Deiodinase is an enzyme that plays a crucial role in the metabolism of thyroid hormones. It helps convert the inactive form of thyroid hormone, thyroxine (T4), into the active form, triiodothyronine (T3). This conversion is essential for regulating various physiological processes, including metabolism, growth, and development. There are three main types of deiodinases: D1, D2, and D3, each with specific functions and locations in the body. D1 and D2 primarily activate T4 to T3, while D3 inactivates T4 and T3. The balance of these enzymes is vital for maintaining proper thyroid hormone levels and overall health.