Illustration of the active site of iodothyronine deiodinase enzyme. Deiodinase enzymes are important in the function of thyroid hormones in the body, producing thyroxine from a precursor hormone and deactivating thyroxine by removal of iodine. Disruption of the function of iodothyronine deiodinase can lead to abnormalities in development and hypothyroid conditions. Iodothyronine deiodinase is one of a small number of proteins which contain selenium in the form of the rare amino acid selenocysteine.