Hyponatremia questions love to look scary (“confused patient, low sodium!”) but most USMLE stems boil down to a fast pattern-recognition game: what’s the serum osmolality, what’s the urine doing, and is ADH acting appropriately? Here are 3 quick, shareable tips to crush the two most commonly confused causes: SIADH vs psychogenic polydipsia.
Tip #1: Sort hyponatremia in 10 seconds — Serum osm → Urine osm → Urine Na
The Step-style flow
-
Confirm hypotonic hyponatremia (most testable kind)
- Serum osmolality low (typically < 275 mOsm/kg)
-
Check urine osmolality (Uosm): dilute or concentrated?
- Uosm < 100 mOsm/kg → ADH is suppressed → think primary/psychogenic polydipsia (or low-solute intake)
- Uosm > 100 mOsm/kg → ADH is on → think SIADH (or other ADH-driven states)
-
Check urine sodium (UNa): kidney retaining salt or wasting it?
- SIADH: usually UNa > 40 mEq/L (euvolemic, natriuresis)
- Polydipsia: UNa is variable, but the giveaway is the very dilute urine
One-liner
“Hyponatremia is mostly an ADH question: dilute urine = ADH off (polydipsia); concentrated urine = ADH on (SIADH).”
Tip #2: Use this mini-table to separate SIADH vs psychogenic polydipsia
| Feature | SIADH | Psychogenic polydipsia (primary polydipsia) |
|---|---|---|
| Core problem | Too much ADH → too much water reabsorption | Too much water intake overwhelms excretion |
| Volume status on exam | Euvolemic (no edema, no orthostasis) | Euvolemic (often) |
| Serum osm | Low | Low |
| Urine osm | Inappropriately high (often > 100, can be much higher) | Very low (< 100) |
| Urine Na | Often > 40 | Often lower/variable |
| Uric acid | Low (classic SIADH clue) | Often normal/low-ish |
| Common associations | CNS disease, malignancy (small cell lung cancer), SSRIs, carbamazepine, cyclophosphamide, pulmonary disease | Psychiatric disease (schizophrenia), compulsive water drinking |
| First-line treatment | Fluid restriction ± salt tabs/loop; consider vaptans in select cases | Water restriction (and address psych cause) |
One-liner
“SIADH = concentrated urine despite hyponatremia; polydipsia = maximally dilute urine.”
Tip #3: Memorize one visual mnemonic: “ADH Makes Pee Dark”
The mnemonic device
- ADH Drives water reabsorption in the collecting duct → Hyper-concentrated urine
- “ADH makes pee dark.”
- SIADH → ADH high → dark (concentrated) urine
- Psychogenic polydipsia → ADH appropriately low → clear (dilute) urine
Add the Step 1 physiology hook (high yield)
ADH works by binding V2 receptors on principal cells → increases cAMP → inserts aquaporin-2 channels.
High-yield clinical tie-ins (USMLE favorites)
When to worry about symptoms
Hyponatremia symptoms are mostly brain swelling:
- Mild: nausea, headache
- Moderate: confusion
- Severe: seizures, coma
Risk rises with rapid drops in sodium.
Treatment pitfalls they love to test
- Severe symptomatic hyponatremia (seizures, severe neuro symptoms): treat with hypertonic (3%) saline.
- Avoid correcting too fast → osmotic demyelination syndrome (ODS)
- Classic: dysarthria, dysphagia, “locked-in” picture after overcorrection.
- Rule-of-thumb limits commonly tested: no more than ~8 mEq/L in 24 hours (sometimes stricter in high-risk patients).
Bonus mini-clue: SIADH labs often show “dilution + natriuresis”
- Serum: low osmolality, low uric acid
- Urine: not appropriately dilute, sodium not being conserved
Rapid-fire practice stems (read like an NBME)
- “Schizophrenia patient drinks gallons of water, Na 124, urine osm 60” → psychogenic polydipsia
- “Small cell lung cancer, Na 118, euvolemic, urine osm 500, urine Na 60” → SIADH
- “Hyponatremia + concentrated urine” → ADH is on → SIADH jumps to the top (after ruling out other ADH-driven causes)
10-second takeaway
- Uosm < 100 = polydipsia (ADH off)
- Uosm > 100 + UNa > 40 = SIADH (ADH on)
- Treat symptoms first; correct sodium slowly to avoid ODS