A 56-year-old woman presented with an acute confusional state and moderate global aphasia. Thyroperoxidase antibody level was elevated (3,890 IU/mL) and SREAT was diagnosed. MRI findings were normal. Cerebrospinal fluid examination revealed only a mildly increased protein. The initial electroencephalogram EEG showed slowing and markedly decreased amplitude over the left hemisphere and left temporal sharp waves. An EEG performed after treatment with intravenous steroids showed a significant improvement of the background slowing, which correlated with clinical improvement. One week later, the patient had an episode of forced head-turning and fencing posture to the right. The EEG shortly afterward showed slowing and a decreased amplitude over the right hemisphere. Continuous EEG monitoring was performed at the time of steroid treatment. Again, there was a significant improvement of the EEG after this treatment that correlated with the clinical condition. SREAT is characterized by fluctuations in mental status and variable EEG findings. These patients often show an excellent clinical improvement to immunosuppressive therapy, including corticosteroids. This case report documents the dramatic clinical and EEG improvement with steroid therapy.