What is the workup for hyponatremia (low sodium levels)?

Workup for Hyponatremia

The workup for hyponatremia should begin with assessment of volume status, measurement of serum and urine osmolality, and urine sodium concentration to determine the underlying cause and guide appropriate treatment. 1

Initial Assessment

Step 1: Confirm True Hyponatremia

• Verify serum sodium level < 135 mEq/L
• Exclude pseudohyponatremia (normal plasma osmolality) and hypertonic hyponatremia (high plasma osmolality, often due to hyperglycemia)

Step 2: Assess Severity and Symptoms

• Classify severity:

  • Mild: 126-135 mEq/L (often asymptomatic)
  • Moderate: 120-125 mEq/L (nausea, headache, confusion)
  • Severe: <120 mEq/L (risk of seizures, coma, respiratory arrest) 1

• Evaluate for symptoms:

  • Mild: fatigue, muscle cramps, constipation, cold intolerance
  • Severe: seizures, altered mental status, coma 1, 2

Step 3: Determine Volume Status

This is the critical step that guides further workup:

1. Hypovolemic Hyponatremia

   • Clinical signs: orthostatic hypotension, dry mucous membranes, tachycardia
   • Laboratory: urine sodium typically <20 mEq/L
   • Potential causes: GI losses, diuretics, cerebral salt wasting, adrenal insufficiency 1

2. Euvolemic Hyponatremia

   • Clinical signs: no edema, normal vital signs
   • Laboratory: urine sodium >20-40 mEq/L
   • Potential causes: SIADH, hypothyroidism, adrenal insufficiency 1

3. Hypervolemic Hyponatremia

   • Clinical signs: edema, ascites, elevated JVP
   • Laboratory: urine sodium <20 mEq/L
   • Potential causes: heart failure, cirrhosis, renal failure 1

Laboratory Investigations

Essential Tests

• Serum sodium concentration
• Serum osmolality
• Urine sodium concentration
• Urine osmolality 1, 2

Additional Tests Based on Clinical Suspicion

• Thyroid function tests (TSH, free T4) to rule out hypothyroidism
• Morning cortisol and ACTH to evaluate for adrenal insufficiency
• Liver function tests and albumin if cirrhosis suspected
• BNP/NT-proBNP if heart failure suspected
• Medication review (diuretics, antidepressants, antipsychotics, anticonvulsants)
• Serum glucose to rule out hyperglycemia causing hypertonic hyponatremia 1, 3

Specific Diagnostic Criteria

For SIADH

• Hyponatremia
• Inappropriately high urine osmolality (>100 mOsm/kg)
• Urine sodium >40 mEq/L
• Normal volume status (CVP 6-10 cm H₂O) 1, 4

For Cerebral Salt Wasting

• Hyponatremia
• Volume contraction
• Excessive natriuresis
• Low central venous pressure (CVP <6 cm H₂O) 1

Common Pitfalls to Avoid

1. Failing to assess volume status accurately

   • This is the most crucial step in determining the cause and appropriate treatment

2. Overlooking medication-induced hyponatremia

   • Common culprits include thiazide diuretics, SSRIs, and certain antiepileptics

3. Correcting sodium too rapidly

   • Can lead to osmotic demyelination syndrome
   • Total correction should not exceed 8 mEq/L over 24 hours 1

4. Neglecting chronic hyponatremia complications

   • Even mild chronic hyponatremia is associated with cognitive impairment, gait disturbances, increased falls and fractures 2

5. Missing adrenal insufficiency

   • Can present as either hypovolemic or euvolemic hyponatremia
   • May require specific testing and hormone replacement

The diagnostic approach to hyponatremia requires systematic evaluation of volume status, laboratory parameters, and consideration of underlying conditions. This structured workup allows for accurate diagnosis and appropriate treatment selection, which is critical as hyponatremia is associated with increased hospital stay and mortality even when mild 2.