thyroid hormone class

Thyroid hormones are an oddity among the amine hormones: they are made from the amino acid tyrosine, yet they behave almost like steroids. The main members are thyroxine (T4), with four iodine atoms, and triiodothyronine (T3), with three. The iodine is essential — without enough dietary iodine the gland cannot build these hormones.

Although thyroid hormones are technically derived from a single amino acid, the iodine and ring structure make the finished molecule fat-soluble. This creates a paradox: a lipophilic hormone that does not dissolve well in watery blood. The body solves this by binding most thyroid hormone to carrier proteins such as thyroxine-binding globulin, leaving only a tiny free fraction available to act.

Once they reach a target cell, thyroid hormones cross the membrane and bind nuclear receptors that regulate genes, much like steroids. T4 acts largely as a reservoir that tissues convert into the more potent T3 using deiodinase enzymes. Their effects — on metabolism, growth, and development — are slow to begin but broad and long-lasting.