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Deiodinase is an enzyme important in the action of thyroid hormones. In the tissues, deiodinase converts the prohormone thyroxine to the active
hormone triiodothyronine by the removal of an iodine atom. The major part of thyroxine
deiodination occurs within the target cells.
There are three types of enzymes (in humans ?) that can deiodinate thyroxine. Iodothyronine deiodinase type I (IDI) is commonly found in the liver, kidney, muscle tissue and
thyroid gland, and type II (IDII) in the testis and thyroid. Type III (IDIII) is found in the fetal tissue, brain
matter and the placenta. IDII can only deiodinate the outer ring of the prohormone thyroxine or the
metabolically inactive reverse triiodothyronine. IDIII can only deiodinate the inner ring of thyroxine or triiodothyronine. IDI 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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