Which pattern distinguishes SIADH from DI?

Which pattern distinguishes SIADH from DI?

• A. SIADH: euvolemic hyponatremia with low serum osmolality and inappropriately concentrated urine; DI: hypernatremia with dilute urine
• B. SIADH: hypernatremia with low urine osmolality
• C. DI: hyponatremia with concentrated urine
• D. SIADH: hypovolemia with hyponatremia and high urine osmolality

Answer: (A)

Water balance patterns are what separate SIADH from DI. In SIADH, there’s too much ADH, so the kidneys reabsorb more water. That excess water lowers serum osmolality and dilutes the blood, causing hyponatremia, while the patient is typically euvolemic. The urine, however, remains inappropriately concentrated because ADH is driving water reabsorption even though the serum is hypotonic. In contrast, DI involves a deficit or resistance to ADH, leading to free water loss. This produces more concentrated urine loss relative to intake, causing hypernatremia (or at least normonatremia in partial DI) and dilute urine. The pattern of hyponatremia with concentrated urine fits SIADH, whereas hypernatremia with dilute urine fits DI. So the distinguishing pattern is hyponatremia with low serum osmolality and inappropriately concentrated urine in SIADH versus hypernatremia with dilute urine in DI.

Water balance patterns are what separate SIADH from DI. In SIADH, there’s too much ADH, so the kidneys reabsorb more water. That excess water lowers serum osmolality and dilutes the blood, causing hyponatremia, while the patient is typically euvolemic. The urine, however, remains inappropriately concentrated because ADH is driving water reabsorption even though the serum is hypotonic.

In contrast, DI involves a deficit or resistance to ADH, leading to free water loss. This produces more concentrated urine loss relative to intake, causing hypernatremia (or at least normonatremia in partial DI) and dilute urine. The pattern of hyponatremia with concentrated urine fits SIADH, whereas hypernatremia with dilute urine fits DI.

So the distinguishing pattern is hyponatremia with low serum osmolality and inappropriately concentrated urine in SIADH versus hypernatremia with dilute urine in DI.