Urine Sodium in SIADH
In SIADH, urine sodium is characteristically HIGH, typically >20 mEq/L and often >40 mEq/L, despite the presence of hyponatremia and low serum osmolality. 1, 2
Diagnostic Criteria and Urine Sodium Thresholds
SIADH is characterized by inappropriately high urinary sodium concentration (>20 mEq/L) in the setting of hyponatremia, with most patients demonstrating levels above 40 mEq/L. 1, 2 This elevated urine sodium occurs because:
- The retained free water from inappropriate ADH secretion leads to volume expansion 2
- Volume expansion triggers physiologic natriuresis to maintain fluid balance, resulting in increased sodium excretion despite low serum sodium 2
- The kidneys continue to excrete sodium normally or even excessively, while inappropriately retaining water 1
Distinguishing SIADH from Hypovolemic Hyponatremia
A urine sodium threshold of 50 mEq/L provides the most accurate separation between SIADH and hypovolemic hyponatremia, with sensitivity 0.89, specificity 0.69, and accuracy 0.82. 3
Key diagnostic distinctions:
- Urine sodium <30 mmol/L has a positive predictive value of 71-100% for hypovolemic hyponatremia that will respond to saline infusion 4, 3
- Urine sodium >20-40 mEq/L with high urine osmolality (>500 mOsm/kg) strongly suggests SIADH 1, 5
- In hypovolemic states, the kidneys appropriately conserve sodium, resulting in low urine sodium (<20 mEq/L) 4
- In SIADH, euvolemia or mild hypervolemia triggers continued sodium excretion despite hyponatremia 2
Clinical Significance of Very High Urine Sodium
Extremely elevated urine sodium concentrations (>130 mmol/L) can occur in severe SIADH and may predict poor response to fluid restriction alone. 6 In these cases:
- Persistence of very high urine sodium despite treatment suggests ongoing inappropriate ADH activity 6
- Fluid administration or consumption worsens hyponatremia in these patients 6
- More aggressive management beyond simple fluid restriction may be necessary 6
Complete Diagnostic Profile
The full diagnostic picture of SIADH includes 1, 2, 5:
- Hypotonic hyponatremia (serum sodium <134 mEq/L) 1
- Low plasma osmolality (<275 mOsm/kg) 1
- Inappropriately high urine osmolality (>500 mOsm/kg) 1, 5
- Elevated urine sodium (>20 mEq/L, usually >40 mEq/L) 1, 2
- Clinical euvolemia (no edema, no orthostatic hypotension, normal skin turgor, moist mucous membranes) 4, 5
- Absence of hypothyroidism, adrenal insufficiency, or diuretic use 1, 7
Common Diagnostic Pitfall
A critical error is assuming that elevated urine sodium always indicates volume overload or adequate hydration. 3 In SIADH, the high urine sodium reflects physiologic natriuresis in response to water retention, not true volume excess. Administering isotonic or hypotonic fluids to patients with SIADH based on elevated urine sodium will worsen hyponatremia, as the kidneys cannot excrete the free water load. 6