ABSTRACT
The thyroid hormones play key regulatory roles in several very important aspects of the life history of vertebrate animals, including organogenesis of the embryo, reproductive physiology, metabolic regulation, and, in poikilotherms, thermeogenesis. Thyroid hormones are very simple molecules, being essentially iodinated thyronine compounds, with iodide being attached to the 3-and 5-positions of the phenolic rings. The three main forms of iodinated tyrosines in the blood of vertebrates are the tetraiodothyronine (3, 3′, 5:5′-thyroxine, T4), the triiodothyronines (3,3′,5-triiodothyronine,T3), and 3, 3′, 5′- triiodothyronine (reverse T3, rT3). Of these, T3 is the biologically active form, and it exerts its hormonal effect by binding to specific T3 receptors (TRs) that are attached to chromatin in the nucleus of the target cells, and by so doing regulating the expression of specific genes. Some of the iodinated thyronines, including some of the forms of diiodothyronine, may also act at extranuclear sites (e.g., the mitochondria or cell membranes) (Lanni et al., 1994).
