Relationship between serum thyrotropin concentrations and metformin therapy in euthyroid patients with type 2 diabetes

Aim A thyrotropin(TSH)-lowering effect of metformin therapy has been recently reported in patients with type 2 diabetes (T2D) and hypothyroidism. We aimed to evaluate the interplay between metformin therapy and serum TSH concentrations in a group of patients with T2D and normal thyroid function. Patients and methods Eight hundred and twenty-eight euthyroid patients with T2D (53% women, mean age 65 center dot 9years, median duration of diabetes 10years) were retrospectively evaluated. There were 250 patients on metformin treatment (30 center dot 2%). Serum concentrations of TSH were measured in all subjects. Results Patients on metformin treatment exhibited significantly higher TSH levels [1 center dot 63 (1 center dot 112 center dot 24)mU/l] than those found in patients without metformin [1 center dot 40 (1 center dot 012 center dot 24)mU/l, P=0 center dot 009]. We found no significant differences in TSH levels in patients who were on therapy with other oral antidiabetics, antihypertensive drugs or hypolipidemic agents in relation to subjects not taking these drugs. Serum TSH was significantly related to gender, body mass index, hyperlipidaemia and the presence of goitre and diabetic macroangiopathy. In multiple regression analysis with TSH as dependent variable, goitre was negatively related to TSH values. Metformin therapy was a nonsignificant variable in this model. Conclusion In summary, this is the first survey analysing the relationship between metformin and thyroid function in a large cohort of patients with diabetes. Our data do not support the presence of an independent and significant relationship between TSH values and metformin treatment in euthyroid patients with T2D.