Impairment of prednisolone disposition in patients with Graves' disease taking methimazole

This study was undertaken to determine the effect of methimazole on the pharmacokinetics of iv prednisolone in patients with Graves' disease. Twenty women were studied, including eight with severe infiltrative ophthalmopathy who had taken methimazole and T4 for at least 4 months, six with severe infiltrative ophthalmopathy who had undergone subtotal thyroidectomy and, therefore, required no antithyroid treatment, and six age-matched normal women. All were euthyroid. Each women received 0.54 mg/kg prednisolone as an iv bolus dose. Plasma total and unbound prednisolone concentrations were measured at multiple times during a 10-h study period by high pressure liquid chromatography and equilibrium dialysis. The clearance of both total and unbound prednisolone was increased significantly in the women receiving methimazole therapy compared to values in both control groups. The volume of distribution at steady state was similar in all groups. These results suggest that patients receiving methimazole have enhanced prednisolone metabolism and, therefore, they may require higher prednisolone doses.