AB - Background: Steroid-responsive encephalopathy associated with autoimmune thyroiditis (SREAT), often termed Hashimoto encephalopathy, is a poorly understood and often misdiagnosed entity. Objective: To characterize the clinical, laboratory, and radiologic findings in patients with SREAT to potentially improve recognition of this treatable entity. Design: Retrospective analysis of clinical features and diagnostic test data. Setting: Two affiliated tertiary care referral institutions. Patients: Twenty consecutive (6 male) patients diagnosed as having SREAT from 1995 to 2003. Main Outcome Measures: Clinical features and ancillary test findings associated with SREAT. Results: The median age at disease onset was 56 years (range, 27-84 years). The most frequent clinical features were tremor in 16 (80%), transient aphasia in 16 (80%), myoclonus in 13 (65%), gait ataxia in 13 (65%), seizures in 12 (60%), and sleep abnormalities in 11 (55%). All patients were assigned an alternative misdiagnosis at presentation, most commonly viral encephalitis (n=5), Creutzfeldt-Jakob disease (n=3), or a degenerative dementia (n=4). The most frequent laboratory abnormalities were increased liver enzyme levels in 11, increased serum sensitive thyroid-stimulating hormone levels in 11, and increased erythrocyte sedimentation rate in 5. In only 5 patients (25%) did cerebrospinal fluid abnormalities suggest an inflammatory process. Magnetic resonance imaging abnormalities believed to be related to the encephalopathy were present in 5 patients (26%). Conclusions: The clinical, laboratory, and radiologic findings associated with SREAT are more varied than previously reported. Misdiagnosis at presentation is common. This treatable syndrome should be considered even if the serum sensitive thyroid-stimulating hormone level and erythrocyte sedimentation rate are normal, the cerebrospinal fluid profile does not suggest an inflammatory process, and neuroimaging results are normal. Until the pathophysiologic mechanism of this and other autoimmune encephalopathies is better characterized, we believe that descriptive terms that reflect an association rather than causation are most appropriate for this syndrome.