A 45 years-old woman presented with loss of iniciative and memory, imotivated ciy and behaviour disturbance with childish traits, quickly progressive along 6 months until total apathy. An EEG showed periodic activity with bilateral triphasic waves against a flattened background activity suggesting Creutzfeldt-Jacob disease (CJD), but investigation for treatable causes of dementia disclosed hypercalcemia and hypophosphatemia. Further investigation showed a mass at the thyroid region that at surgery was identified as an oxyphilic ceils adenoma. With electrolyte disturbance correction and posterior surgery there was normalization of both EEG and clinical status. Though it was previously reported mental confusion and EEG alteration associated with hyperparathyroidism we do not know of any previous case o>: confusion associated with periodic activity in EEG as in this disease. Hyperparathyroidism should be a differential diagnosis in every «de novo» case of CJD.