Types
In most vertebrates, there are three types of enzymes that can deiodinate thyroid hormones:
| Type | Location | Function |
| type I (DI) | is commonly found in the liver and kidney | DI can deiodinate both rings |
| type II deiodinase (DII) | is found in the heart, skeletal muscle, CNS, fat, thyroid, and pituitary | DII can only deiodinate the outer ring of the prohormone thyroxine and is the major activating enzyme (the already inactive reverse triiodothyronine is also degraded further by DII) |
| type III deiodinase (DIII) | found in the fetal tissue and the placenta | DIII can only deiodinate the inner ring of thyroxine or triiodothyronine and is the major inactivating enzyme |
The following is a list of the three human iodothyronine deiodinases:
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Read more about this topic: Iodothyronine Deiodinase
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