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.