Iodothyronine

General term for thyroid hormones that have an iodine-containing amine structure.

Iodothyronines include the classical thyroid hormones

• Thyroxine (T4)
• triiodothyronine (T3)

Furthermore the "non-classical" thyroid hormones, which are partly produced as by-products of thyroid hormone synthesis, partly by regulated deiodination from the classical thyroid hormones. The "non-classical" thyroid hormones include:

• reverse T3 (rT3: biologically inactive form of T3).
• Diiodthyronines (T2: 3,5-Diiodthyronine is an active thyroid hormone with L-thyroxine-like action)
• Monoiodthyronins (T1: the physiological function of these monoiodinated metabolites is still largely unexplained)
• Iodothyroacetates: formed by deamination from iodothyronines. Examples are TRIAC and TETRAC. They show partial agonistic effects to the iodothyronines.
• Thyronamines are formed from iodothyronines by decarboxylation. Their function is largely unknown.

Iodothyronines are lipophilic. They're poorly soluble in plasma. Consequently, they bind to different transport proteins (especially thyroxine-binding globulin, transthyretin and albumin). Iodothyronines are produced by different transport proteins. Iodothyronines are actively transported into the cells by various iodothyronine transporters (transport proteins - see below thyroxine, triiodothronine). Due to this active transport, the intracellular iodothyronine level is higher than in the blood plasma or interstitium.