Inorganic iodine enters the body primarily as iodide, I-. After entering the thyroid follicle (or thyroid follicular cell) via a Na+/I- symporter (NIS) on the basolateral side, iodide is shuttled across the apical membrane into the colloid via pendrin, after which thyroid peroxidase oxidizes iodide to atomic iodine (I) or iodinium (I+). The "organification of iodine," the incorporation of iodine into thyroglobulin for the production of thyroid hormone, is nonspecific; that is, there is no TPO-bound intermediate, but iodination occurs via reactive iodine species released from TPO. The chemical reactions catalyzed by thyroid peroxidase occur on the outer apical membrane surface and are mediated by hydrogen peroxide.
Stimulation and inhibition
TPO is stimulated by TSH, which upregulates gene expression.
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