A 79-year-old female was admitted to our hospital because of unconsciousness and convulsion following mental deterioration. On admission, she exhibited myoclonic movement of the right side of the face and right fingers in addition to rigospasticity and tremors in the right arm and leg. Laboratory tests revealed hyperthyroidism with an increased anti-TSH-R antibody titer. In addition, an echogram indicated excessive blood flow at the thyroid; hence, the patient was diagnosed with Basedow's disease. Interestingly, the tests also revealed increased titer of anti-TPO antibody, anti-Tg antibody, and anti-NH2 terminal of alpha-enolase (NAE) antibody; in addition, an EEG showed abnormal findings potentially indicating periodic synchronous discharge. Brain MRI showed cerebral atrophy, and brain 99mTc-ECD-SPECT images demonstrated an overall decrease in the accumulation of 99mTc in the cerebrum. The abovementioned findings are common to patients with Creutzfeldt-Jakob disease (CJD). We initiated treatment for hyperthyroidism with thiamazole and lugol, but this did not regain consciousness. Because she had anti-thyroid antibody was observed, we considered a differential diagnosis of Hashimoto's encephalopathy and, in fact, methylprednisolone pulse therapy alleviated her symptoms and normalized the EEG findings. The condition in this case clinically mimicked CJD; therefore, the differentiated diagnosis is important because Hashimoto's encephalopathy is treatable disease.