In SIADH, which electrolyte abnormality is most characteristic?

In SIADH, which electrolyte abnormality is most characteristic?

• A. Hypernatremia
• B. Hypokalemia
• C. Hyponatremia
• D. Hypercalcemia

Answer: (C)

SIADH causes the kidneys to hang onto too much water because ADH is secreted inappropriately. This dilutes the serum sodium, so the hallmark is hyponatremia with a low plasma osmolality. Even though sodium is low, the body remains euvolemic (often with concentrated urine), and the urine is inappropriately rich in sodium and osmolality. So the characteristic electrolyte abnormality is hyponatremia. Hypernatremia would suggest a water deficit or lack of ADH, not this syndrome, and potassium or calcium abnormalities aren’t defining features here.

SIADH causes the kidneys to hang onto too much water because ADH is secreted inappropriately. This dilutes the serum sodium, so the hallmark is hyponatremia with a low plasma osmolality. Even though sodium is low, the body remains euvolemic (often with concentrated urine), and the urine is inappropriately rich in sodium and osmolality. So the characteristic electrolyte abnormality is hyponatremia. Hypernatremia would suggest a water deficit or lack of ADH, not this syndrome, and potassium or calcium abnormalities aren’t defining features here.