The Effects of Metformin on Thyroid Function among Patients with Subclinical Hypothyroidism and Coexisting Metabolic Syndrome

Background: The interference of metformin with thyroid function has been recently reported in several studies. In the present research, we assessed the effect of metformin on thyroid function tests in patients with subclinical hypothyroidism associated with metabolic syndrome. Method: In a double-blind clinical trial, 60 patients were selected among who referred to outpatient endocrine clinics and met the inclusion and exclusion criteria were considered for the study. Inclusion criteria were the presence of metabolic syndrome and subclinical hypothyroidism (2.5<TSH<10 mIU/l). Screened patients used no medications interfering with TSH level. Pregnancy, GFR less than 50 ml/min and intolerance of metformin were also considered as exclusion criteria. Patients were divided into case and a placebo groups. In the case group, patients received 1000 mg/day of metformin for 12 weeks. Anthropometry, liver and thyroid function tests, and lipid profile were evaluated before and after the intervention. Results: A total of 44 patients fully participated for the whole study period. The mean age was 44±14 years and 15 patients (34.1%) were male. The mean TSH and FT4 levels before and after intervention were 5.8±2.15, 4.8±2.7 and 1.10±0.19, 1.14±0.26, respectively. The positive TPO-ab was seen among 18 (40.9%) patients. TSH (4.12±2.07, p=0.013) and FT4 (1.18±0.23, p=0.007) levels were decreased and increased, respectively, compared to the placebo group. Furthermore, a reduction in metabolic element was observed. Conclusion: Our data showed that metformin reduced the TSH level in subclinical hypothyroid patients, especially in patients with TSH baseline level higher than 5mIU/l and TPO-ab positive patients.