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. 2023 Jul 27;37(5):1626–1640. doi: 10.1111/jvim.16823

FIGURE 2.

FIGURE 2

A thyroid follicular cell demonstrating the main steps of thyroid hormone synthesis and secretion. (1) In the basolateral membrane, circulating iodide is actively concentrated in the thyroid follicular cell by NIS. (2) Following diffusion across the cytosol, iodide enters the colloid via PDS. In the apical membrane, TPO catalyzes (3a) oxidation of iodide to iodine, (3b) binding of iodine to thyroglobulin (organification), generating MIT and DIT, and (3c) coupling of MIT and DIT to form T3 and T4. (4) Colloid‐containing thyroglobulin is endocytosed by the thyroid follicular cell. (5) Proteolytic enzyme‐containing lysosomes fuse with vesicles ferrying thyroglobulin, resulting in T4, T3, DIT, and MIT release. (6a) Thyroid hormones are secreted into circulation via MCT8. (6b) Iodine within DIT and MIT is recycled to be used for the organification of new thyroglobulin molecules.