What is triiodothyronine?

Triiodothyronine (T3) is a thyroid hormone. It is one of the two main hormones produced by the thyroid gland, the other being thyroxine (T4). T3 is the more active form of thyroid hormone and is responsible for most of the physiological effects of thyroid hormones.

T3 is produced by the removal of one iodine atom from T4 by the enzyme iodothyronine deiodinase. T3 can also be produced from the deiodination of reverse triiodothyronine (rT3).

T3 circulates in the blood bound to thyroid-binding globulin (TBG) and transthyretin (TTR). Only a small amount of T3 is free and unbound in the blood. The free T3 is the biologically active form of the hormone.

T3 binds to receptors in the nucleus of cells and regulates the expression of genes. T3 stimulates the synthesis of proteins, increases the body's metabolic rate, and promotes growth and development.

T3 levels are regulated by a negative feedback loop involving the hypothalamus, pituitary gland, and thyroid gland. When T3 levels are low, the hypothalamus releases thyrotropin-releasing hormone (TRH), which stimulates the pituitary gland to release thyroid-stimulating hormone (TSH). TSH stimulates the thyroid gland to produce and release T3 and T4.

When T3 levels are high, the hypothalamus and pituitary gland decrease the release of TRH and TSH, respectively, which in turn decreases the release of T3 and T4 by the thyroid gland.

T3 levels can be affected by a number of factors, including diet, stress, illness, and medications. High T3 levels can cause hyperthyroidism, while low T3 levels can cause hypothyroidism.