into thyroid cells against a chemical and electric gradient. It diffuses rapidly across the cells and is transported into the lumen of thyroid follicles, where T4 and T3 are produced (Figure 2-1). Iodide transport into the cells is mediated by a specific protein molecule, the sodium (Na+)/iodide (I−) symporter (NIS) (Dohan et al. 2003). The symporter is also present in substantial quantities in the salivary glands, stomach, and mammary glands; the iodide that is transported into these tissues is not further metabolized, as it is in the thyroid gland, but instead is secreted unchanged into saliva, gastric juice, or milk. Very small amounts of the symporter have been found in other tissues (see the last section of this chapter and Chapter 4).