Thyroid hormones

Thyroid hormones are two hormones produced and released by the thyroid gland, namely triiodothyronine (T3) and thyroxine (T4). They are tyrosine-based hormones that are primarily responsible for regulation of metabolism. T3 and T4 are partially composed of iodine. A deficiency of iodine leads to decreased production of T3 and T4, enlarges the thyroid tissue and will cause the disease known as simple goitre.

Production and regulation

The major form of thyroid hormone in the blood is thyroxine (T4), which has a longer half-life than T3. The ratio of T4 to T3 released into the blood is roughly 20 to 1. T4 is converted to the active T3 (three to four times more potent than T4) within cells by deiodinases (5'-iodinase). These are further processed by decarboxylation and deiodination to produce iodothyronamine (T1a) and thyronamine (T0a). All three isoforms of the deiodinases are selenium-containing enzymes, thus dietary selenium is essential for T3 production.

Function

Thyroid hormones (T3 and T4) are essential for brain maturation, and for bone growth and development. In adults, thyroid hormones help maintain brain function, food metabolism, and body temperature, among other effects.

Medical use

Thyroid hormones are used therapeutically, primarily in the treatment of hypothyroidism, a condition in which the body does not produce enough thyroid hormones. They can also be used to treat goitre via its ability to lower thyroid-stimulating hormone (TSH), a hormone that is considered goitre-inducing.

See also

• Thyroid function tests
• Thyroid disease
• Hyperthyroidism
• Hypothyroidism

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  Diagram of the thyroid system
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  Thyroid system overview
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  Thyroid hormone synthesis process