We report the clinical laboratory, electroencephalography (EEG), magnetic resonance imaging (MRI) and single photon emission computerized tomography (SPECT) findings in a 15 year-old euthyroid girl with autoimmune thyroiditis and encephalopathy. She had stupor, coma and generalized tonic clonic seizure preceded by tremor and myoclonus with a previous misdiagnosis of epilepsy and encephalitis. Response to steroid after the 3rd relapse was excellent. Another four children in the literature are also discussed.