Resistance to thyroid hormone (RTH) is a syndrome caused by a mutation in the carboxyl-terminal domain of the thyroid hormone receptor beta (TRbeta) gene. 3,5,3'-triiodothyroacetic acid (Triac) has been used on an empirical basis to treat RTH but its efficacy is still controversial. In previous studies, we demonstrated that Triac has TR isoform- and TRE-specific effects. In this report, we used five natural RTH mutations of the ligand-binding domain in both TRbeta1 and TRbeta2 isoforms for the evaluation of the effect of T3 and Triac on regulation of transcription and binding affinity. We show that Triac has superior activity on negatively and positively regulated promoters and higher binding affinity than T3 for a majority of TRbeta1 and TRbeta2 mutants. However, the difference of transcriptional activity and binding affinity between both ligands is less for RTH mutants than for wild type receptors. These results suggest that Triac could be a potential treatment for RTH patients.