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Deiodinase () is an enzyme important in the action of thyroid hormones. In the tissues, deiodinases can either activate or inactivate thyroid hormones. Activation occurs by conversion of the prohormone thyroxine (T4) to the active hormone triiodothyronine (T3) through the removal of an iodine atom on the outer ring. Inactivation of thyroid hormones occurs by removal of an iodine atom on the inner ring which converts thyroxine to the inactive reverse triiodothyronine (rT3), or which converts the active triiodothyronine to the inactive diiodothyronine (T2). The major part of thyroxine deiodination occurs within the target cells. In most vertebrates, there are three types of enzymes that can deiodinate thyroid hormones. Iodothyronine deiodinase type I (DI) is commonly found in the liver, kidney, muscle tissue and thyroid gland, type II deiodinase (DII) mostly in the brain but also in the testis and thyroid and type III deiodinase (DIII) is found in the fetal tissue and the placenta. DII can only deiodinate the outer ring of the prohormone thyroxine (or the metabolically inactive reverse triiodothyronine) and is the major activating enzyme. DIII can only deiodinate the inner ring of thyroxine or triiodothyronine and is the major inactivating enzyme. DI can deiodinate both rings. Deiodinases are unusual in that the enzyme contains selenium, in the form of an otherwise rare amino acid selenocysteine.
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