Evaluation of the effect of metformin therapy on tsh serum levels in diabetic patients

In the last few years, a great interest has aroused in the effects of metformin on thyroid function in diabetic patients. This study aims to evaluate the effect of metformin on serum thyrotropin (TSH) levels in euthyroid type 2 diabetic patients. The study included forty euthyroid diabetic patients, aged between (20–70) years, randomly selected from the outpatient section of endocrinology departments who started metformin treatment at recruitment. Serum TSH levels were measured before and after 6-months of metformin treatment using immune chemiluminescent assay. After 6 months of metformin treatment, mean TSH levels were insignificantly lower than basal TSH levels (p value > 0.05). We divided the study group according to basal TSH levels into two groups, and we found that TSH levels after six months of treatment were significantly higher than basal levels in group 1 (basal TSH < 1.8μIU/ml) but in group 2 (basal TSH > 1.8μIU/ml), TSH levels after six months of treatment were significantly lower than basal levels. In conclusion, our data strengthen the previous observation that metformin treatment does not produce a significant change in TSH levels in euthyroid patients. We also found that metformin may have a buffer effect on TSH levels as its effect differs according to basal levels. This effect may cause some complications in the diagnosis of thyroid dysfunction especially if clinicians rely only on TSH measurement in diagnosis. In conclusion, we should take into account this effect of metformin when we study thyroid function.