Deiodinase (or iodide peroxidase or "Monodeiodinase") is a peroxidase enzyme that is involved in the activation or deactivation of thyroid hormones.

Types of deiodinases include:

Iodotyrosine deiodinase contributes to breakdown of thyroid hormones. It releases iodine, for renewed use, from iodinated tyrosines resulting from catabolism of iodothyronines. Iodotyrosine deiodinase employs a flavin mononucleotide cofactor and belongs to the NADH oxidase/flavin reductase superfamily.

Iodothyronine deiodinases catalyze release of iodine directly from the thyronine hormones. They are selenocysteine-dependent membrane proteins with a catalytic domain resembling Peroxiredoxins (Prx). Three related isoforms, deiodinase type I, II, and III, contribute to activation and inactivation of the initially released hormone precursor T₄ (thyroxine) into T₃ (triiodothyronine) or rT₃ (reverse triiodothyronine) in target cells. The enzymes catalyze a reductive elimination of iodine (the different isoforms attack different thyronine positions), thereby oxidizing themselves similar to Prx, followed by a reductive recycling of the enzyme.

In starvation, deiodinase (to be specific, Deiodinase I) is inhibited thus lowering basal metabolic rate. However, in the brain, heart, skeletal muscle and thyroid, this is not so, as these organs must maintain homeostasis (skeletal muscle through shivering can increase temperature). This is achieved by the latter organs expressing deiodinase II rather than Deiodinase I as in most peripheral tissues.

Selenium in iodotyrosine deiodinase, as selenocysteine, plays a crucial role in determining the free circulating levels of T₃. Selenium deficiency can have implications in fall of T₃ levels.

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