A patient following head injury was admitted in Intensive care ward with signs of raised intracranial pressure. He was put on ventilator and staed on Intravenous fluids and diuretics. Twenty four hours later his urine output was 3.5 liters, serum sodium 156 mEq/1 and serum osmolality of 316 mOsm/kg. The most likely diagnosis based on these parameters is-