Thyroid hormone regulates the balance between lipolysis and lipogenesis. We previously reported that male mice with a dominant-negative P398H mutation introduced into the TR gene have visceral obesity, hyperleptinemia and reduced catecholamine-stimulated lipolysis in white adipose tissue. Based on our observation of hepatic steatosis in the TR P398H male mice, we used in vitro and in vivo models to investigate the influence of the TR P398H mutant on PPAR signaling. wild-type (wt) TR and the P398H mutant significantly reduced PPAR-mediated transcription in transient transfection assays. Triiodothyronine (T3) reversed the inhibition of PPAR action by wt TR, but not the P398H mutant. Chromatin immunoprecipitation (ChIP) assays demonstrated that the P398H mutant reduces PPAR binding to PPREs. In gel shift assays the P398H mutant directly bound the PPRE and inhibited PPAR binding, which was not reversed by addition of RXR. The TR R384C and PV dominant-negative mutants are not associated in vivo with a metabolic phenotype, and had reduced (PV), or absent (R384C), PPAR-inhibition, compared with P398H. The metabolic phenotype of the P398H mutant mice is due, in part, to unique properties of the P398H mutant receptor interfering with PPAR signaling. The P398H mutant is a potential probe to characterize the physiological role of TR/PPAR interactions.