TSH-Lowering Effect of Metformin in Type 2 Diabetic Patients Differences between euthyrold, untreated hypothyroid, and euthyrold on L-T4 therapy patients

OBJECTIVE - To assess the interplay between metformin treatment and thyroid function in type 2 diabetic patients. RESEARCH DESIGN AND METHODS - The acute and long-term effects Of metformin on thyroid axis hormones were assessed in diabetic patients with primary hypothyroidism who were either untreated or treated With levothyroxine (L-T4), as well as in diabetic patients With normal thyroid function. RESULTS- No acute changes were found in 11 patients with treated hypothyroidism. After I year of metformin administration, a significant thyrotropin (TSH) decrease (P < 0.001) was observed in diabetic subjects with hypothyroidism who were either treated (n 29; from 2.37 +/- 1.17 to 1.41 +/- 1.21 mIU/l) or untreated (n = 18; 4.5 +/- 0.37 vs. 2.93 +/- 1.48) with L-T4, but not in 54 euthyroid Subjects. No significant change in free T4 (FT4) was observed in any group. CONCLUSIONS - Metformin administration influences TSH without change of FT4 in patients with type 2 diabetes and concomitant hypothyroidism. The need for reevaluation of thyroid function in these patients within 6-12 months after starting metformin is indicated.