or... read more Heart Failure (HF) . 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 medications 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. Some experts recommend screening for thyroid disease every year in people older than 70 years. However, several medical organizations have examined the pros and cons of such screening and recommend against such screening in people without any suggestive symptoms or signs of thyroid disease.

Diagnosis 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 supports the diagnosis of hypothyroidism if the free level of T4 is also low. In that case, evaluation of the pituitary with pituitary function tests and imaging is generally performed.

Treatment 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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