SIADH is one of those Step “gotcha” diagnoses: the labs scream dilution, the patient looks euvolemic, and the correct next step is often fluid restriction—not dumping in more saline “because sodium is low.” Here are 3 quick, shareable tips to lock it in for test day.
Tip 1: Memorize the core pattern (it’s a dilution problem)
The one-liner
SIADH = too much ADH → too much free water reabsorption → dilutional hyponatremia with inappropriately concentrated urine.
High-yield lab signature (Step 1/2 classic)
| Finding | SIADH |
|---|---|
| Serum Na | Low |
| Serum osmolality | Low ( mOsm/kg) |
| Urine osmolality | High (inappropriately concentrated; often mOsm/kg) |
| Urine Na | High (often mEq/L) |
| Volume status | Euvolemic (no edema) |
| Uric acid / BUN | Often low (dilution + natriuresis) |
Why “euvolemic” if ADH is high?
ADH initially expands intravascular volume a bit → the body compensates by dumping sodium in the urine (natriuresis) to restore near-normal volume. Net effect: water retained > sodium retained → hyponatremia, but without big edema.
Tip 2: Use a visual mnemonic: “The Sponge + Salt Dump”
Quick mental image
Picture the kidney as a sponge soaking up free water under ADH influence, while the body opens a salt chute to dump sodium to stay euvolemic.
- Sponge effect (ADH on V2 receptors) → aquaporin insertion in collecting ducts → water reabsorption
- Salt dump (compensatory natriuresis) → urine Na high, patient looks euvolemic
Micro-phys tie-in (high yield)
- ADH acts on V2 receptors (Gs → ↑cAMP) on principal cells → inserts aquaporin-2 channels.
- ADH also increases urea reabsorption in the inner medullary collecting duct → helps maintain medullary gradient → more concentrated urine.
Tip 3: Don’t miss the Step triggers + treatment thresholds
Common USMLE triggers (think “SAD causes SIADH”)
- Small cell lung carcinoma (ectopic ADH)
- CNS pathology: stroke, hemorrhage, infection, trauma
- Pulmonary disease: pneumonia, TB
- Meds (classic culprits): SSRIs, carbamazepine, cyclophosphamide, desmopressin, vincristine, MDMA
Treatment in one tight framework
Treat SIADH based on symptoms + severity, not just the sodium number.
- First-line (most cases): Fluid restriction
- If severe symptoms (seizures, severe confusion) or very low Na:
- Hypertonic (3%) saline + loop diuretic (to avoid worsening free-water retention)
- Chronic/refractory SIADH:
- Demeclocycline (induces nephrogenic DI)
- Vaptans (ADH receptor antagonists; e.g., tolvaptan/conivaptan)
The correction pitfall they love to test
Overcorrecting chronic hyponatremia → osmotic demyelination syndrome (central pontine myelinolysis).
- Classic risk: chronic hyponatremia + rapid correction
- Classic presentation (delayed): dysarthria, dysphagia, “locked-in” features
- Practical rule students memorize: don’t raise Na too fast (commonly cited limit: 8–10 mEq/L in 24 hours)
Rapid-fire SIADH checklist (shareable)
- Low serum osm, high urine osm, high urine Na, euvolemic
- Think small cell lung cancer, CNS, pulmonary, SSRIs/carbamazepine
- Treat: fluid restriction → 3% saline if severe symptoms
- Avoid: rapid correction → osmotic demyelination