Diabetes insipidus (DI) is one of those Step “gotcha” diagnoses: the patient is drowning in urine, not sugar—so the key is figuring out whether the problem is no ADH being made/released (central) or ADH is there but the kidney ignores it (nephrogenic). Here’s a quick, shareable way to lock it in fast.
The core idea (one-liner)
Central DI = “No ADH” → fix it with desmopressin.
Nephrogenic DI = “No response to ADH” → fix the kidney/causes (thiazides, amiloride, NSAIDs).
The mnemonic (visual + sticky)
“C = Can’t Create; N = No nephron response”
Think of ADH as a text message and the kidney as the receiver:
- Central DI: the phone can’t create/send the text (↓ ADH).
- Nephrogenic DI: the receiver has no signal / ignores it (kidney resistant).
Even shorter:
- Central = Can’t Create ADH
- Nephrogenic = Nephron says “No” to ADH
Rapid comparison table (USMLE-style)
| Feature | Central DI | Nephrogenic DI |
|---|---|---|
| Primary defect | ↓ ADH production/release | ADH resistance at collecting duct |
| Serum ADH | Low | Normal/high |
| Urine | Very dilute (↓ urine osmolality), polyuria | Very dilute (↓ urine osmolality), polyuria |
| Serum | ↑ serum osmolality, often hypernatremia | ↑ serum osmolality, often hypernatremia |
| Water deprivation test | Urine stays dilute | Urine stays dilute |
| Desmopressin test | Urine osmolality increases (kidney responds) | Minimal/no increase (kidney doesn’t respond) |
| High-yield causes | Head trauma, pituitary surgery, tumors, infiltrative disease | Lithium, demeclocycline, hypercalcemia, hypokalemia, inherited (V2 receptor/AQP2) |
| Treatment | Desmopressin | Treat cause; thiazide, amiloride (esp. lithium), NSAIDs |
High-yield physiology you actually need
ADH (vasopressin) binds V2 receptors on principal cells in the collecting duct → inserts aquaporin-2 channels → reabsorbs water.
- If you lack ADH (central), you can’t insert AQP2.
- If the kidney can’t respond (nephrogenic), ADH is “shouting,” but AQP2 doesn’t show up.
Step 1/2 “classic” clue patterns
Central DI clues
- Recent head trauma, neurosurgery, pituitary/hypothalamic mass
- Improves dramatically with desmopressin
- Can be associated with other pituitary issues depending on the lesion
Nephrogenic DI clues
- Lithium use (biggest test favorite)
- Electrolytes: hypercalcemia and hypokalemia can cause ADH resistance
- Does not improve with desmopressin (or only minimal change)
The 10-second test-taking algorithm
- Polyuria + polydipsia + dilute urine → think DI (after excluding osmotic diuresis like hyperglycemia).
- Water deprivation: still dilute → DI confirmed.
- Give desmopressin:
- Concentrates urine → Central DI
- No change → Nephrogenic DI
Quick shareable recap (tweet-length)
Central DI: “Can’t Create ADH” → ↓ADH, dilute urine, responds to desmopressin.
Nephrogenic DI: “Nephron says No” → ADH resistant (lithium!), dilute urine, no response to desmopressin.