SIADH is most accurately explained by which mechanism leading to hyponatremia in the setting of small cell lung carcinoma?

• A. Decreased ADH
• B. Increased ADH causes water retention and dilutional hyponatremia
• C. Increased aldosterone
• D. Decreased ADH and water loss

Answer: (B)

Ectopic ADH production by the tumor drives water retention, leading to dilutional hyponatremia. In small cell lung carcinoma, ADH is released in excess, so the kidneys reabsorb more free water via V2 receptors in the collecting ducts, increasing aquaporin-2 channels. This expands total body water without a proportional increase in sodium, lowering serum osmolality and diluting serum sodium. The patient often remains euvolemic because natriuresis helps offset the added water, so urine remains relatively concentrated with a high urine sodium. The result is hyponatremia from dilution rather than sodium loss. Decreased ADH would cause dehydration and hypernatremia, and increased aldosterone would promote sodium and water retention without the same dilutional effect, so they don’t explain this scenario.