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Hypothyroidism is a condition characterized by abnormally decreased activity of the thyroid gland and deficient production of thyroid hormones. The thyroid gland secretes hormones that play an essential role in regulating growth, maturation, and the rate of metabolism.
Specific symptoms and findings associated with hypothyroidism may be variable, depending upon the age at symptom onset, the degree of thyroid hormone deficiency, and/or other factors. In many adults with hypothyroidism, the condition may be characterized by generalized fatigue and lack of energy (lethargy), muscle weakness and cramping, dryness of the skin and hair, incomplete or infrequent passing of stools (constipation), sensitivity to cold, and other symptoms. If the condition is present at birth (congenital hypothyroidism), associated symptoms and findings may become apparent during early infancy. These may include respiratory and feeding difficulties, listlessness, protrusion of the abdomen, constipation, dry skin, coarse hair, progressive accumulation of fluid within bodily tissues, and other associated abnormalities. Some affected infants may have progressive retardation of physical and mental development that becomes increasingly severe (cretinism) without early recognition of the condition and prompt treatment.
There are several different causes of hypothyroidism. The condition may result from an underlying defect that is present at birth (congenital), such as improper development (dysplasia) or absence (aplasia) of the thyroid gland or biochemical (enzymatic) abnormalities. The condition may also develop later during childhood or adulthood (acquired) due to certain underlying disorders, the use of particular medications, or surgical removal of the thyroid gland. Although hypothyroidism most frequently affects adult females, the condition occurs in both genders and may become apparent at any age.
Hypothyroidism most commonly develops during adulthood. The range and severity of associated symptoms are variable, depending upon the degree of thyroid hormone deficiency and/or other factors. In some adults with mild hypothyroidism, no symptoms or only minimal symptoms may be apparent. However, without recognition and treatment of the condition, affected individuals may go on to develop more severe thyroid dysfunction.
Symptoms associated with hypothyroidism commonly include a lack of energy, drowsiness, or generalized fatigue (lethargy); loss of interest in regular activities (apathy); declining appetite; weight gain; sensitivity to cold; muscle cramping and weakness; and incomplete or infrequent passing of stools (constipation). In some affected individuals, there may also be abnormal enlargement of the thyroid gland (goiter). Additional symptoms may include forgetfulness, particularly in elderly individuals; unusual hoarseness of the voice; slowed or impaired articulation of speech (dysarthria); dryness and coarseness of the scalp hair; carpal tunnel syndrome; and/or menstrual abnormalities in affected women, such as increased menstrual flow. Affected individuals may also develop a dull facial expression; puffiness and swelling of the face and around the eyes; dryness, scaling, waxiness, and thickening of the skin; and unusually sparse hair. If untreated, hypothyroidism may eventually lead to abnormally decreased levels of the oxygen-carrying pigment (hemoglobin) in red blood cells (anemia), a slowed heart rate, low body temperature, and progressive confusion, lack of response to surroundings (stupor), and coma (i.e., myxedema coma), resulting in potentially life-threatening complications. Evidence suggests that certain factors may appear to precipitate myxedema coma, such as trauma, infection, exposure to the cold, or particular medications that serve to depress brain function.
If hypothyroidism is present at birth (congenital), symptom onset, severity, and progression may be variable, depending upon the degree of thyroid hormone deficiency and/or other factors. Rarely, congenital hypothyroidism may be temporary (transient).
Symptoms and findings associated with congenital hypothyroidism may include feeding difficulties; diminished muscle tone (hypotonia); listlessness; an unusually hoarse cry, yellowish discoloration of the skin, whites of the eyes, and mucous membranes (jaundice); coarsening and dryness of the skin; and episodes characterized by temporary cessation of spontaneous breathing. In some cases, there may also be abnormal enlargement of the thyroid gland (goiter). Congenital hypothyroidism may also be characterized by constipation; anemia, coldness and mottling of the skin; a slow pulse; a large abdomen; a soft swelling at the navel due to protrusion of the intestines through a weak area of the abdominal wall (umbilical hernia); abnormal wideness of the two membrane-covered gaps or "soft spots" between bones of the skull (fontanels); and/or other findings. Without prompt detection and treatment, affected infants may develop progressive, increasingly severe retardation of physical and mental development (cretinism). Associated symptoms and findings may include slowed bone growth, abnormally short limbs, wide hands with short fingers; increasing lethargy; delays in reaching developmental milestones (e.g., sitting up, crawling, learning to speak); and progressively severe mental retardation. Affected infants may also have an unusually thickened, protruding tongue, delayed tooth eruption; widely set eyes; a broad, flat nose; and a thickened, short neck. Additional characteristic features may include dry, scaly skin; abnormal puffiness of the skin, particularly of the eyelids, the external genitals, and the backs of the hands; thickening of the scalp; coarse, scanty scalp hair; and/or other abnormalities.
