The discovery that there is tight linkage between the affected family members of resistance to thyroid hormone (RTH) and the thyroid hormone receptor β (TRβ) gene locus ushered in an exciting opportunity to study the molecular basis of RTH (1). Indeed, shortly thereafter, a Pro453His mutation was identified in the TRβ gene of one kindred (2) and a Gly345rg mutation in another (3), establishing that mutations of the TRβ gene cause RTH. To date, about 100 different mutations in the TRβ gene have been reported in more than 300 families (4).