Obtained in the diet, iodine is essential for the synthesis of thyroxine. Iodine is normally contained by the commercial salt as an additive, and seafood is a source of iodine. Absorbed iodine is transported through the blood to the thyroid gland, where an active transport mechanism named an iodine pump moves the iodine (anions of iodine) into the follicle cells. Then the iodines are converted (oxidized) to iodine and secreted into the colloid. Within the polypeptide chain of protein named thyroglobulin, the iodine attaches to specific amino acids once in the colloid.
monoiodotyrosine (MIT) is resulted from the attachment of one iodine to a tyrosine; whereas, diiodotyrosine (DIT) is resulted from the attachment of two iodines. Enzymes connect DIT and MIT together within the colloid. Tetraiodothyronine-T4, or thyroxine is produced by two linked DITs. Triiodothyronine or T3 is produced by a coupling of MIT and DIT. T3 and T4 are still attached to thyroglobulin at this point. Upon stimulation by TSH, the cells of the follicle take up a small volume of colloid by pinocytosis, hydrolyze the T3 and T4 from the thyroglobulin, and secrete the free hormones into the blood. Thyroid hormones stimulate protein synthesis, promote maturation of the nervous system, and increase the rate of energy utilization by the body through the activation of genes.