Hashimoto encephalopathy (HE) is a term used to describe an encephalopathy of presumed autoimmune origin characterized by high titers of antithyroid peroxidase antibodies, an essential feature of this disorder. Pertinent laboratory findings include an elevated serum level of antithyroid peroxidase antibody (TPOAb) and/or antithyroglobulin antibody (TgAb) and negative autoimmune markers of encephalitis in cerebrospinal fluid (CSF). The analysis of CSF may show increased cellularity due to lymphocytic pleocytosis. However, there is no clear relationship between the severity of neurologic symptoms and the type or serum concentration of antibodies. Approximately two-thirds of patients of HE will experience seizures. Patients can present with a recurrence/remission or a diffuse, progressive pattern characterized by slowly progressive cognitive impairment with dementia, confusion, or hallucinations. We present a rare case of Hashimoto's encephalopathy associated with hyperthyroidism in a 61-year-old man evaluated for left facial droop and right upper extremity weakness, successfully treated with corticosteroids and methimazole. We conclude that the diagnosis of HE should be suspected early and be on the differential in patients who present with encephalopathy, which workup shows positive anti-thyroid antibodies and CSF with lymphocytosis. Treatment for meningitis or other life-threatening conditions should never be postponed, as HE is a diagnosis of exclusion that usually requires an extensive laboratory and diagnostic workup. Early diagnosis has an excellent outcome once treatment is started as it is a steroid-responsive encephalopathy associated with autoimmune thyroiditis. Presentation: Saturday, June 11, 2022 1:00 p.m. - 3:00 p.m.