 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).