A sodium/iodide symporter (NIS), also known as a sodium/iodide cotransporter[5] or solute carrier family 5, member 5 (SLC5A5) is a protein that in humans is encoded by the SLC5A5 gene.[6][7][8] 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.[9] NIS mediated uptake of iodide into follicular cells of the thyroid gland is the first step in the synthesis of thyroid hormone.[9]