Thyroid hormone (T3) and peroxisome proliferators have overlapping metabolic effects in the maintenance of lipid homeostasis. Their actions are mediated by their respective receptors: thyroid hormone receptors (TR) and peroxisome proliferator-activated receptors (PPAR). We recently found that a dominantly negative TRβ mutant (PV) that causes a genetic disease, resistance to thyroid hormone, acts to repress the ligand (troglitazone)-mediated transcriptional activity of PPARγ in cultured thyroid cells. This finding suggests that TRβ mutants could crosstalk with PPARγ-signaling pathways. The present study explored the molecular mechanisms by which PV represses the PPARγ transcriptional activity. Gel-shift assays show that the PV, similar to wild-type TRβ, bound to the peroxisome proliferator response element (PPRE) as homodimers and heterodimers with PPARγ or the retinoid X receptor (RXR), thereby competing with PPARγ for binding to PPRE and for sequestering RXR. Association of PPRE-bound PV with corepressors [e.g., nuclear receptor corepressor (NCoR)] that led to transcriptional repression was independent of T3 and troglitazone. Chromatin immunoprecipitation assay further demonstrated that, despite the presence of ligands, NCoR was recruited to PPRE-bound PV on a PPARγ-target gene, the lipoprotein lipase, in vivo, suggesting the dominant action of PV on PPARγ-mediated transcriptional activity. Thus, the dominant negative action of PV is not limited on the wild-type TRs. The findings that TRβ mutants affect PPARγ functions through dominant negative action provide insights into the molecular mechanisms by which TR regulates the PPARγ-target genes involved in metabolic pathways, lipid homeostasis, and carcinogenesis.