Antagonists have been developed for several nuclear receptors but not for others, including TRs. TR antagonists may have significant clinical utility for treating hormone excess states and other conditions. A structure derived "extension hypothesis" was applied to synthesize a TR antagonist. The principal design feature was to attach an extension group to a TR agonist whose structure would perturb formation of the TR coactivator-binding surface. The compound, 3,5-dibromo-4-(3',5'-diisopropyl-4'-hydroxyphenoxy)benzoic acid, has no (TRalpha) or very weak partial (TRbeta) TR agonist activity and blocks TR binding of T3, formation of the coactivator-binding surface, and both a positive T3 response on a thyroid hormone response element and a negative T3 response on the TSHbeta promoter in cultured cells. The results suggest that 3,5-dibromo-4-(3',5'-diisopropyl-4'-hydroxyphenoxy)benzoic acid is a TR antagonist for thyroid hormone response element-mediated responses, this approach can be used more generally to generate nuclear receptor antagonists, and this compound or analogues may have medical and research utility.