To investigate chronic effects of polychlorinated biphenyls (PCBs) and polychlorinated dibenzofurans (PCDFs) on thyroid functions, serum concentrations of thyroid stimulating hormone (TSH), triiodothyronine (T_3), thyroxine (T_4) and free T_4 were studied in serum of 197 patients with Yusho and 42 control subjects in 2015. Serum levels of T_3 were decreased in patients with Yusho compared with controls. A significant relationship between concentrations of PCB in the blood and serum levels of T_3 was observed, while there was no correlation between blood PCB concentrations and TSH, T_4 or free T_4 levels. Serum levels of T_3 were significantly decreased in the subjects with high PCB concentrations compared with those with low PCB concentrations. Furthermore, serum levels of T_3 were significantly decreased in the subjects with high 2,3,4,7,8-pentachlorodibenzofuran (PeCDF) concentrations compared with those with low 2,3,4,7,8-PeCDF concentrations. We conclude that serum T_3 levels decrease in patients with Yusho, and it may be associated with PCBs and 2,3,4,7,8-PeCDF in the blood.