Polychlorinated biphenyls (PCBs) have been reported to cause a variety of toxic effects. In order to assess the thyroid function after exposure to PCBs and investigate whether PCBs induce autoimmune process in the thyroid gland, we determined the levels of serum thyroid hormones (FT3, FT4, and T4), thyroid-stimulating hormone (TSH), and thyroid peroxidase antibody (TPOAb) in Sprague-Dawley rats treated with a commercial mixture of PCBs, Aroclor 1,254 (PCBs group), or the antithyroid drug, propylthiouracil (PTU group). The histopathology of the thyroid was also examined. Serum FT3, FT4, and T4 concentrations were significantly reduced, while TSH values were dramatically increased in PCBs group and PTU group compared with control rats (p < 0.05). TPOAb levels were significantly elevated in PCBs-treated rats (p < 0.05) but not in PTU group (p > 0.05). In contrast to the controls, treatment with PCBs lead to distinct histopathological changes in the thyroid gland, such as hyperplasia of the epithelia in follicles, colloid content reduction, vascularization, and lymphocytic infiltration in the perifollicular areas, whereas the major changes in the thyroid in PTU-treated rats were follicles shrinkage or collapse and colloid content reduction compatible with induced hypothyroidism. The results indicate that PCBs affect thyroid function via the induction of autoimmunity, which is a mechanism different from the effect of antithyroid drug on the thyroid gland.