The major source of iodine contained in our body comes from our diet. If the iodine taken via diet is in the molecular form (I2), it is absorbed by facilitated diffusion, if in the iodide form, then the (I-) absorption takes place with the help of sodium-iodide symporter carrier protein present in the gastric mucosa. Since, in humans, no information exists on tubular secretion or active transport, renal clearance first of all depends on the glomerular filtration. Inorganic iodine is ingested into the cell through iodide symporter which is located in the basolateral membrane, close to the capillary, of follicle cells and enables cell ingestion of two Na+ iodine and one I- iodine against electrochemical gradient. The activity of this pump is enhanced by TSH and this pump is the rate-limiting step in the synthesis of thyroid hormones. The second transport system required for transporting the iodide into the follicle lumen is the passive transport which enables it to enter into colloid after passing through thyroid cell apical membrane. The first step of the thyroid hormone synthesis is the addition of iodide to the tyrosyl residues in the thyroglobuline. This addition takes place at the boundary of apical plasma membrane and follicle lumen and in existence of H2O2, thyroperoxidase (TPO), iodide and thyroglobuline. First iodide becomes oxidised and then combines with tyrosyl residues of thyroglobuline. The next step in the hormone synthesis is the amalgamation of two neighbouring monoiododetyrosyl (MIT) residues to generate diiodothyronine (DIT). Two DITs amalgamate to generate T4, and amalgamation of one DIT and one MIT produces T3. TPO catalyzes this reaction.