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 2 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 adults, 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. In the United States, the most common cause is
Hashimoto thyroiditis: Hypothyroidism develops as the thyroid gland is gradually destroyed.
Other causes of primary hypothyroidism include
Thyroid inflammation (thyroiditis)
Treatment of hyperthyroidism or thyroid cancer
Lack of iodine
Radiation to the head and neck
Inherited disorders that prevent the thyroid gland from making or secreting enough hormones
Thyroid inflammation (thyroiditis) can cause permanent or temporary hypothyroidism depending upon the cause. Hashimoto thyroiditis gradually destroys the thyroid gland and usually causes permanent hypothyroidism. Subacute thyroiditis is probably caused by an infection with a virus. Autoimmune inflammation that occurs after childbirth (silent lymphocytic thyroiditis) is another cause. In subacute thyroiditis and silent lymphocytic thyroiditis, 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 medications 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 countries where iodine is not added to salt. 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.
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 adults.
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 adults, may appear confused or forgetful—signs that can easily be mistaken for Alzheimer disease or other forms of dementia. Women with hypothyroidism may have changes in their menstrual periods.
By permission of the publisher. From Burman K, Becker K, Cytryn A, et al. In Atlas of Clinical Endocrinology: Thyroid Diseases. Edited by SG Korenman (series editor) and MI Surks. Philadelphia, Current Medicine, 1999.
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 medications such as sedatives that depress brain function.
Diagnosis of Hypothyroidism
Measurement of thyroid-stimulating hormone (TSH) 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
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 adults, 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.
