  iodothyronine deiodinases ( and ) are a subfamily of deiodinase enzymes important in the activation and deactivation of thyroid hormones. thyroxine (t4), the precursor of 3,5,3â-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. these enzymes are not to be confused with the iodotyrosine deiodinases that are also deiodinases, but not members of the iodothyronine family. the iodotyrosine deiodinases (unlike the iodothyronine deiodinases) do not use selenocysteine or selenium. the iodotryrosine enzymes work on iodinated single tyrosine residue molecules to scavenge iodine, and do not use as substrates the double-tyrosine residue molecules of the various iodothyronines. == activation and inactivation == in tissues, deiodinases can either activate or inactivate thyroid hormones: * activation occurs by conversion of the prohormone thyroxine (t4) to the act
EC_1.97.1 Genes_on_chromosome_1 Genes_on_chromosome_14 Selenoproteins