Thyroid hormone and thyroid hormone receptor (TR) play an essential role in metabolic regulation. However, the role of TR in adipogenesis is not established. We previously reported that TR sumoylation is essential for TR-mediated gene regulation and mutation of either of the two sites in TRα, or any of the three sites in TRβ, reduced TR sumoylation. We transfected TR sumoylation site mutants into human primary preadiocytes, and the mouse 3T3L1 preadipocyte cell line to determine the role of TR sumoylation in adipogenesis. Reduced sumoylation of TRα or TRβ resulted in fewer and smaller lipid droplets and reduced proliferation of preadipocytes. TR sumoylation mutations compared to wild-type TR, results in reduced C/EBPs expression and reduced PPARγ2 mRNA and protein levels. TR sumoylation mutants recruit NCoR and disrupted PPARγ-mediated perilipin 1 (Plin1) gene expression, associated with impaired lipid droplet formation. Expression of NCoRΔID, a mutant NCoR lacking the TR interaction domain, partially "rescued" the delayed adipogenesis and restored Plin1 gene expression and adipogenesis. TR sumoylation site-mutants impaired Wnt/β-catenin signaling pathways and the proliferation of primary human preadipocytes. Expression of the TRβ K146Q sumoylation site mutant down-regulated essential genes required for canonical Wnt signal-mediated proliferation, including Wnt ligands, Fzds, β-catenin, LEF1 and CCND1. Additionally, the TRβ K146Q mutant enhanced the canonical Wnt signaling inhibitor, Dickkopf-related protein 1 (DKK1). Our data demonstrate that TR sumoylation is required for activation of the Wnt canonical signaling pathway during preadipocyte proliferation and enhances PPARγ signaling that promotes differentiation.