We present a 58-year-old female with gradual cognitive decline and gait instability over 6 months. Her motor examination was notable for myoclonus, brisk reflexes with flexor plantar responses, and a cautious gait without ataxia. Cognitive testing revealed mildly impaired attention, but profoundly impaired calculation, judgment and visual memory. There were no manifestations of autoimmune thyroid disease. Routine laboratory analysis was unrevealing. Cerebrospinal fluid analysis was remarkable only for an elevated protein of 0.64 g/L (normal <0.45 g/L). Electroencephalography demonstrated intermittent bitemporal slowing. Brain MRI with gadolinium demonstrated extensive bilateral subcortical and periventricular white matter T2-weighted and hyperintensity on fluid attenuated inversion recovery MRI. Elevated anti-thyroperoxidase antibody of 8.07 IU/mL (<5.61 IU/mL) and thyroglobin antibody of 9.85 IU/mL (<4.11 IU/mL) were found and steroid responsive encephalopathy associated with autoimmune thyroiditis was diagnosed. Methylprednisolone (1 g daily for 3 days) resulted in dramatic improvement in cognition and mobility, which remained on follow-up.