Effect of metformin on thyroid stimulating hormone and thyroid volume in patients with prediabetes: A randomized placebo-controlled clinical trial

Background: The people with prediabetes have insulin resistance (IR). IR may affect thyroid function, size and nodules. We investigated the effects of metformin on the thyroid gland in prediabetic people. Materials and Methods: In a randomized, double-blind placebo-control clinical trial, 89 people with prediabetes, aged 18-65 years were studied for 3 months. They were divided into two, metformin (n = 43) and placebo (n = 46) treated groups. Serum thyroid stimulating hormone (TSH) was measured and thyroid nodules and volume was studied by ultrasonography. The data were compared between and within groups, before and after the study. Results: Mean of the baseline characteristics in metformin and placebo-treated groups had no statistically significant difference. At the end of the study, serum TSH was not significantly different between the two groups. However, if the TSH range was divided into two low normal (0.3-2.5 μU/ml) and high-normal (2.6-5.5 μU/ml) ranges, significant decrease was observed in metformin-treated group with a high-normal basal serum TSH (P = 0.01). Thyroid volume did not change in metformin-treated group. However, in placebo-treated group, the thyroid was enlarged (P = 0.03). In 53.9% of participants, thyroid nodule was observed. There was just a decrease in the volume of small solid (not mixed) nodules from median of 0.07 ml to 0.04 ml in metformin-treated group (P = 0.01). Conclusion: In prediabetic people, metformin decreases serum TSH, only, in those people with TSH >2.5 μU/ml and reduces the size of small solid thyroid nodules. It also prevents an increase in the thyroid volume.