FIGURE 2-1 Diagram of thyroid cells and a thyroid follicle, showing key steps in thyroxine (T4) and triiodothyronine (T3) synthesis and secretion. A follicle consists of a single layer of thyroid follicular cells surrounding a lumen, which is filled with thyroglobulin (Tg). Iodide (I) and sodium (Na+) ions are transported into cells via the sodium (Na+)/iodide (I) symporter (NIS) in the basolateral membrane of the cells. Iodide diffuses to the luminal side of the cell and is transported into the lumen of the follicle, where it is oxidized and then used to form T4 and T3 (within Tg). Tg is taken up by cells and broken down, freeing its constituent T4 and T3 molecules, which then diffuse out of the cell and into the bloodstream.

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).



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