SIADH is one of those Step 1 “small topic, huge payoff” conditions: it shows up in renal physiology, neuro, pulm, pharm, and endocrine all at once—and the question writers love it because the labs look confusing unless you truly understand the logic. Once you internalize the pathophysiology, the diagnosis and treatment become almost automatic.
What SIADH Is (Definition in One Sentence)
Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is excess ADH activity (often due to ectopic production or CNS/pulmonary disease) causing euvolemic hyponatremia from inappropriate water retention.
Key Step framing: The body keeps too much free water, not too much sodium.
Quick Physiology Refresher: What ADH Normally Does
ADH (vasopressin) is released from the posterior pituitary (made in hypothalamic supraoptic/paraventricular nuclei) and acts on:
- V2 receptors (kidney collecting duct) → ↑ cAMP → insertion of aquaporin-2 channels
→ ↑ water reabsorption → concentrated urine - (Less Step 1–common but important) V1 receptors (vascular smooth muscle) → vasoconstriction
Normal triggers: ↑ plasma osmolality, ↓ effective circulating volume.
Pathophysiology: Why SIADH Causes Euvolemic Hyponatremia
Step-by-step logic (high yield)
- ↑ ADH → ↑ water reabsorption in collecting ducts
- Total body water increases → serum osmolality decreases → hyponatremia by dilution
- Mild ECF expansion would normally trigger RAAS suppression:
- ↓ renin, ↓ aldosterone → kidneys excrete sodium (“natriuresis”)
- Result: you end up not frankly volume overloaded on exam (no major edema) → euvolemic appearance
What’s happening in the urine?
- ADH is telling the kidney to retain water → urine becomes inappropriately concentrated
- Meanwhile, natriuresis increases urinary sodium → urine sodium often elevated
Clinical Presentation: What You See (and What You Don’t)
Symptoms are mostly from hyponatremia
Severity depends on how low and how fast the sodium fell.
- Mild/moderate: nausea, headache, cramps, dizziness
- Neuro symptoms (from cerebral edema): confusion, lethargy, seizures, coma
- Severe acute hyponatremia is an emergency
Volume status on physical exam
- Typically euvolemic
- No peripheral edema, no ascites
- No signs of dehydration (unless another process is present)
High-Yield Causes & Associations (Classic USMLE Patterns)
Think: CNS + pulmonary + cancer + drugs.
Ectopic ADH (classic)
- Small cell lung carcinoma (paraneoplastic SIADH)
CNS disorders
- Stroke/hemorrhage, trauma
- Meningitis/encephalitis
- Brain tumors
Pulmonary disease
- Pneumonia (including atypicals)
- TB
- Acute respiratory failure
Drugs (commonly tested)
- Carbamazepine (increases ADH effect)
- SSRIs
- Cyclophosphamide
- Vincristine
- MDMA (ecstasy) (often paired with polydipsia)
- Desmopressin (ADH analog)
Post-op/pain/nausea (real-life common)
- Surgery, pain, stress can increase ADH transiently
First Aid cross-references (where you’ll see it):
- Endocrine—Posterior pituitary: SIADH vs central diabetes insipidus (DI)
- Renal—Acid/base & electrolytes: hyponatremia workup patterns
- Pulmonary/Onc: small cell lung carcinoma paraneoplastic syndromes
- Pharm: meds associated with SIADH (notably carbamazepine, SSRIs, cyclophosphamide)
Diagnosis: The Pattern You Need to Recognize
Core diagnostic features (classic lab constellation)
SIADH is typically:
- Hyponatremia: low serum Na⁺
- Low serum osmolality: hypotonic plasma
- Inappropriately concentrated urine: urine osmolality > 100 mOsm/kg
- Urine sodium often > 40 mEq/L (because RAAS is suppressed and natriuresis occurs)
- Euvolemia on exam
- Low BUN and low uric acid are supportive clues (dilution + increased excretion)
Table: SIADH lab pattern (memorize-worthy)
| Parameter | SIADH (Typical) | Why |
|---|---|---|
| Serum Na⁺ | ↓ | Dilution from retained water |
| Serum osmolality | ↓ | Hypotonic state |
| Urine osmolality | ↑ (inappropriately) | ADH keeps urine concentrated |
| Urine Na⁺ | ↑ | Mild volume expansion → ↓ RAAS → natriuresis |
