Sodium-iodide symporter

A sodium/iodide symporter (NIS), also known as a sodium/iodide cotransporter or solute carrier family 5, member 5 (SLC5A5) is a protein that in humans is encoded by the SLC5A5 gene. It is a transmembrane glycoprotein with a molecular weight of 87 kDa and 13 transmembrane domains, which transports two sodium cations (Na⁺) for each iodide anion (I⁻) into the cell. NIS mediated uptake of iodide into follicular cells of the thyroid gland is the first step in the synthesis of thyroid hormone.

Iodine uptake

Iodine uptake mediated by thyroid follicular cells from the blood plasma is the first step for the synthesis of thyroid hormones. This ingested iodine is bound to serum proteins, especially to albumins. The rest of the iodine which remains unlinked and free in bloodstream, is removed from the body through urine (the kidney is essential in the removal of iodine from extracellular space).

Iodine uptake is a result of an active transport mechanism mediated by the NIS protein, which is found in the basolateral membrane of thyroid follicular cells. As a result of this active transport, iodide concentration inside follicular cells of thyroid tissue is 20 to 50 times higher than in the plasma. The transport of iodide across the cell membrane is driven by the electrochemical gradient of sodium (the intracellular concentration of sodium is approximately 12 mM and extracellular concentration 140 mM). Once inside the follicular cells, the iodide diffuses to the apical membrane, where it is metabolically oxidized through the action of thyroid peroxidase to iodinium (I⁺) which in turn iodinates tyrosine residues of the thyroglobulin proteins in the follicle colloid. Thus, NIS is essential for the synthesis of thyroid hormones (T₃ and T₄).