<P> Iodine is an essential trace element for life, the heaviest element commonly needed by living organisms, and the second - heaviest known to be used by any form of life (only tungsten, a component of a few bacterial enzymes, has a higher atomic number and atomic weight). </P> <P> Iodine's main role in vertebrate biology is as constituents of the thyroid hormones, thyroxine (T4) and triiodothyronine (T3). These are made from addition condensation products of the amino acid tyrosine, and are stored prior to release in an iodine - containing protein called thyroglobulin . T4 and T3 contain four and three atoms of iodine per molecule, respectively . The thyroid gland actively absorbs iodine from the blood to make and release these hormones into the blood, actions which are regulated by a second hormone TSH from the pituitary . Thyroid hormones are phylogenetically very old molecules which are synthesized by most multicellular organisms, and which even have some effect on unicellular organisms . </P> <P> Thyroid hormones play a basic role in biology, acting on gene transcription to regulate the basal metabolic rate . T3 acts on small intestine cells and adipocytes to increase carbohydrate absorption and fatty acid release, respectively . A deficiency of thyroid hormones can reduce basal metabolic rate up to 50%, while in excessive production of thyroid hormones the basal metabolic rate can be increased by 100% . T4 acts largely as a precursor to T3, which is (with minor exceptions) the biologically active hormone . </P>

Where is the most of the iodine in the body concentrated