• Carlo Cappelli, Mario Rotondi, Ilenia Pirola, Barbara Agosti, Elena Gandossi, Umberto Valentini, Elvira De Martino, Antonio Cimino, Luca Chiovato, Enrico Agabiti-Rosei, and Maurizio Castellano.
• Internal Medicine and Endocrinology Unit, Department of Medical and Surgical Sciences, University of Brescia, Brescia, Italy. cappelli@med.unibs.it
• Diabetes Care. 2009 Sep 1;32(9):1589-90.

ObjectiveTo assess the interplay between metformin treatment and thyroid function in type 2 diabetic patients.Research Design And MethodsThe 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.ResultsNo 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.ConclusionsMetformin 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.

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