Although blood thyroid hormone levels are useful for detection of antithyroid chemicals in rodent toxicity studies, they are highly variable depending on conditions of blood sampling, so the development of more appropriate method is still expected. Our recent study showed that in rats treated with thyroid peroxidase inhibitors for 28 days, histopathological and immunohistochemical analyses can be more efficient indicator than blood hormone levels. Here, we administered promoters of thyroid hormone metabolism, phenobarbital sodium salt (PB) and nicardipine hydrochloride (NCD), to six-week-old male and female SD rats (5 rats/group) at doses of 10, 30, 100 mg/kg and 15, 50, 150 mg/kg, respectively, for 28 days. Decreased serum T4 and increased TSH were observed in males of 100 mg/kg PB and 150 mg/kg NCD and in both sexes of 150 mg/kg NCD, respectively, and thyroid weights were increased in these groups. Histopathological analysis revealed that hypertrophy of thyroid follicular cells was found in both sexes of >30 mg/kg PB and >50 mg/kg NCD groups. In immunohistochemistry, TSH-positive area in the anterior pituitary gland was significantly increased in the groups with increased serum TSH levels, and was considered useful for detection of antithyroid chemicals. Histopathological findings in the thyroid were observed from lower doses without significant changes in serum T4 and TSH, which was consistent with that obtained using thyroid peroxidase inhibitors, suggesting that they can be more sensitive parameters for detecting antithyroid effects by promoting hormone metabolism.