Thyroxine, abbreviated as T4, is the major hormone produced by the follicular cells of the thyroid gland. Since it has four iodine molecules attached to its molecular structure, thyroxine is synthesized via the iodination and covalent bonding of the phenyl portions of tyrosine residues found in an initial peptide, thyroglobulin, which is secreted into thyroid granules. These iodinated diphenyl compounds are cleaved from their peptide backbone upon being stimulated by thyroid stimulating hormone. More in the T3 and T4 section of thyroid.
Thyroxine is carried in the blood stream, with 99.95% of the secreted thyroxine being protein-bound, principally to thyroxine-binding globulin (TBG), and, to a lesser extent, to transthyretin and serum albumin. Thyroxine is a prohormone and a reservoir for the active thyroid hormone triiodothyronine (T3), which is about four times more potent. T4 is converted in the tissues by deiodinases, including thyroid hormone iodine peroxidase (TPO), to T3.
Function: thyroxine is involved in controlling the rate of metabolic processes in the body and influencing physical development. Administration of thyroxine has been shown to significantly increase the concentration of nerve growth factor in the brains of adult mice. Thyroxine increases the number and activity of mitochondria in cells by binding to the cells' DNA, raising the basal metabolic rate. Administration of thyroxine causes an increase in the rate of carbohydrate metabolism and a rise in the rate of protein synthesis and breakdown. This hormone excites the nervous system, leading to increased activity of the endocrine system. T4 remains active in the body for more than a month.