If hypothyroidism develops later during childhood, symptoms may also be variable, depending upon the degree of thyroid dysfunction, the age at symptom onset, and/or other factors. In some cases, associated symptoms and findings may include a decreased rate of growth, delayed bone maturation, decreased energy, intolerance to cold, constipation, puffiness of the skin, and/or other abnormalities. If hypothyroidism becomes apparent during adolescence, symptoms may tend to be similar to those seen in affected adults. In addition, affected adolescents often have delayed puberty.
Recent evidence suggests that children born to mothers with untreated hypothyroidism during pregnancy may tend to score lower on intelligence tests when compared to children of mothers without the condition. Such tests include studies relating to intelligence, reading ability, language, visual-motor performance, attention, and school performance. However, children of mothers with treated hypothyroidism score similarly to children of unaffected mothers. According to researchers, such results suggest that prompt diagnosis and treatment of hypothyroidism in pregnant women may play an important part of appropriate prenatal care. (For further information, please see the section entitled "Standard Therapies" below.)
Hypothyroidism results from decreased activity of the thyroid gland, an endocrine gland comprised of a pair of lobes on either side of the windpipe (trachea) within the neck. Certain cells within the thyroid gland (i.e., follicular cells) secrete the thyroid hormones known as thyroxine (or T4) and triiodothyronine (or T3). These hormones assist in regulating the rate of chemical processes within the body (metabolism). In addition, thyroid hormones play a vital role in regulating physical and mental development during infancy and childhood. Thyroid hormone production is regulated by secretion of thyroid-stimulating hormone (TSH), which is produced by the pituitary gland (also known as the "master gland").
Several underlying disorders, conditions, or other factors may be responsible for decreased activity of the thyroid gland and deficient production of thyroid hormones. If hypothyroidism is present at birth (congenital), the condition may be caused by abnormalities of certain enzymes within thyroid cells or improper development of the thyroid gland (thyroid dysgenesis), including absence (aplasia) or malformation (dysplasia) of the thyroid or the presence of incompletely developed (rudimentary) thyroid tissue in an abnormal location (ectopic thyroid tissue). Such abnormalities may appear to occur spontaneously for unknown reasons (sporadically) or may be familial. In addition, in some affected newborns, congenital hypothyroidism may result from fetal exposure to certain therapies during pregnancy (e.g., radioiodine therapy, "antithyroid" medications) or other causes.
Hypothyroidism may also develop later during childhood or adulthood (acquired) due to the administration of particular medications, surgical removal or radiation therapy of the thyroid gland, or certain underlying disorders. For example, a common cause of hypothyroidism is Hashimoto's Thyroiditis, an autoimmune disorder characterized by the development of antibodies against cells of the thyroid gland. Autoimmune disorders result when the body's natural defenses against foreign substances or invading microorganisms are directed against the body's own cells. (For more information on this disorder, please see the "Related Disorders" section of this report below.) In addition, in some cases, hypothyroidism may be due to abnormalities of the pituitary gland (e.g., tumor development), resulting in deficient production or secretion of thyroid-stimulating hormone (secondary hypothyroidism).
Hypothyroidism most commonly affects adult females. However, the condition occurs in both genders and may become apparent at any age. According to reports in the medical literature, congenital hypothyroidism occurs in approximately one in 4,000 to 5,000 infants worldwide, affecting females about twice as often as males.
As mentioned above (see "Causes"), Hashimoto's thyroiditis is thought to be the most common cause of hypothyroidism. Also known as chronic lymphocytic thyroiditis, it is an autoimmune disorder in which antibodies attack the thyroid gland, leading to deficiency of thyroid cells required to produce sufficient levels of thyroid hormones. Affected individuals characteristically develop abnormal enlargement of the thyroid gland (goiter) and eventual hypothyroidism. The disorder is more common in women than men and increases in frequency with age. (For more information on this disorder, choose "Hashimoto*" as your search term in the Rare Disease Database.)
Graves' disease is a disease of the thyroid gland. As with Hashimoto's thyroiditis, it more frequently affects women than men. It is thought to represent an abnormal autoimmune response in which certain antibodies are produced that bind to thyroid cells, leading to excessive secretion of thyroid hormones (hyperthyroidism). The disorder is also commonly characterized by enlargement of the thyroid gland (goiter) and/or protrusion of the eyeballs (exophthalmos). Symptoms and findings associated with hyperthyroidism may include nervousness, sleep disturbances, heat intolerance, weight loss, and/or tremor. Those who receive certain treatments for hyperthyroidism associated with Graves' disease (e.g., radioactive iodine to destroy thyroid cells, surgery) may tend to develop hypothyroidism. (For more information on this disorder, choose "Graves*" as your search term in the Rare Disease Database.)
