In SIADH, which of the following laboratory findings is characteristic?

• A. Hyponatremia with low serum osmolality and inappropriately concentrated urine.
• B. Hypernatremia with high serum osmolality and dilute urine.
• C. Hypernatremia with low serum osmolality.
• D. Normonatremia with high urine osmolality.

Answer: (A)

SIADH produces euvolemic hyponatremia because excess ADH causes the kidneys to retain water. This dilutes the blood, lowering serum sodium and serum osmolality. However, the ADH effect keeps the kidneys from making maximally dilute urine, so the urine remains inappropriately concentrated despite the hyponatremia. The combination of low serum sodium, low serum osmolality, and concentrated urine is the hallmark pattern of SIADH, which is why the described lab findings fit best.

Other patterns don’t fit SIADH: hypernatremia with high serum osmolality would point to water loss or insensible losses rather than ADH-driven water retention; hypernatremia with low serum osmolality isn’t consistent; normonatremia with high urine osmolality doesn’t reflect the hyponatremia that characterizes SIADH.