Tetraiodothyronine 5' deiodinase

Tetraiodothyronine 5' deiodinase is an enzyme. The amino acid selenocysteine is one of its constituent parts.

It creates the active form of triiodothyronine (T3) from thyroxine (T4) and is one of three selenocysteine-containing iodothyronine deiodinases that work in concert to guarantee adequate local (in the peripheral tissues) and systemic (from the thyroid gland) levels of the active thyroid hormone T(3). It is known as D1. It also is capable of deactivating T3 by reduction to T2.

The other two deiodinases are known as D2 and D3. D2 is the primary activator from T4 to T3 (found mostly in the thyroid, brain and testis), while D3 is the primary inactivator from T4 and T3 (found mostly in fetal tissue and placenta).