| Volume status | Euvolemic | Water retention + sodium loss reach steady state |
| ADH level | ↑ (often) | Ectopic or dysregulated secretion |
Step-style diagnostic criteria (conceptual)
You should be thinking:
- Confirm hypotonic hyponatremia
- Check urine osmolality
- If > 100, ADH is “on” (appropriate or inappropriate)
- Assess volume status
- Euvolemic + concentrated urine strongly suggests SIADH
- Rule out major mimics:
- Hypothyroidism
- Adrenal insufficiency (low cortisol)
- Diuretic use (esp thiazides)
Differential Diagnosis: How SIADH Differs from Other Hyponatremias
SIADH vs psychogenic polydipsia
- Psychogenic polydipsia: kidneys can still dilute urine
→ urine osmolality very low (often < 100) - SIADH: urine stays concentrated despite hyponatremia
SIADH vs cerebral salt wasting (CSW) (board-favorite confusion)
Both can occur with CNS disease and show:
- hyponatremia
- high urine sodium
Key difference: volume status
- SIADH: euvolemic (or slight hypervolemia without edema)
- CSW: hypovolemic (true salt loss → dehydration signs)
Treatment differs:
- SIADH → water restriction
- CSW → volume and salt repletion
SIADH vs adrenal insufficiency
- Adrenal insufficiency can cause hyponatremia via ↑ ADH + salt loss
- Look for hypotension, hyperkalemia, low cortisol (primary AI)
Treatment: What to Do and Why (Step 1 + Step 2 High Yield)
Treatment depends on:
- symptom severity and
- chronicity (acute vs chronic hyponatremia).
1) Address the cause
- Treat pneumonia/CNS pathology
- Stop offending drugs
- Evaluate for malignancy if suspicious (e.g., smoker + lung mass)
2) Fluid restriction (first-line for most stable patients)
- Mainstay for chronic, mild/moderate SIADH
- Goal: create negative free-water balance
3) Hypertonic saline for severe symptoms
Indications you should recognize immediately:
- seizures
- severe confusion/coma
- signs of herniation risk
Use 3% hypertonic saline (often with close monitoring).
4) ADH antagonism (selected cases)
- Conivaptan (V1/V2 antagonist)
- Tolvaptan (V2 selective)
These “vaptans” increase free-water excretion (aquaresis). Often considered when fluid restriction fails (more commonly Step 2/clinical management).
5) Demeclocycline (older board classic)
- Tetracycline that induces nephrogenic DI
- Used sometimes for chronic SIADH (less common clinically now but still testable)
The Dangerous Pitfall: Correcting Sodium Too Fast
The classic complication is osmotic demyelination syndrome (ODS) (formerly central pontine myelinolysis).
Why it happens (concept)
In chronic hyponatremia, brain cells adapt by decreasing intracellular osmolytes. If you rapidly raise serum sodium, water shifts out of neurons → cellular injury/demyelination.
High-yield clinical clue
- Delayed neuro deterioration after correction: dysarthria, dysphagia, weakness, “locked-in” features.
Testable correction limits (know the numbers)
A commonly tested safe limit is:
- No more than 8 mEq/L in 24 hours (some references allow 10–12 in select cases, but boards increasingly emphasize conservative correction)
HY Question Stems You Should Instantly Translate
Stem pattern 1: Lung cancer paraneoplastic
- Older smoker, weight loss, lung mass + Na⁺ 118 + low serum osm + high urine osm
→ SIADH from small cell lung carcinoma
Stem pattern 2: On an SSRI/carbamazepine
- New SSRI, confusion, headache, hyponatremia with concentrated urine
→ drug-induced SIADH
Stem pattern 3: “Euvolemic hyponatremia”
- No edema, no orthostasis, labs consistent
→ SIADH is top of list (after excluding thyroid/adrenal issues)
Rapid Review Box (What You Actually Need on Test Day)
- Dx: hypotonic hyponatremia + inappropriately concentrated urine + euvolemia
- Urine osm: high (ADH effect)
- Urine Na⁺: often high
- Causes: small cell lung carcinoma, CNS disease, pulmonary disease, SSRIs, carbamazepine, cyclophosphamide, vincristine, MDMA
- Tx: fluid restriction; 3% hypertonic saline if severe symptoms; consider vaptans or demeclocycline for chronic refractory cases
- Do not correct too fast → osmotic demyelination syndrome