Transport of iodide, an essential constituent of the thyroid hormones T3 and T4, represents the first and main rate-limiting step in thyroid hormone synthesis. The thyroid gland shares its capacity to accumulate iodide (I−) with several other tissues, including salivary glands, gastric mucosa, lactating mammary gland, choroid plexus and the ciliary body of the eye. Although the transport systems in extrathyroidal tissues exhibit functional similarities with their thyroid counterpart, they lack the capacities to accumulate I− in a TSH-dependent fashion and to organify accumulated I−. Its exclusive capacity to efficiently trap and organify I−, a scarce trace element in many areas of the world, makes the thyroid gland a highly specialized and truely unique organ. Physiologically, the thyroid gland is capable of concentrating I− by 20–40-fold with respect to the concentration of the anion in plasma. Iodide is transported transcellularly from the basolateral to the apical membrane.