Hypovolemic Hyponatremia
This clinical presentation—serum sodium 125 mEq/L, urine sodium <20 mEq/L, and urine osmolality 305 mOsm/kg—indicates hypovolemic hyponatremia due to extrarenal fluid losses. The low urine sodium (<20 mEq/L) demonstrates appropriate renal sodium conservation in response to volume depletion, while the concentrated urine (305 mOsm/kg) reflects appropriate ADH secretion triggered by hypovolemia 1, 2.
Diagnostic Interpretation
The combination of these laboratory values points to volume depletion from extrarenal causes such as gastrointestinal losses (vomiting, diarrhea), severe burns, third-spacing, or inadequate fluid intake 3, 4. The kidneys are responding appropriately by retaining sodium (urine sodium <20 mEq/L) and concentrating the urine (osmolality 305 mOsm/kg) in an attempt to restore intravascular volume 2, 3.
Key Distinguishing Features
- Urine sodium <30 mmol/L has a positive predictive value of 71-100% for response to isotonic saline infusion, confirming this is true volume depletion rather than SIADH or other causes 1, 2
- The urine osmolality of 305 mOsm/kg is appropriately elevated (>100 mOsm/kg) in response to hypovolemia, distinguishing this from primary polydipsia where urine would be maximally dilute 2, 5
- If this were SIADH (euvolemic hyponatremia), the urine sodium would typically be >20-40 mEq/L despite normal volume status, and urine osmolality would be inappropriately high (>500 mOsm/kg) 2, 3
Common Causes of This Pattern
The most likely etiologies producing this laboratory constellation include:
- Gastrointestinal losses: Vomiting, diarrhea, nasogastric suction 3, 4
- Third-spacing: Pancreatitis, peritonitis, bowel obstruction 3
- Severe burns with significant fluid sequestration 3, 4
- Inadequate oral intake in elderly or debilitated patients 1
- Remote diuretic use (if diuretics were stopped days ago, allowing urine sodium to normalize to <20 mEq/L) 4
Treatment Approach
Immediate volume repletion with isotonic (0.9%) saline is the correct treatment, not fluid restriction 1, 3. The management algorithm is:
- Administer isotonic saline (0.9% NaCl) for rapid intravascular volume replacement 1, 3
- Correction rate should not exceed 8 mmol/L in 24 hours to prevent osmotic demyelination syndrome 1, 5
- For severe symptoms (seizures, altered mental status), correct 6 mmol/L over 6 hours or until symptoms resolve 1
- Monitor serum sodium every 2-4 hours during active correction 1
Critical Pitfall to Avoid
Do not use fluid restriction in this patient—this is hypovolemic hyponatremia requiring volume expansion, not SIADH 1, 3. Fluid restriction would worsen the volume depletion and potentially lead to hemodynamic compromise 6. The low urine sodium (<20 mEq/L) definitively excludes SIADH and confirms the need for saline administration 2, 3.