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The article describes a case of Graves' disease treated with methimazole and examines the influence of methimazole-induced alterations of thyroid hormone concentrations during warfarin therapy. A 22-year-old woman presented at our endocrinology outpatient clinic with palpitations, sweating, fatigue, tremors, and diarrhea. She had a pain in her right leg and had difficulty walking. Her thyroid profile was consistent with hyperthyroidism. The patient was treated with warfarin 5 mg once a day for deep vein thrombosis for 2 days. Since a therapeutic range of International Normalized Ratio levels could not be achieved, methimazole was stopped due to drug-drug interaction. Lithium was started instead. A euthyroid state was obtained in 2 weeks together with a therapeutic International Normalized Ratio level. Interactions between warfarin and drugs that alter thyroid hormone concentrations have been reported; however, the extent and significance of the interaction between methimazole and warfarin have been inadequately described. Concomitant therapy with warfarin and antithyroid drugs should be managed by frequent monitoring of both thyroid function and the International Normalized Ratio. Lithium is employed only to provide temporary control of thyrotoxicosis in patients who cannot take thionamide and iodide. The administration of lithium alone or in combination with other drugs is shown to be an effective method of controlling hyperthyroidism when conventional antithyroid drugs show adverse effects or become insufficient. When warfarins are used together with antithyroid medications, adequate anticoagulation may not be obtained due to drug-drug interactions. Lithium can be an alternative drug for antithyroid medication in patients on warfarin therapy. © 2008 Lippincott Williams & Wilkins, Inc.