Studies have shown that there is an association between type 2 diabetes mellitus and thyroid dysfunction although, the mechanism is poorly understood. This study assessed the thyroid function in euthyroid T2DM patients on metformin and its possible relationship with duration of metformin use. Thirty-five euthyroid T2DM patients who have been on metformin for ≥5 years, 10 euthyroid T2DM patients who have been on non-metformin oral antidiabeteic drugs for ≥5 years and 19 apparently healthy euthyroid controls were individuals served as controls. Serum levels of thyroid stimulating hormone (TSH), free thyroxine (FT4) and free triiodothyronine (FT3) were determined using ELISA. Levels of TSH, FT3 and FT4 were similar in patients on metformin, non-metformin and the controls. The median level of TSH was insignificantly lower in patients who have taken metformin for ≥10 years compared with patients who have taken metformin for <10 years. Mean levels of FT3 and FT4 were similar between the 2 groups. Metformin use does not alter thyroid function in euthyroid T2DM patients. Also, prolonged use of metformin does not have significant effect on the levels of thyroid stimulating hormone and thyroid hormones in euthyroid T2DM patients.
