The molecular genetic events underlying thyroid carcinogenesis are poorly understood. Mice harboring a knock-in dominantly negative mutant thyroid hormone receptor beta (TRbetaPV/PV mouse) spontaneously develop follicular thyroid carcinoma similar to human thyroid cancer. Using this mutant mouse, we tested the hypothesis that the peroxisome proliferator-activated receptor gamma (PPARgamma) could function as a tumor suppressor in thyroid cancer in vivo. Using the offspring from the cross of TRbetaPV/+ and PPARgamma+/- mice, we found that thyroid carcinogenesis progressed significantly faster in TRbetaPV/PV mice with PPARgamma insufficiency from increased cell proliferation and reduced apoptosis. Reduced PPARgamma protein abundance led to the activation of the nuclear factor-kappaB signaling pathway, resulting in the activation of cyclin D1 and repression of critical genes involved in apoptosis. Treatment of TRbetaPV/PV mice with a PPARgamma agonist, rosiglitazone, delayed the progression of thyroid carcinogenesis by decreasing cell proliferation and activation of apoptosis. These results suggest that PPARgamma is a critical modifier in thyroid carcinogenesis and could be tested as a therapeutic target in thyroid follicular carcinoma.