Urine Concentration in SIADH
In SIADH, urine is inappropriately concentrated with osmolality >500 mOsm/kg and urinary sodium concentration >20 mEq/L (typically >40 mEq/L), despite the presence of hyponatremia and low plasma osmolality. 1
Diagnostic Urine Characteristics
The hallmark urinary findings in SIADH include:
- Urine osmolality >500 mOsm/kg - This represents inappropriately concentrated urine given the patient's hypotonic plasma state 1
- Urinary sodium concentration >20 mEq/L, usually >40 mEq/L - This reflects continued natriuresis despite hyponatremia 1, 2
- Urine osmolality exceeds plasma osmolality - The kidney fails to appropriately dilute urine in response to low serum osmolality 1, 3
Clinical Context
These urinary findings occur in the setting of:
- Plasma osmolality <275 mOsm/kg 1
- Serum sodium <134 mEq/L 1
- Clinical euvolemia (no edema, orthostatic hypotension, or volume depletion) 1, 3
- Normal renal, adrenal, and thyroid function 1, 3
Very High Urine Sodium Concentrations
In severe SIADH cases, urinary sodium can exceed 130 mmol/L, and persistence of very high urine sodium concentrations may predict poor response to fluid restriction alone 4. These extremely elevated urinary sodium levels can occur with traumatic brain injury or recurrent SIADH from various triggers 4.
Pathophysiology
The concentrated urine with high sodium content results from persistent or elevated plasma arginine vasopressin (AVP) despite hyponatremia and low plasma osmolality 3. This leads to water retention and a physiologic natriuresis where fluid balance is maintained at the expense of plasma sodium 5.
Common Diagnostic Pitfall
A urine sodium <30 mmol/L suggests hypovolemic hyponatremia with expected good response to saline infusion, not SIADH 1, 4. Failing to recognize the inappropriately high urine osmolality and sodium concentration in the context of hyponatremia can lead to misdiagnosis and inappropriate treatment 1.