A 64-year-old man with hilar densities has biopsy showing small round cells with little cytoplasm. Which electrolyte abnormality would you expect if this represents a small cell carcinoma with SIADH?

A 64-year-old man with hilar densities has biopsy showing small round cells with little cytoplasm. Which electrolyte abnormality would you expect if this represents a small cell carcinoma with SIADH?

• A. Hypernatremia
• B. Hyponatremia
• C. Hyperkalemia
• D. Hypophosphatemia

Answer: (B)

Small cell carcinoma often secretes ADH ectopically, causing SIADH. The excess ADH makes the kidneys reabsorb more water, diluting the blood and lowering serum sodium. So you get hyponatremia with inappropriately concentrated urine and typically a euvolemic patient. This is the classic electrolyte pattern for SIADH, unlike hypernatremia that would come from ADH deficiency or insensible water loss, or unrelated electrolyte shifts like hyperkalemia or hypophosphatemia.

Small cell carcinoma often secretes ADH ectopically, causing SIADH. The excess ADH makes the kidneys reabsorb more water, diluting the blood and lowering serum sodium. So you get hyponatremia with inappropriately concentrated urine and typically a euvolemic patient. This is the classic electrolyte pattern for SIADH, unlike hypernatremia that would come from ADH deficiency or insensible water loss, or unrelated electrolyte shifts like hyperkalemia or hypophosphatemia.