FIGURE 1.

Iodine pathway in the thyroid cell. Iodide (I⁻) is transported into the thyrocyte by NIS at the basal membrane and migrates to the apical membrane. I⁻ is oxidized by the enzymes TPO and H₂O₂ and attached to tyrosyl residues in thyroglobulin to produce the hormone precursors MIT and DIT. The residues then couple to form T4 and T3 within the thyroglobulin molecule in the follicular lumen. Thyroglobulin enters the cell by endocytosis and is digested. T4 and T3 are released into the circulation, and nonhormonal iodine on MIT and DIT is recycled within the thyrocyte. Adapted from reference 79 with permission. DIT, diiodotyrosine; H₂O₂, hydrogen peroxidase; MIT, monoiodotyrosine; NIS, sodium/iodide symporter; TPO, thyroperoxidase; T3, triiodothyronine; T4, thyroxine.