Which laboratory finding is typical of SIADH?

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Multiple Choice

Which laboratory finding is typical of SIADH?

In SIADH, excess ADH causes the kidneys to reabsorb water, which dilutes the blood and leads to dilutional hyponatremia. The most characteristic lab finding is a low serum sodium, reflecting this dilution. You’ll often see a low serum osmolality as well, since the plasma is made more dilute. Urine studies typically show that the urine is inappropriately concentrated (increased urine osmolality) and that sodium loss in the urine is ongoing (urine sodium typically >20 mEq/L) despite hyponatremia. Hypernatremia would not fit this pattern, as the issue is water retention, not water loss. So the hyponatremia best captures the systemic abnormality seen with SIADH, with supportive urine findings that reinforce the diagnosis.

In SIADH, excess ADH causes the kidneys to reabsorb water, which dilutes the blood and leads to dilutional hyponatremia. The most characteristic lab finding is a low serum sodium, reflecting this dilution. You’ll often see a low serum osmolality as well, since the plasma is made more dilute.

Urine studies typically show that the urine is inappropriately concentrated (increased urine osmolality) and that sodium loss in the urine is ongoing (urine sodium typically >20 mEq/L) despite hyponatremia. Hypernatremia would not fit this pattern, as the issue is water retention, not water loss.

So the hyponatremia best captures the systemic abnormality seen with SIADH, with supportive urine findings that reinforce the diagnosis.

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