Thyroid hormones
The thyroid hormones function via a well-studied set of nuclear receptors, termed the thyroid hormone receptors. These receptors, together with corepressor molecules, bind DNA regions called thyroid hormone response elements (TREs) near genes. This receptor-corepressor-DNA complex can block gene transcription. Triiodothyronine (T3), which is the active form of thyroxine(T4), goes on to bind to receptors. The deiodinase catalyzed reaction removes an iodine atom from the 5' position of the outer aromatic ring of Thyroxine's (T4) structure. When triiodothyronine (T3) binds a receptor, it induces a conformational change in the receptor, displacing the corepressor from the complex. This leads to recruitment of coactivator proteins and RNA polymerase, activating transcription of the gene.
Liothyronine
Liothyronine is a synthetic form of triiodothyronine (T3), a thyroid hormone used to treat hypothyroidism and myxedema coma. T3 is the metabolically active thyroid hormone, which causes feedback inhibition, and lowers elevated TSH levels. It increases metabolism in peripheral tissues and is indicated when there is an impaired conversion of T4 to T3 in peripheral tissues. It is also used as an augmentation strategy in treating major depressive disorder when used in combination with antidepressants. It is marketed as the sodium salt under the brand name Cytomel (or Tertroxin in Australia).