A 65-year-old male with small cell lung cancer presents with confusion. Serum sodium is 118 mEq/L, serum osmolality is low (245 mOsm/kg), and urine osmolality is inappropriately high (550 mOsm/kg). The patient is euvolemic. What is the diagnosis?
Answer and explanation
Correct answer: D. Syndrome of inappropriate antidiuretic hormone secretion (SIADH)
- Step 1: SIADH is defined by hypotonic hyponatremia, inappropriate urine concentration (urine osmolality > 100 mOsm/kg), elevated urine sodium (> 20-40 mEq/L), and clinical euvolemia.
- Step 2: Small cell lung cancer is the classic paraneoplastic malignancy producing ectopic ADH (vasopressin).
- Step 3: Diabetes insipidus presents with hypernatremia and dilute urine.
Answer
Syndrome of inappropriate antidiuretic hormone secretion (SIADH).
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