Deiodinases are enzymes involved in the activation and deactivation of thyroid hormones by removing iodine atoms from their molecules. They play a key role in regulating the levels of active triiodothyronine (T3) and inactive thyroxine (T4) in body tissues.

There are three types of deiodinases (D1, D2, and D3), each with its own distribution and function. D1 and D2 activate T4, converting it to T3, whereas D3 provides hormone inactivation, protecting tissues from excess thyroid activity. Disorders in the work of these enzymes can affect metabolism and overall hormonal balance.