A 53-year-old man was admitted to the hospital medical service after an outpatient evaluation for encephalopathy revealed a serum sodium level of 166 mmol/L. The duration of the patient's hypernatremia was unknown. The patient had been previously healthy but in the past year had experienced progressive fatigue and neurologic disturbances including somnolence, constructional apraxia, cognitive impairment, narcolepsy, cataplexy, and rapid eye movement sleep behavior symptoms. Several months before the current admission, he was noted to be hyponatremic, likely as a consequence of syndrome of inappropriate antidiuretic hormone secretion, and he was treated with water restriction and demeclocycline. In the course of evaluating his symptoms, he was found to have a single lymph node on the right side of his neck that was positive for metastatic squamous cell carcinoma; a tonsillar origin was suspected, but ultimately no primary lesion was identified. Additional imaging evaluation at that time (approximately 2 months before admission) prompted a hypothalamic biopsy that revealed noncaseating granulomas in the absence of infection. In the time leading up to admission, his symptoms continued to progress, with fluctuating hypernatremia and hyponatremia, polydipsia, polyuria, and worsening hypersomnia.The patient's outpatient medications included treatments for neurosarcoidosis (initiated 2 months before the current admission), including prednisone (40 mg daily), methotrexate, cyclosporine, hydroxychloroquine, and infliximab. In addition, he was receiving demeclocycline (600 mg twice daily) for treatment of hyponatremia. He was not on fluid restriction but had unreliable oral intake because of his progressive encephalopathy. When aroused and prompted, he had an intact thirst mechanism and was able to drink.Laboratory evaluation at admission yielded the following (reference ranges provided parenthetically): serum sodium, 166 mmol/L (135-145 mmol/L); serum potassium, 4.0 mmol/L (3.6-5.2 mmol/L); serum glucose, 92 mg/dL (70-140 mg/dL); creatinine, 1.4 mg/dL (0.8-1.3 mg/dL); and serum urea nitrogen, 31 mg/dL (8-24 mg/dL).On examination, the patient was somnolent but arousable to voice (Glasgow Coma Scale score, 13). He was oriented to person but not place or time. His mentation precluded a comprehensive review of systems, but collateral information from his family was positive for progressive inability to perform activities of daily living and unintentional 11.25-kg weight loss.