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Thyroxine is the principal hormone produced by the thyroid gland. It promotes protein synthesis (blending) and growth, and also helps regulate the body's metabolism.

Thyroid Stimulating Hormones

Thyroxine is produced by the thyroid gland in a very complex way. When the blood's thyroxine level is low, the brain's hypothalamus (the part of the brain that regulates body functions) produces a thyrotropin-releasing hormone. This stimulates the pituitary gland to produce thyrotropin. Thyrotropin is a thyroid-stimulating hormone (TSH) that excites the thyroid gland. When the blood's thyroxine level is high, the hypothalamus releases a hormone that inhibits TSH production.


Hyperthyroidism is a condition caused by an overactive thyroid. The syndrome can cause weight loss, nervousness, and protruding eyes. Called Graves's disease, it was first identified by Irish physician Robert James Graves (1796-1853).

Treating Underactive Glands

German chemist Eugen Baumann (1846-1896) was the first researcher to treat underactive thyroids using extracts made from animal thyroid glands. In 1914 American biochemist Edward Kendall isolated and used the crystalline form of the hormone which was later named thyroxine. In 1926 the British chemist C. R. Harington (1897-1972) determined thyroxine's exact structure and synthesized it out of materials in a laboratory.

Treating Overactive Glands

Today, overactive glands can be treated with medication, removed surgically, or destroyed by radiation. When the gland is removed or destroyed the patient must always take thyroid hormones as replacement therapy. Hormone therapy is also used for underactive glands.

The first widely-used test to measure peoples' thyroid levels was developed in the 1930s by American biochemist Evelyn B. Man (1904-1992). Called the protein-bound iodine test, it soon showed that many "demented" patients in mental hospitals (those having deteriorated mental capabilities) actually had underactive thyroid glands. Treatments with thyroxine helped many of these patients regain normal mental abilities.

Doctors now routinely evaluate an infants' thyroid function by testing blood from newborn babies' umbilical cords. This allows correction of any thyroid gland problem before mental or physical damage occurs.

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thyroxine (thīrŏk´sēn), substance secreted by the thyroid gland. The hormone thyroxine forms by combining the amino acid tyrosine with iodine. Complexed to a protein, it is stored in the follicle stems between thyroid cells. Thyroxine enters into the bloodstream complexed to another protein, plasma globulin. Thyroxine increases the number and activity of mitochondria in cells by binding to the cells' DNA, increasing the basal metabolic rate. Administration of thyroid hormones, such as thyroxine, causes an increase in the rate of carbohydrate metabolism and a rise in the rate of protein synthesis and breakdown. The hormone, which excites the nervous system and leads to increased activity of the endocrine system, remains active in the body for more than a month. Thyroxine activity is controlled by thyrotropin, a substance released from the pituitary gland. Conversely, thyroxine regulates the effect of thyrotropin by feedback inhibition, i.e., high levels of thyroxine depress the rate of thyrotropin secretion. Synthetically prepared thyroxine is used clinically in the treatment of thyroid gland deficiency diseases in adults and in the treatment of cretinism in children.

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thyroxine An iodinated amino acid, formed in the thyroid gland from the proteolysis of thyroglobulin. It is the major hormone of this gland and exerts a positive control of the basal metabolic rate of virtually all tissues and organs in the body.

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thy·rox·ine / [unvoicedth]īˈräksēn; -sin/ (also thyroxin / -sin/ ) • n. Biochem. the main hormone, C15H11NO4I4, produced by the thyroid gland, acting to increase metabolic rate and so regulating growth and development.

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thyroxine (T4) The principal hormone of the thyroid gland. See also thyroglobulin.

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thyroxine Hormone secreted by the thyroid gland. It contains iodine and helps regulate the rate of metabolism; it is essential for normal growth and development.

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thyroxine (T4) (th'y-roks-een) n. see thyroid hormone.

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thyroxine One of the thyroid hormones.

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