All of the following are clinical features of SIAD except:

• A. Increased urinary osmolality
• B. Increased urinary sodium
• C. Hyponatremia
• D. Hypervolemia

Answer: (D)

In SIADH, excess antidiuretic hormone causes water retention that dilutes serum sodium, leading to hyponatremia with low serum osmolality. The kidneys respond by producing urine that is inappropriately concentrated, so urine osmolality is increased. Sodium balance is also altered because, to maintain euvolemia, the kidneys excrete sodium, so urinary sodium is elevated. The typical clinical picture is euvolemic hyponatremia with no edema. Hypervolemia, or fluid overload, is not a feature of SIADH and is more characteristic of conditions like heart failure or cirrhosis. Therefore, hypervolemia is the exception.