Iodine cycle: Ingested iodine is trapped in the thyroid, oxidized and bound to tyrosine to form iodotyrosines in the thyroglobulin, coupling of iodotyrosyl residues forms T3 and T4. Hormone secreted by the gland is transported into the serum. The hormone exerts its effects and ultimately deionidated. The iodine is either reused or excreted