Hypothyroidism may be diagnosed based upon a thorough clinical evaluation; detection of characteristic symptoms and findings; a detailed patient and family history; and laboratory testing. Laboratory studies to determine thyroid function may include blood tests that measure thyroid hormone levels, particularly thyroxine (T4), and the concentration of thyroid-stimulating hormone (TSH). For example, such results may confirm low T4 and high TSH in primary hypothyroidism (i.e., due to primary disease of the thyroid gland); low T4 and low TSH in secondary (pituitary) hypothyroidism; or normal T4 and high TSH in some patients with no obvious symptoms or minimal symptoms suggestive of hypothyroidism (subclinical hypothyroidism). In some cases, additional specialized studies may also be conducted, such as thyroid antibody tests to help determine whether the immune system is affecting thyroid functioning; imaging of the thyroid; and/or other tests.
In most of North America, all newborns are routinely screened for congenital hypothyroidism based on measurement of T4 levels, followed by TSH levels when T4 levels are low. Such newborn screening programs have been implemented due to awareness that prompt diagnosis and treatment of congenital hypothyroidism is vital for normal brain development during infancy.
As noted above, according to some researchers, physicians should consider routinely providing TSH testing for pregnant women, since high TSH levels may provide an early warning of impaired thyroid functioning. Because undiagnosed hypothyroidism during pregnancy may have adverse effects on the developing fetus, such researchers indicate that prompt detection and appropriate treatment of hypothyroidism in pregnant women may be an important factor in their children's intellectual and physical development.
The treatment of hypothyroidism typically includes thyroid hormone replacement therapy by mouth (orally). In newborns and infants with congenital hypothyroidism, it is essential that such treatment be initiated as early as possible and closely monitored to promote proper physical and mental development and prevent irreversible adverse effects. Various preparations are available for thyroid hormone replacement therapy. These include preparations of synthetic T4, T3, and combinations of the two hormones as well as dried (desiccated) thyroid from the thyroid glands of animals. Preferred thyroid hormone therapy typically consists of synthetic T4 preparations (levothyroxine sodium).
Initial starting doses are determined by age and other factors and adjusted as needed, based upon regular assessment of thyroid function. (Young children require disproportionately large doses relative to body size as compared to adolescents or adults.) In some affected infants, the diagnosis may later need to be confirmed to eliminate the possibility of temporary (transient) hypothyroidism. In other cases, however, daily replacement therapy is often required for life. Thyroid function must be regularly monitored to ensure that dose levels are appropriate, maintaining TSH and T4 levels in the normal range for age, and to adjust the dose to meet a person's changing needs as required. For example, a previously effective dose may need to be adjusted under certain circumstances, such as due to changes in thyroid function or when such therapy is used in association with other medications, in the elderly, or during pregnancy. As noted above, if hypothyroidism is initially diagnosed during pregnancy, hormone replacement therapy is started and adjusted as needed based on regular assessment of TSH levels.
Emergencies (e.g., myxedema coma) may require intravenous administration of thyroid hormones and steroid therapy. The Food and Drug Administration (FDA) approved Triostat (liothyronine sodium injection) for the treatment of myxedema coma and precoma. This drug is the synthetic injectable form of the thyroid hormone triiodothyronine (T3).
In some affected individuals, surgery to remove goiters or diseased thyroid tissue may be required. If the use of certain medications is responsible for the suppression of thyroid function, such therapy may be discontinued, if possible.
In some cases, genetic counseling may be of benefit for affected individuals and their families. Additional treatment for the condition is symptomatic and supportive.
Information on current clinical trials is posted on the Internet at www.clinicaltrials.gov. All studies receiving U.S. government funding, and some supported by private industry, are posted on this government website.
For information about clinical trials being conducted at the National Institutes of Health (NIH) in Bethesda, MD, contact the NIH Patient Recruitment Office:
Tollfree: (800) 411-1222
TTY: (866) 411-1010
For information about clinical trials sponsored by private sources, contact:
The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) is sponsoring an evaluation of patients with thyroid disorders, including hypothyroidism, hyperthyroidism, thyroid hormone resistance, Graves' Dermopathy, and thyroid-stimulating hormone (TSH) secreting pituitary adenomas. The purpose of the study is to enhance understanding of the natural history, clinical presentation, and genetics of thyroid function disorders. This evaluation has been in progress since 1977 and is currently (2004) still seeking healthy volunteers. Information on this study can be found by using the NIH listing at the beginning of the section.
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Report last updated: 2011/07/11 00:00:00 GMT+0