In diabetes insipidus, which statement about urine osmolality is true?

In diabetes insipidus, which statement about urine osmolality is true?

• A. It is typically high (>600 mOsm/kg)
• B. It is variable but usually normal
• C. It is typically low (<200 mOsm/kg)
• D. It is always equal to serum osmolality

Answer: (C)

In diabetes insipidus, the lack of effective antidiuretic hormone means the kidneys can’t reabsorb water in the collecting ducts, so a large volume of dilute urine is produced. Because the urine is dilute, its osmolality falls well below normal, typically under 200 mOsm/kg. This distinguishes DI from situations with concentrated urine (high osmolality) or urine that matches serum osmolality. Even though serum osmolality may rise from free-water loss, the urine remains dilute, which is why the correct statement is that urine osmolality is typically low, less than 200 mOsm/kg. In partial DI, values can vary, but the overall pattern remains dilute urine due to impaired concentrating ability.

In diabetes insipidus, the lack of effective antidiuretic hormone means the kidneys can’t reabsorb water in the collecting ducts, so a large volume of dilute urine is produced. Because the urine is dilute, its osmolality falls well below normal, typically under 200 mOsm/kg. This distinguishes DI from situations with concentrated urine (high osmolality) or urine that matches serum osmolality. Even though serum osmolality may rise from free-water loss, the urine remains dilute, which is why the correct statement is that urine osmolality is typically low, less than 200 mOsm/kg. In partial DI, values can vary, but the overall pattern remains dilute urine due to impaired concentrating ability.