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:
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.
Hypothyroidism may be
Primary hypothyroidism results from a disorder of the thyroid gland itself. The most common cause is
Hashimoto thyroiditis Hashimoto Thyroiditis Hashimoto thyroiditis is chronic, autoimmune inflammation of the thyroid. Hashimoto thyroiditis results when the body attacks the cells of the thyroid gland—an autoimmune reaction. At first... read more : Hypothyroidism develops as the thyroid is gradually destroyed.
Other causes of primary hypothyroidism include
Thyroid inflammation (thyroiditis)
Treatment of hyperthyroidism Treatment Hyperthyroidism is overactivity of the thyroid gland that leads to high levels of thyroid hormones and speeding up of vital body functions. Graves disease is the most common cause of hyperthyroidism... read more or thyroid cancer Thyroid Cancer The cause of thyroid cancer is not known, but the thyroid gland is very sensitive to radiation, which may cause cancerous changes. Thyroid cancer is more common among people who were treated... read more
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 Subacute Thyroiditis Subacute thyroiditis is acute inflammation of the thyroid, probably caused by a virus. (See also Overview of the Thyroid Gland.) Subacute thyroiditis usually begins suddenly. In this disorder... read more is probably caused by an infection with a virus. Autoimmune inflammation that occurs after childbirth (silent lymphocytic thyroiditis Silent Lymphocytic Thyroiditis Silent lymphocytic thyroiditis is painless, autoimmune inflammation of the thyroid that typically develops after childbirth and usually goes away on its own. (See also Overview of the Thyroid... read more ) is another cause. The hypothyroidism is usually temporary because the thyroid gland is not destroyed.
Treatment of hyperthyroidism Drug treatment Hyperthyroidism is overactivity of the thyroid gland that leads to high levels of thyroid hormones and speeding up of vital body functions. Graves disease is the most common cause of hyperthyroidism... read more or thyroid cancer Thyroid Cancer The cause of thyroid cancer is not known, but the thyroid gland is very sensitive to radiation, which may cause cancerous changes. Thyroid cancer is more common among people who were treated... read more 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 Iodine Deficiency Iodine deficiency, which is common worldwide, can lead to enlargement of the thyroid gland. The thyroid gland contains most of the iodine in the body. Iodine in the thyroid gland is necessary... 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.
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 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 , 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.
People with hypothyroidism frequently have high levels of cholesterol Dyslipidemia Dyslipidemia is a high level of lipids (cholesterol, triglycerides, or both) or a low high-density lipoprotein (HDL) cholesterol level. Lifestyle, genetics, disorders (such as low thyroid hormone... read more in their blood.
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 . 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.
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.
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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