The synthesis of the iodothyronines is dependent on iodine from the diet. The follicular cells of the thyroid gland concentrate iodine in the form of iodide using an iodide trap. The iodide trap is a pump in the follicles that actively pumps iodide from the blood into the follicles. This is shown in the diagram below. Once the follicular cells have taken up the iodide, it is activated into a reactive form by a peroxidase enzyme. Once activated, this reactive iodine associates with a protein rich in the amino acid tyrosine (called thyroglobulin). Initially, the iodine associates with thyroglobulin either singly or in pairs (mono- or di- units). Then, as a result of coupling reactions, units with three or four iodine ions (tri- or tetra- units) are formed. The protein containing the iodothyronines is then stored as colloid in the follicular cells.