Thyroid related autoantibodies have been related to the development of encephalopathy, known as Hashimoto's encephalopathy. However, their relation with the encephalopathy occurring in patients with Graves' disease has not been well established. The case is reported of a 51 year old woman presenting with subacute progressive dementia with evidence of hyperthyroidism. She had Graves' disease associated with high titres of thyroid related autoantibodies. Her encephalopathy was not improved by antithyroid drugs, but promptly responded to corticosteroid treatment, and stabilised with a gradual reduction of thyroid related autoantibody titres. Brain positron emission tomography initially showed a diffuse and multifocal cerebral hypometabolism with subsequent normalisation on her clinical recovery, which was consistent with the acute and reversible cerebral inflammation probably mediated by autoimmune mechanisms.