Context It has been reported that metformin might modify thyroid hormone economy. In two retrospective studies, initiation of treatment with metformin caused suppression of TSH to subnormal levels. Objective To prospectively evaluate if administration of metformin to obese, diabetic patients with primary hypothyroidism on stable thyroxine replacement doses modifies TSH levels. Patients and methods Eight obese, diabetic postmenopausal women with primary hypothyroidism participated in the study. They received 1,700 mg of metformin daily for 3 months. Weight, TSH, free T4, and free T3 levels were measured at baseline, 3 months after metformin initiation and 3 months after its withdrawal. Results After 3 months of on metformin, mean TSH was significantly lower than basal TSH (3.11 +/- 0.50 microUI/ml vs. 1.18 +/- 0.36 microUI/ml; P = 0.01). Mean TSH 3 months after metformin withdrawal was 2.21 +/- 0.37 microUI/ml, significantly higher than TSH after metformin (P = 0.05), but not different from basal TSH. Mean fT4 level increased during metformin administration (basal fT4: 1.23 +/- 0.06 ng/dl, fT4 after metformin: 1.32 +/- 0.04 ng/dl; P = ns), and decreased after its withdrawal (fT4 3 months after metformin withdrawal: 1.15 +/- 0.05 ng/dl; vs. 3 months after metformin, P = 0.04; vs. basal; P = ns). Conclusions In obese, diabetic patients with primary hypothyroidism on thyroxine replacement treatment, short-term metformin administration is associated with a significant fall in TSH.