Almost all patients who developed autoimmune thyroid disease associated with encephalopathy were diagnosed to have Hashimoto's thyroiditis, but a few patients with Graves' disease who developed encephalopathy have been reported. A 36-year-old female with a 10-year history of Graves' disease had experienced three episodes of tonic-clonic seizure. At admission, the patient's status was confused, and she also developed tactile and visual hallucinations. The cranial MRI confirmed white-matter lesion and showed subcortical high signal lesions on T2-weighted images. In EEG record, diffuse slow activity was noticed in both sides. T3 and TSH were decreased, T4 remained normal and thyroid peroxidase antibody (TPO) was evaluated to be more than 2,000 (T4 = 8.4, T3 = 12/9, TSH = 0/14, TPO >2,000). The diagnosis of autoimmune thyroid disease is probable in all patients with signs of encephalopathy with unknown origin, while they have a previous history of thyroid disease.