The sodium-iodide symporter plays a role in the accumulation of iodide in the thyroid gland. here, one iodide gets converted to one iodine, which is utilized for the formation of either of the two types of thyroid hormones, t3 and t4. t3 and t4 are named after the number of iodines found in each of these hormones. to produce a single molecule of t3, a total of sodium ions must move down their concentration gradients by secondary active transport. the movement of iodide ions occurs in the direction as sodium ions.