Introduction
The Syndrome of Inappropriate Antidiuretic Hormone secretion (SIADH) is one of the most common causes of euvolemic hyponatremia in clinical practice.
The key to managing SIADH is:
Understand the physiology → Confirm diagnosis → Correct sodium safely.
What is SIADH?
SIADH is a condition in which there is excess release of ADH (vasopressin) despite:
- Normal or low plasma osmolality
- No dehydration
- No hypotension
Result :
- Water retention
- Dilutional hyponatremia
- Normal body sodium content
- Euvolemic status
Pathophysiology
Normally: Low serum osmolality → ADH suppressed
In SIADH: ADH secretion continues inappropriately →Water reabsorption in collecting ducts →Dilution of serum sodium → Hyponatremia
Urine becomes:
- Concentrated
- High urine sodium
Stepwise Approach to Diagnosing SIADH :
Step 1: Confirm Hyponatremia
- Serum sodium <135 mEq/L
Step 2: Check Serum Osmolality
- <275 mOsm/kg → True hypotonic hyponatremia
- Normal/high → Think pseudohyponatremia or hyperglycemia
(SIADH = Hypotonic hyponatremia)
Step 3: Assess Volume Status
Patient appears:
- No edema
- No dehydration
- Normal BP→ Euvolemic
Step 4: Check Urine Studies
Classic SIADH findings:
- Urine osmolality >100 mOsm/kg
- Urine sodium >30–40 mEq/L
- Serum uric acid low
- BUN low
Diagnostic Criteria
All must be present:
- Hypotonic hyponatremia
- Euvolemia
- Urine osmolality >100
- Urine sodium >30
- Normal adrenal function
- Normal thyroid function
Always rule out:
- Hypothyroidism
- Adrenal insufficiency
Causes of SIADH
CNS Causes
- Stroke
- Meningitis
- Tumors
- Head trauma
Pulmonary Causes
- Pneumonia
- Tuberculosis
Malignancy
- Small cell lung carcinoma
Drugs
- SSRIs
- Carbamazepine
- Cyclophosphamide
Management of SIADH (Stepwise)
1) Severe Symptoms (Seizures, coma)
Give: 3% Hypertonic saline,Correct slowly.
Target: Increase 4–6 mEq initially if severe symptoms.
2)Mild/Moderate Cases
First line: Fluid restriction (800–1000 mL/day)
3)If Persistent
Options:
- Salt tablets
- Loop diuretics
- Demeclocycline
- Vasopressin receptor antagonists (e.g., tolvaptan)
Safe Correction Rule ⚠️
Do not increase sodium more than:
- 8–10 mEq/L in 24 hours
- 18 mEq/L in 48 hours
Rapid correction → Risk of Osmotic Demyelination Syndrome.
SIADH vs Cerebral Salt Wasting (Quick Comparison)
| Feature | SIADH | CSW |
| Volume | Euvolemic | Hypovolemic |
| Urine Sodium | High | High |
| Serum Uric acid | Low | High |
| Treatment | Fluid Restriction | IV Fluids |
Clinical Algorithm Summary :
- Confirm hypotonic hyponatremia
- Assess volume status
- Check urine sodium and osmolality
- Exclude thyroid & adrenal causes
- Manage based on symptoms
Frequently Asked Questions(FAQ)
1. Why is urine sodium high in SIADH?
Because body sodium is normal, kidneys excrete sodium appropriately.
2. Is SIADH always due to lung cancer?
No. Many CNS and drug causes exist.
3. What is the first-line treatment?
Fluid restriction in stable patients.
4. Can SIADH cause seizures?
Yes, in severe acute hyponatremia.
5. Why must sodium correction be slow?
Rapid correction can cause osmotic demyelination syndrome.
Final Clinical Takeaway
SIADH is a diagnosis of exclusion.
Remember:
Hypotonic + Euvolemic + High urine sodium + High urine osmolality = Think SIADH.
Mastering this algorithm makes sodium disorders much easier.