Steroid responsive encephalopathy associated with autoimmune thyroiditis is a well-recognized complication of autoimmune thyroid disease. However, the characterization of the histopathological features of steroid responsive encephalopathy associated with autoimmune thyroiditis are limited to six cases. Reported pathological features include vasculitis involving venules and arterioles, lymphocytic perivascular cuffs and microglial activation. We report a case of SREAT with biopsy proven (on two occasions) primary CNS demyelination and radiological evidence of steroid responsiveness, identifying primary CNS demyelination as a complication of autoimmune thyroid disease.