In hypoosmolar euvolemic hyponatremia due to SIADH, which urine osmolality is typically observed?

• A. Urine osmolality <100 mOsm/kg
• B. Urine osmolality 150-250 mOsm/kg
• C. Urine osmolality >300 mOsm/kg
• D. Urine osmolality equal to serum osmolality

Answer: (C)

In SIADH, hyponatremia occurs because ADH is released inappropriately, causing the kidneys to retain water. The collecting ducts remain permeable to water, so the urine stays concentrated even when serum osmolality is low. This makes urine osmolality markedly high, typically well above 300 mOsm/kg. That high urine concentration distinguishes SIADH from conditions where the body tries to excrete free water, which produce dilute urine (urine osmolality much less than 300). The other scenarios—dilute urine (<100 mOsm/kg) or urine osmolality closely matching serum—do not fit the pattern of SIADH-associated hyponatremia.