A 75-year-old woman presents with a complaint of a headache and mild confusion. Her medical history is remarkable only for hypertension, which is well controlled with hydrochlorothiazide. Her examination and laboratory tests show no signs of infection, but her serum sodium level is 119 mEq/L, and plasma osmolarity is 245 mOsm/kg.

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