A 49-year-old woman (previous history of childhood asthma, no medication) presented to the emergency department with nausea and vomiting that had occurred for 5 days and slurred speech for 1 day prior to presentation. The patient denied use of alcohol and illicit drugs. Physical examination revealed her blood pressure to be 125/70 mmHg; she had no postural drop and had a regular pulse of 72 beats/min. She had no fever and no signs of contracted extracellular fluid volume. Results of further physical and neurological examination were unremarkable and revealed no goiter, pigmentation, or vitiligo. Her laboratory results are shown in Table 1⇓ . Additional diagnostic tests included chest x-ray, abdominal ultrasound, and brain computed tomography, none of which revealed abnormalities. The syndrome of inappropriate antidiuretic hormone secretion (SIADH)1 was suspected. However, fluid restriction (500 mL/day) did not lead to increased serum sodium.