TSH-LOWERING EFFECT OF METFORMIN IN TYPE 2 DIABETIC PATIENTS: DIFFERENCES BETWEEN EUTHYROID, UNTREATED HYPOTHYROID AND EUTHYROID 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 1 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.