A 75-year-old woman presented with a complaint of a headache and mild confusion. The patient had a history of hypertension, which was well controlled with hydrochlorothiazide. Her examination and laboratory tests showed no signs of infection, but her serum sodium level was 119 mEq/L and her plasma osmolarity was 245 mOsm/kg.

She appeared to be clinically hypovolemic. Source: Case Files Internal Medicine