In SIADH, which combination of serum osmolality and urine osmolality is most typical?

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Multiple Choice

In SIADH, which combination of serum osmolality and urine osmolality is most typical?

Excess ADH in SIADH causes the body to retain water, which dilutes the blood and lowers serum osmolality. Yet the kidneys keep reabsorbing water due to the continued ADH effect, so the urine remains concentrated. That combination—dilute serum with concentrated urine—is typical of SIADH. In numbers, serum osmolality tends to be below normal, while urine osmolality stays inappropriately high (often well above 100 mOsm/kg and frequently over 300 mOsm/kg) despite hyponatremia. So, the pattern you’d expect is low serum osmolality with high urine osmolality.

Excess ADH in SIADH causes the body to retain water, which dilutes the blood and lowers serum osmolality. Yet the kidneys keep reabsorbing water due to the continued ADH effect, so the urine remains concentrated. That combination—dilute serum with concentrated urine—is typical of SIADH. In numbers, serum osmolality tends to be below normal, while urine osmolality stays inappropriately high (often well above 100 mOsm/kg and frequently over 300 mOsm/kg) despite hyponatremia.

So, the pattern you’d expect is low serum osmolality with high urine osmolality.

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