Thyroid hormone (TH) modulates metabolic efficiency by controlling the coupling of mitochondrial oxidative phosphorylation. However, its uncoupling mode of action is still enigmatic. Treatment of Jurkat or GH3 cells by triiodothyronine (T₃) is reported here to result in limited, cyclosporin A-sensitive mitochondrial depolarization, conforming to low-conductance gating of the mitochondrial transition pore (MTP). MTP protein components induced by T₃ treatment were verified in T₃-treated and hypothyroid rat liver as well as in Jurkat cells. T₃ treatment resulted in increase in mitochondrial bax and bak together with decreased mitochondrial bcl2. T₃-induced mitochondrial depolarization was aborted by over expression of bcl2. In contrast to Bax-Bcl₂ family proteins, some other MTP components were either not induced by T₃ (e.g. voltage-dependent anion channel (VDAC)) or were induced but not involved in cyclosporin A-sensitive MTP gating (e.g. cyclophilin D, adenine nucleotide translocase2) Hence, TH-induced mitochondrial uncoupling may be ascribed to low-conductance MTP gating mediated by TH-induced increase in mitochondrial proapoptotic combined with decrease in mitochondrial antiapoptotic proteins of the Bax-Bcl2 family.