Iodothyronine deiodinase

Iodothyronine deiodinases (EC1.21.99.4 and EC1.21.99.3) are a subfamily of deiodinase enzymes important in the activation and deactivation of thyroid hormones. Thyroxine (T₄), the precursor of 3,5,3’-triiodothyronine (T₃) is transformed into T₃ by deiodinase activity. T₃, through binding a nuclear thyroid hormone receptor, influences the expression of genes in practically every vertebrate cell. Iodothyronine deiodinases are unusual in that these enzymes contain selenium, in the form of an otherwise rare amino acid selenocysteine.