Iodide Transport by Follicular Cells and Iodine Trapping Within Follicles

The thyroid follicular cells transport I" across the cell and secrete the precursor protein, Tg, into the fol-licular lumen. In addition, these cells contain an apical membrane-bound enzyme, thyroperoxidase (TPO), and the enzymatic machinery to produce hydrogen peroxide (H2O2). In the presence of H2O2, TPO catalyzes the incorporation of I" into tyrosyl residues of Tg to form monoiodotyrosine (MIT) and diiodotyrosine (DIT) and the coupling of these iodotyrosyl residues to form T4 and T3.

Thyroid follicular cells actively transport iodide into the cell against both a concentration gradient and a negative potential (Fig. 65.1). At the basal (blood side) fol-licular cell membrane, an iodide pump actively transports

Extracellular Fluid

Follicular Cell

Follicular Cell

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