The<unk>  function via a well-studied set of nuclear receptors, termed the<unk> receptors. These receptors, together with corepressor molecules, bind DNA regions called<unk> 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. Although this general functional model has considerable experimental support, there remain many open questions. More recently genetic evidence has been obtained for a second mechanism of<unk> action involving one of the same nuclear receptors, TRβ, acting rapidly in the cytoplasm through the PI3K. This mechanism is conserved in all mammals but not fish or amphibians, and regulates brain development and adult metabolism. The mechanism itself parallels the actions of the nuclear receptor in the nucleus: in the absence of<unk>  TRβ binds to PI3K and inhibits its activity, but when<unk> binds the complex dissociates, PI3K activity increases, and the<unk> bound receptor diffuses into the nucleus. Thyroxine, iodine and apoptosis Thyroxine and iodine stimulate the spectacular apoptosis of the cells of the larval gills, tail and fins in amphibian metamorphosis, and stimulate the evolution of their nervous system transforming the aquatic, vegetarian tadpole into the terrestrial, carnivorous frog. In fact, amphibian frog Xenopus laevis serves as an ideal model system for the study of the mechanisms of apoptosis. Effects of triiodothyronine Effects of triiodothyronine (T3) which is the metabolically active form: * Increases cardiac output * Increases heart rate * Increases ventilation rate * Increases basal metabolic rate * Potentiates the effects of catecholamines (i.e. increases sympathetic activity) * Potentiates brain development * Thickens endometrium in females * Increases catabolism of proteins and carbohydrates
Thyroid hormones
Sex hormone
Gastrointestinal hormone
Hormone
Xenohormone
Thyroglobulin
Thyroid-stimulating hormone
Lipoprotein