FIGURE 2-2 Structures of thyroxine (T4), triiodothyronine (T3), and reverse triiodothyronine (reverse T3). Note that T3 is missing an iodine atom on its outer ring and reverse T3 is missing an iodine atom on its inner ring. T3 is most potent thyroid hormone. T4 is active only after conversion to T3, and reverse T3 has no biologic activity.

The NIS is a glycoprotein composed of 643 amino acids and a small amount of carbohydrate. It is in the outer (plasma) membrane at the basolateral surface of the thyroid follicular cells (Figure 2-1). It is not present in the membrane at the apical surface of the cell, the part of the cell that is adjacent to the lumen of the thyroid follicle. The symporter uses the inward movement of sodium ions to transport iodide into the cells; two sodium ions are transported for each iodide ion.

NIS has a very high affinity for iodide, which allows transport of iodide into the cells against a high concentration gradient. It also transports other ions with a similar shape and electric charge, such as perchlorate and thiocyanate. The affinity of NIS for those substances is higher than its affinity for iodide, so they can block iodide transport into thyroid cells, which can result in a decrease in the iodide concentration in the cells and therefore in a decrease in the availability of iodide for synthesis of T4 and T3. Among those substances, perchlorate is the best studied. It is a true competitive inhibitor of iodide transport: it blocks iodide transport through the symporter in a dose-dependent manner. Conversely, iodide blocks



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