Deiodinases

Deiodinase is an enzyme important in the action of thyroid hormones.

Deiodinases are unusual in that the enzyme contains selenium, in the form of an otherwise rare amino acid selenocysteine.

Activation and inactivation
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.

Types
In most vertebrates, there are three types of enzymes that can deiodinate thyroid hormones: