We report on a 35-year-old woman who complained of myasthenia in ocular, bulbar, and limb muscles, but who was negative for antibodies against acetylcholine receptor, muscle-specific kinase, or low-density lipoprotein receptor-related protein 4, accompanied by the suppression of thyroid-stimulating hormone with elevated free T3 and free T4. Administration of edrophonium significantly ameliorated blepharoptosis, and electromyography revealed 13.9% waning after 3 Hz repetitive stimulation in the left accessory nerve. Thus, she was suspected of having triple seronegative myasthenia gravis or thyrotoxic myasthenia. She was remitted after the resection of her enlarged thymus with an elevated uptake in fluorine-18 fluorodeoxyglucose positron emission tomography, suggesting an unknown autoimmune target that escaped detection by current autoantibody screens. Consequently, thymectomy may still be effective in patients with seronegative MG plus hyperthyroidism.