Thyroid hormone receptors (TRs) play a major role in animal physiology. TRs are important and very interesting regulators of diverse aspects, including brain development, hearing, bone growth, morphogenesis, metabolism, intestine, and heart rate in vertebrates (Fig. 1). Aberrant functions of TRs induce tremendous defects in these pathways. For example, the human disease of Resistance to Thyroid Hormone (RTH) (see Chapter 8 by Yoh and Privalsky) is a genetically autosomal dominant inherited syndrome that is caused by mutations in the gene encoding the TRβ. The role of the ligand of TRs, the thyroid hormone, is to modulate the activity and functionality of TRs
