Subchapter 93A – 3,3′,5-Triiodothyronine

Abstract 3,3′,5-Triiodothyronine (T 3 ) is an iodinated amino acid derivative, a small proportion of which is primarily synthesized in thyroglobulin molecules in the thyroid gland. T 3 is an active form of thyroid hormone (TH) but less is produced than thyroxine (T 4 ). Most T 3 is secondarily produced by deiodination of T 4 in peripheral tissues. Physiological effects of THs are exerted via genomic and non-genomic actions: genomic actions depend on nuclear thyroid receptors (TRs), which act as ligand-dependent transcription factors. T 3 is responsible for the genomic actions and some of the non-genomic actions: growth and development in bone and central nervous system, regulation of lipid and carbohydrate metabolism, and protein breakdown. The genomic actions appear within several hours or more accompanied by gene transcriptional activation or suppression. TR knock-out and knock-in studies result in the failure of some of the TH actions, in the TR subtype-specific manner.