In diabetes insipidus, urine osmolality is typically

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In diabetes insipidus, the kidneys cannot concentrate urine because ADH (vasopressin) action is deficient or the renal response to ADH is impaired. Without ADH-driven water reabsorption in the collecting ducts, large amounts of free water are excreted, producing urine that is very dilute. Urine osmolality reflects this dilute urine and is typically low, usually well under 300 mOsm/kg, even when fluids are available. As a result, you see polyuria with low urine osmolality, while serum osmolality may rise due to water loss, driving thirst.

High urine osmolality would imply the kidneys are concentrating urine, which isn’t seen in DI. Normal urine osmolality would not fit the typical pattern of DI. Urine osmolality changing with hydration status doesn’t align with the hallmark defect of DI, where the urine remains dilute despite fluid intake.