iodidedependent

Iodide-dependent, or iodide-dependent, is an adjective used to describe enzymes, pathways, or biological processes that require iodide (I−) as a substrate, cofactor, or essential reactant. The term is most commonly applied in the context of thyroid hormone biosynthesis in vertebrates, where iodide is indispensable for producing thyroid hormones.

In the thyroid, iodide-dependent processes begin with the uptake of iodide from the bloodstream into thyroid follicular cells via the sodium-iodide symporter (NIS). Thyroid peroxidase (TPO) then oxidizes iodide to reactive iodine species, enabling organification, where iodide is covalently attached to tyrosine residues on thyroglobulin to form monoiodotyrosine (MIT) and diiodotyrosine (DIT). Coupling of MIT and DIT molecules yields the principal hormones thyroxine (T4) and triiodothyronine (T3). Stored thyroglobulin serves as a reservoir, and hormonal release occurs after proteolysis of thyroglobulin and subsequent secretion into circulation. The overall process is iodide-dependent because iodide is the substrate that ultimately leads to hormone production.

Beyond thyroid hormone synthesis, iodide-dependent enzymes such as lactoperoxidase and certain peroxidases utilize iodide to generate reactive iodine species that mediate antimicrobial activity or catalyze iodination reactions. In these contexts, iodide serves as a substrate for enzyme-catalyzed reactions rather than as a structural cofactor.

Clinical and nutritional relevance centers on iodine sufficiency. Inadequate iodide intake can impair thyroid hormone production, causing hypothyroidism and goiter, particularly in developing regions. Excess iodide can also disrupt thyroid function in some individuals. Iodide status is routinely addressed through dietary iodization programs and monitoring of thyroid health. See also iodine, thyroid hormones, thyroid peroxidase, and Wolff-Chaikoff effect.