SIADH: The Daily PANCE Blueprint
A 68-year-old woman with small cell lung cancer reports mild fatigue over the past two weeks. She has no headache, confusion, or seizures. She is clinically euvolemic — no edema, normal skin turgor, normal blood pressure without orthostasis. Serum sodium is 126 mEq/L and serum osmolality is 254 mOsm/kg, while urine osmolality is 480 mOsm/kg and urine sodium is 55 mEq/L. Thyroid and adrenal function are normal. Which of the following is the most appropriate initial management?
A. Infuse normal saline
B. Infuse 3% hypertonic saline
C. Fluid restriction
D. Give IV furosemide
E. Give desmopressin
Answer and topic summary
The answer is C. Fluid restriction
Euvolemic hyponatremia with a low serum osmolality but inappropriately concentrated urine and a high urine sodium is SIADH — here a paraneoplastic syndrome of small cell lung cancer, which secretes antidiuretic hormone independent of normal feedback. The kidney holds onto free water, diluting the serum while the urine stays concentrated. Because she is mildly symptomatic and chronic, the first-line treatment is fluid restriction, typically under 800 to 1,000 mL per day, plus treating the underlying tumor. Salt tablets, urea, or a vaptan are added if restriction alone fails. Two traps live here. Normal saline can paradoxically worsen the sodium in SIADH, because the concentrated urine excretes the salt and retains the water. And hypertonic saline is reserved for severe symptoms — seizures, obtundation — which she does not have; correcting chronic hyponatremia faster than 8 mEq/L in 24 hours risks osmotic demyelination.
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