A 15-year-old girl is described with encephalopathy associated with Hashimoto thyroiditis which subsequently developed into autoimmune thyrotoxicosis (hashitoxicosis) and distal renal tubular acidosis at age 5 years, pernicious anemia at age 9, and encephalopathy at age 12. Thyroid hormone levels were within the normal ranges at the time of neurologic presentation. Serum IgG concentration and oligoclonal IgG bands in cerebrospinal fluid closely correlated with the severity of neurologic symptoms. Treatment with glucocorticoids improved the level of consciousness but her mental state continued to deteriorate. Repeated cranial computed tomographic scans revealed progressive symmetric atrophy in the gray matter without infarction. These findings suggest that encephalopathy associated with Hashimoto thyroiditis is caused by an antineuronal antibody-mediated reaction.