Hypothyroidism occurs in 10 to 15% of type 2 diabetic patients. Many of these diabetic patients may also be treated with metformin, which is associated with a significant reduction (up to 50%) in the serum levels of TSH. Decreased TSH levels are observed in both untreated diabetic patients and those treated with levothyroxine replacement therapy. TSH lowering develops slowly and is detectable after a few months of treatment. Endocrinologists recommend rechecking thyroid function within 6 to12 months after starting metformin in diabetic patients with concomitant hypothyroidism.
TSH reduction is not associated with reciprocal changes in any other thyroid function test such as Total T4, free T4 or T3. Patients do not exhibit any clinical signs of hyperthyroidism. The mechanism by which metformin lowers TSH level is unknown. Metformin does not effect any thyroid function tests in euthyroid diabetic patients.
C. Cappelli. TSH-Lowering Effect of Metformin in Type 2 Diabetic Patients, Diabetes Care September 2009 vol. 32 no. 9 1589-1590
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