To determine whether low dose ketoconazole (KTZ) has antithyroid action, we studied thyroid function tests in the 13 rats treated with KTZ (20 mg/kg twice daily) for thirty days. Serum triiodothyronine and thyroxine levels were decreased (P<0.05) and serum thyroid-stimulating hormone levels were mildly increased (P>0.05) at the end of treatment. Histopathological analysis of the thyroid glands demonstrated an increase in cylindrical cells in study group, but the epithelial cells were mainly cubical in control group. These findings showed that low dose KTZ had antithyroid effect in rats. The responsible mechanisms may be direct effect of the drug on thyroid gland.