Abstract

The reset osmostat syndrome, a form of inappropriate antidiuretic hormone secretion (SIADH), occurs when the threshold for antidiuretic hormone secretion is moved downward. There is evidence to suggest a “reset osmostat phenomenon” in normal pregnancies, whereby the average plasma-osmolality is decreased by 5–10 mOsm/kg. We present a case of a non-physiologic reset osmostat in a pregnant patient, thought to be caused by large intracranial arteriovenous malformations and intraventricular hemorrhage. The presence of a reset osmostat should be suspected in any patient with apparent SIADH who has mild hyponatremia that is stable over many days despite variations in sodium and water intake. Therapeutic efforts to raise the serum sodium concentration appear to be unnecessary.