Thyroid hormone receptors (TRs) are ligand dependent members of the steroid/retinoic acid superfamily of transcription factors. The two genes encoding TRs identified to date, TR alpha and TR beta, have been mapped to human chromosomes 17 and 3, respectively. TRs bind to thyroid hormone response elements (TREs) with half site binding motifs in the orientation of palindromes, direct repeats, or inverted palindromes. The affinities of binding are both variable and influenced differentially by 3,5,3 triiodo L thyronine (T3). Thyroid hormones, through their interaction with the thyroid receptor (TR), effect metabolic processes, growth and development in many tissues by regulating the expression of genes for growth hormone, malic enzyme and several hepatic proteins.