Figure 2: Iodine metabolism. When ingested, iodide is trapped and oxidized in the thyroid gland and binds to tyrosine to form iodotyrosines; in Tg, coupling of iodotyrosyl residues forms T4 and T3; the hormone secreted by the gland is transported in the serum and one part of T4 is deiodinated to T3. The hormone plays out its metabolic effects in the cell and is finally deiodinated; iodide is reutilized or excreted in the kidneys. A second cycle occurs inside the thyroid through deiodination of iodotyrosines, giving rise to iodides, some of which are reutilized without leaving the gland.