The sodium iodide symporter and thyroid disease.

The movement of a variety of species from the iodine-rich environment of the sea to the relatively iodine-deficient land has necessitated the development of mechanisms which will trap more efficiently inorganic iodide (I – ) necessary for the formation of thyroid hormones (Venturi et al. , 2000). In man as in other mammals iodide uptake by the thyroid is mediated via a TSHdependent transmembrane protein, the sodium iodide symporter (NIS), so-called because it co-transports Na + with I – into the thyroid against the concentration gradient (De La Vieja et al. , 2000). The NIS is located in the basolateral membrane of thyroid follicular cells, and accumulated I – is organified to molecular iodine (I 2 ) through the action of the enzyme thyroid peroxidase (TPO) in the presence of H 2 O 2 , which takes place mainly at the apical membrane of the follicular cell. Organified iodine iodinates the thyroid protein thyroglobulin, which when stored in the lumen of thyroid follicles represents a storage site for the hormones thyroxine (T4) and triiodothyronine (T3) (Taurog, 1996). Possession of NIS enables the thyroid to concentrate I –