A 32-year-old man is feeling unwell and brought to the hospital. His serum sodium is 125 mEq/L. On examination, his blood pressure is 110/70 mmHg, pulse 110/min supine, and sitting up causes him to feel lightheaded with a drop in his blood pressure. His urine sodium is 5 mEq/L and urine osmolality is 800 mOsm/kg.For the above patient with hyponatremia, select the most likely diagnosis.
congestive heart failure (CHF)
extrarenal sodium and fluid losses
SIADH
polydipsia
The combination of ECF volume contraction and low urinary sodium (<10 mmol/L) suggests extrarenal sodium loss. Common causes are vomiting, diarrhea, or excessive sweating.
Oncology