Thyrotropin releasing hormone (TRH) is secreted by the hypothalamus, and acts on the anterior pituitary to stimulate release of thyroid stimulating hormone (TSH). The release of TRH is inhibited in the presence of high levels of circulating T3 and T4.

Thyroid stimulating hormone (TSH) is secreted by the anterior pituitary, in response to the stimulating effect of TRH. TSH acts on the thyroid gland to stimulate release of T3 and T4. The release of TSH is inhibited in the presence of high levels of circulating T3 and T4.

Thyroxine (T4) is produced by the thyroid gland in response to the stimulating effect of TSH. Thyroxine is transported in plasma in either its free form or bound to protein. Thyroxine is converted to T3 and reverse T3 (rT3) in both the circulation and in tissues.

Triiodothyronine (T3) is both directly secreted by the thyroid and converted from T4.

Reverse triiodothyronine (rT3) is an inactive form of T3 that is secreted in stress states.

Thyroid binding globulin (TBG) is a plasma protein that binds and transports the majority (~75%) of thyroid hormones. Thyroid hormones are also transported to a lesser degree by transthyretin and albumin.

Thyroglobulin (Tg) is a storage form of the thyroid hormones produced by follicular cells of the thyroid, and is released into serum in conjunction with T3 and T4.

Thyroperoxidase (TPO) is an enzyme that catalyses the oxidation of iodide on tyrosine residues in thyroglobulin, enabling production of the thyroid hormones.