Normal diluting capacity in hyponatremic patients. Reset osmostat or a variant of the syndrome of inappropriate antidiuretic hormone secretion

Four patients with chronic illnesses and stable hyponatremia and plasma hypotonicity had normal urinary diluting capacity, with excretion of > 80% of a standard water load (20 ml/kg) within 4 hours and maintenance of a urine osmolality < 100 mosmol/kg, during sustained water diuresis. Administration of a chronic salt load did not correct the hyponatremia. However, it was stabilized after treatment of the underlying medical condition. These subjects may represent a true resetting of the osmostat or a variant of the syndrome of inappropriate antidiuretic hormone secretion.