When should oral sodium chloride tablets be started in a patient with SIADH who has chronic mild‑to‑moderate hyponatraemia, mild or no neurological symptoms, and inadequate fluid restriction?

When to Start Oral Sodium Chloride Tablets in SIADH

Oral sodium chloride tablets (100 mEq three times daily) should be added when fluid restriction to 1 L/day fails to adequately correct chronic mild-to-moderate hyponatremia in SIADH patients with minimal or no neurological symptoms. 1

Primary Treatment: Fluid Restriction First

• Fluid restriction to ≤1 L/day (or <800 mL/day for refractory cases) is the cornerstone first-line therapy for euvolemic hyponatremia (SIADH) and must be attempted before adding sodium tablets 1, 2
• Nearly half of SIADH patients do not respond adequately to fluid restriction alone, making second-line therapy necessary 2
• Fluid restriction should be maintained for at least several days to assess response before escalating treatment 1, 3

Indications for Adding Oral Sodium Tablets

Add oral sodium chloride 100 mEq three times daily when:

• Serum sodium remains <120-125 mmol/L despite adequate fluid restriction 1, 4
• Mild symptoms (nausea, vomiting, headache, weakness) persist despite fluid restriction 1, 5
• The patient cannot tolerate or comply with strict fluid restriction 3, 6
• Chronic hyponatremia requires ongoing management and fluid restriction alone is insufficient 4, 7

Specific Clinical Scenarios

Chronic Mild-to-Moderate Hyponatremia (Sodium 120-130 mmol/L)

• This is the ideal population for oral sodium supplementation as they are typically asymptomatic or mildly symptomatic and do not require hypertonic saline 4, 5
• Combine sodium tablets with continued fluid restriction (1 L/day) for synergistic effect 1, 4
• Augment with a high-protein diet to increase solute intake 4, 2

When NOT to Use Sodium Tablets

• Severe symptomatic hyponatremia (seizures, altered mental status, coma) requires immediate 3% hypertonic saline, not oral tablets 1, 5
• Hypervolemic hyponatremia (heart failure, cirrhosis) where sodium supplementation would worsen fluid overload 1, 4
• Acute hyponatremia (<48 hours) with severe symptoms requires urgent intravenous therapy 4, 5

Dosing and Monitoring Protocol

• Standard dose: 100 mEq (approximately 6 grams) of sodium chloride orally three times daily 1, 4
• Each 1 gram of sodium chloride contains approximately 17 mEq of sodium 4
• Monitor serum sodium every 4-6 hours initially, then daily once stable 4
• Target correction rate: 4-8 mmol/L per day, never exceeding 8 mmol/L in any 24-hour period to prevent osmotic demyelination syndrome 1, 4

Safety Considerations

• Avoid potassium-containing salt substitutes as patients are at risk for hyperkalemia, especially with renal impairment 4
• Monitor for hyperkalemia when using oral sodium supplements 4
• Calculate sodium deficit using: Desired increase in Na (mEq/L) × (0.5 × ideal body weight in kg) 4
• Home preparation of sodium chloride supplements using table salt is not recommended due to potential formulation errors 1

Alternative Second-Line Options

If sodium tablets fail or are not tolerated:

• Oral urea is considered very effective and safe for SIADH refractory to fluid restriction 2
• Vaptans (tolvaptan 15 mg once daily) may be considered for persistent hyponatremia despite fluid restriction and sodium supplementation 1, 3
• Demeclocycline, lithium, or loop diuretics are less commonly used due to side effects 1, 6

Common Pitfalls to Avoid

• Starting sodium tablets before attempting adequate fluid restriction 2
• Using sodium tablets in hypervolemic patients (cirrhosis, heart failure) where they worsen fluid retention 1, 4
• Correcting chronic hyponatremia faster than 8 mmol/L in 24 hours, risking osmotic demyelination 1, 4
• Failing to monitor serum sodium frequently during initial correction 4
• Using oral tablets for severe symptomatic hyponatremia requiring urgent intravenous therapy 5