Polydipsia and polyuria with serum Na 152 mmol/L, serum osmolality 310 mOsm/kg, urine Na 10 mmol/L, and urine osmolality 190 mOsm/kg indicate which disorder?

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Multiple Choice

Polydipsia and polyuria with serum Na 152 mmol/L, serum osmolality 310 mOsm/kg, urine Na 10 mmol/L, and urine osmolality 190 mOsm/kg indicate which disorder?

When plasma osmolality is high, the kidneys should respond by concentrating urine to conserve water. Here the serum Na is 152 and serum osmolality is 310, showing a hyperosmolar, water-deficit state. Yet the urine osmolality is only 190 mOsm/kg, which is inappropriately dilute for that plasma osmolality. That inability to concentrate urine despite hypernatremia points to diabetes insipidus, where an insufficient antidiuretic hormone action (central DI) or a renal insensitivity (nephrogenic DI) prevents the kidneys from concentrating urine. The very low urine Na (10) supports volume depletion from water loss, consistent with DI. The other disorders don’t fit as well: diabetes mellitus would typically show osmotic diuresis with different signs such as glycosuria and hyperglycemia; SIADH causes hyponatremia with concentrated urine; primary polydipsia usually presents with low serum osmolality due to excessive water intake, not the high osmolality seen here.

When plasma osmolality is high, the kidneys should respond by concentrating urine to conserve water. Here the serum Na is 152 and serum osmolality is 310, showing a hyperosmolar, water-deficit state. Yet the urine osmolality is only 190 mOsm/kg, which is inappropriately dilute for that plasma osmolality. That inability to concentrate urine despite hypernatremia points to diabetes insipidus, where an insufficient antidiuretic hormone action (central DI) or a renal insensitivity (nephrogenic DI) prevents the kidneys from concentrating urine.

The very low urine Na (10) supports volume depletion from water loss, consistent with DI. The other disorders don’t fit as well: diabetes mellitus would typically show osmotic diuresis with different signs such as glycosuria and hyperglycemia; SIADH causes hyponatremia with concentrated urine; primary polydipsia usually presents with low serum osmolality due to excessive water intake, not the high osmolality seen here.

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