Metformin is a widely described drug in treatment and prevention of type 2 diabetes. Although its mechanism of action is not well understood, new evidence supports that metformin could affect thyroid profile in patients with type 2 diabetes. Aim of this study was to access the effect of metformin treatment on thyroid axis hormones in diabetic patients with normal thyroid function. Thus, one hundred eight euthyroid subjects were selected for being healthy controls, obese insulin resistant not treated with metformin, and diabetic non-obese or diabetic obese treated with metformin. Circulating hormones were determined by ELISA. Our data show that thyroid function parameters were not significantly different in diabetic subjects treated with metformin compared to healthy controls. However, diabetics showed significantly lower thyrotropin levels than those of obese insulin resistant subjects untreated with metformin. Further, no significant change in free T4 was observed. In conclusions metformin administration may affect thyroid axis in type 2 diabetic patients. Interestingly, it seems to bring some benefits by maintaining plasma thyrotropin levels to subnormal levels in diabetics. However, more investigations are needed to elucidate the exact mechanism at a cellular and molecular level.