Which electrolyte abnormality is characteristically seen in SIADH?

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

Answer: (A)

In SIADH the problem is too much ADH causing the kidneys to reabsorb water. This extra water dilutes the serum sodium, leading to hyponatremia with low serum osmolality. Because the body adjusts to retain water with some salt loss, patients are typically euvolemic, and the urine remains inappropriately concentrated with elevated urine sodium. So the electrolyte abnormality you’d see is low serum sodium—hyponatremia.