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Concept
Triiodothyronine
Summary
Triiodothyronine, also known as T3, is a thyroid hormone. It affects almost every physiological process in the body, including growth and development, metabolism, body temperature, and heart rate.
Production of T3 and its prohormone thyroxine (T4) is activated by thyroid-stimulating hormone (TSH), which is released from the anterior pituitary gland. This pathway is part of a closed-loop feedback process: Elevated concentrations of T3, and T4 in the blood plasma inhibit the production of TSH in the anterior pituitary gland. As concentrations of these hormones decrease, the anterior pituitary gland increases production of TSH, and by these processes, a feedback control system stabilizes the level of thyroid hormones in the bloodstream.
T3 is the true hormone. Its effects on target tissues are roughly four times more potent than those of T4. Of the thyroid hormone that is produced, just about 20% is T3, whereas 80% is produced as T4. Roughly 85% of the circulating T3 is later formed in the liver and anterior pituitary by removal of the iodine atom from the carbon atom number five of the outer ring of T4. In any case, the concentration of T3 in the human blood plasma is about one-fortieth that of T4. The half-life of T3 is about 2.5 days. The half-life of T4 is about 6.5 days.
Iodothyronine deiodinase#Types
T3 is the more metabolically active hormone produced from T4. T4 is deiodinated by three deiodinase enzymes to produce the more-active triiodothyronine:
Type I present in liver, kidney, thyroid, and (to a lesser extent) pituitary; it accounts for 80% of the deiodination of T4.
Type II present in CNS, pituitary, brown adipose tissue, and heart vessel, which is predominantly intracellular. In the pituitary, it mediates negative feedback on thyroid-stimulating hormone.
Type III present in placenta, CNS, and hemangioma. This deiodinase converts T4 into reverse T3, which, unlike T3, is inactive.
T4 is synthesised in the thyroid follicular cells as follows.
Official source
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