Objective To evaluate the effects of metformin on thyrotropin(TSH)levels. Methods From the patients with type 2 diabetes mellitus or metabolic syndrome, 48 patients with primary hypothyroidism were enrolled and grouped. 17 patients were treated only with metformin(group A), 19 patients with metformin and stable L-T4substitution(group B), and the remaining 12 patients with antidiabetic drugs(other than metformin)and L-T4(group C). Meanwhile, 20 euthyroid patients with other thyroid abnormalities(group D)and 30 patients without thyroid diseases(group E)served as control. TSH, FT3, FT4, TT3, TT4, and blood glucose were determined regularly in all these subjects. Results After administration of metformin for 12 months, serum TSH were decreased in group A [(5.05±1.07 vs 2.61±0.91)mU/L, P＜0.01] and group B [(2.67±1.03 vs 1.35±0.74)mU/L, P＜0.01]. No difference was found in FT3and FT4in both groups. TSH levels were raised from(1.30±0.71)to(2.58±1.02)mU/L(P＜0.01)within 8～12 weeks in 13 out of 15 patients after metformin withdrawal. Serum TSH and thyroid hormones in the other 3 groups were not significantly changed. Conclusion Administration of metformin may lead to reduction of serum TSH level.