noun, plural: triiodothyronines
In animals, hormones are substances produced and secreted by an endocrine gland, the ductless gland of the endocrine system. In humans, the thyroid gland located at the front of the neck is an endocrine gland. The follicular cells (thyrocytes) of the thyroid gland, in particular, make use of iodine to biosynthesize hormones. In humans, the two major thyroid hormones are the thyroxine and the triiodothyronine. The production of thyroid hormones is activated by the thyroid-stimulating hormone (TSH) released by the anterior pituitary gland.
Triiodothyronine, with a chemical formula of C15H12I3NO4, is considered as the most potent thyroid hormone. It plays a role in various processes in the body, such as body temperature, growth, and heart rate.
Circulating thyroxines are greater in number (about 80% of the thyroid hormones) than the circulating triiodothyronine. The half-life of thyroxines is also longer than that of the triiodothyronines. Thyroxines that enter the cell by way of ATP-dependent carrier-mediated transport. Inside the cell, the thyroxines are converted into triiodothyronines by deiodinase enzymes. These enzymes act by deiodinating thryroxine.
The triiodothyronine may be further processed to produce iodothyronamine and thyronamine.
The triiodothyronine binds to the nuclear receptor, thyroid hormone receptor. The binding causes conformational change in the receptor, displacing the corepressor molecules. The corepressor together with the receptor binds DNA regions called thyroid hormone response elements near genes. The complex blocks the transcription of certain genes. Thus, the displacement of corepressor from the complex leads to the recruitment of coactivator proteins and RNA polymerase, and the subsequent activation of gene transcription.