Hypernatremia associated with urine osmolality less than 300 mOsm/kg is most consistent with which condition?

• A. Diabetes insipidus
• B. Syndrome of inappropriate ADH secretion (SIADH)
• C. Acute dehydration due to vomiting
• D. Chronic kidney disease

Answer: (A)

When hypernatremia is present, the kidneys’ ability to concentrate urine helps identify the cause. A urine osmolality below 300 mOsm/kg means the urine is dilute despite high serum sodium, indicating a defect in antidiuretic hormone action or response. This pattern is classic for diabetes insipidus, where insufficient ADH effect (central DI) or renal insensitivity (nephrogenic DI) leads to large volumes of dilute urine and free water loss that raises the plasma sodium.

In contrast, SIADH produces hyponatremia with inappropriately concentrated urine (high urine osmolality), not hypernatremia with dilute urine. Vomiting and dehydration typically trigger ADH release, concentrating the urine (high urine osmolality) to conserve water. Chronic kidney disease can impair concentrating ability but does not usually present as hypernatremia with very dilute urine in the same way as diabetes insipidus.