Diabetes Insipidus
Massive free water loss from ADH deficiency (central) or resistance (nephrogenic). Classic: polyuria (> 3L/day), polydipsia, dilute urine (osm < 300), rising serum Na⁺.
🔍 Overview
Overview
Diabetes insipidus (DI) is the inability to concentrate urine, causing massive free water loss. Two types: Central DI -deficient ADH production from posterior pituitary (post-pituitary surgery #1, head trauma, tumors, idiopathic, Sheehan syndrome). Nephrogenic DI -kidneys resistant to ADH (lithium #1 cause, hypercalcemia, hypokalemia, tubulointerstitial disease, medications). Key features: polyuria > 3L/day (can exceed 15-20L), dilute urine (osm < 300), hypernatremia if free water access is restricted. Triphasic response post-pituitary surgery: DI (days 1-5) → SIADH (days 5-10, transient ADH release from dying neurons) → permanent DI (if > 80% of ADH neurons destroyed). This is a dangerous pattern -the SIADH phase can cause fatal hyponatremia if you're giving DDAVP for the initial DI phase.
🧪 Workup
Workup
- Serum Na⁺ + serum osmolality -Na usually > 145, osm > 295. If patient has free water access, Na may be normal (compensated by polydipsia).
- Urine osmolality + urine specific gravity -Uosm < 300 mOsm/kg (often < 100) in the presence of elevated serum osm = inappropriately dilute = DI. Sp. gravity < 1.005.
- 24-hour urine volume -> 3L/day (often 5-15L). UOP > 250 mL/hr should prompt immediate evaluation.
- Water deprivation test -gold standard for diagnosis (rarely needed in obvious cases). [Miller et al., 1970 Withhold fluids, monitor urine osm and body weight. In DI, urine remains dilute despite rising serum osm. Then give DDAVP 2 mcg IV: if Uosm rises > 50% → central DI (responds to exogenous ADH). If no response → nephrogenic DI.
- Copeptin level -newer test, co-secreted with ADH. Low copeptin + hyperosmolality = central DI. Fenske et al., 2018
- MRI pituitary -if central DI suspected. Look for absent posterior pituitary bright spot (normal T1 hyperintensity from ADH-containing vesicles).
- Calcium + potassium -hypercalcemia and hypokalemia cause nephrogenic DI
- Lithium level -most common drug cause of nephrogenic DI (40% of chronic lithium users develop some concentrating defect)
- Medication review -lithium, amphotericin B, foscarnet, demeclocycline, cidofovir
🚨 Management
Management
- Central DI:
- Desmopressin (DDAVP) -10-40 mcg daily intranasal, 0.1-0.4 mg BID PO, or 1-4 mcg q12h IV. Selective V2 agonist, so unlike vasopressin it has no vasopressor effect and will not raise blood pressure or cause splanchnic ischemia.
- Titrate to the lowest dose that controls thirst and nocturia, and allow a period of breakthrough polyuria before each dose. Continuous full antidiuresis in a patient who keeps drinking is what causes iatrogenic hyponatremia.
- Let the patient drink to thirst -an intact thirst mechanism protects the sodium far better than any dosing schedule. The dangerous patient is the one who cannot drink (intubated, obtunded, adipsic), who needs measured free water replacement instead.
- Find the cause -pituitary MRI, and check the other pituitary axes. Isolated central DI is uncommon, and a missed cortisol deficiency can mask DI until steroids are replaced and the polyuria suddenly appears.
- Monitor Na⁺ at least daily while titrating (q4-6h in the acute post-operative setting) -both under- and over-treatment show up in the sodium before the patient reports symptoms.
- Nephrogenic DI:
- Remove cause -stop lithium (if possible), correct hypercalcemia, correct hypokalemia
- Thiazide diuretics (paradoxical effect) -hydrochlorothiazide 25 mg daily. Causes mild volume depletion → increased proximal Na⁺/H₂O reabsorption → less water delivered to collecting duct → reduced urine volume. Counterintuitive but effective.
- Amiloride 5-10 mg daily -specifically for lithium-induced NDI. Blocks lithium entry through ENaC in collecting duct.
- Low-sodium, low-protein diet -reduces solute load → reduces obligatory urine volume
- NSAIDs (indomethacin) -reduce prostaglandin-mediated antagonism of ADH. Adjunct, not first-line.
- ICU management of acute DI: D5W or free water via NG tube. Match UOP with replacement (mL for mL replacement initially). DDAVP 1-2 mcg IV q12h. BMP q4-6h.
💊 Medications
Medications
| Drug | Dose | Route | Notes |
|---|---|---|---|
| DDAVP (desmopressin) | 10-40 mcg daily (nasal); 0.1-0.4 mg BID (PO); 1-4 mcg q12h (IV) | IN/PO/IV | Central DI treatment. V2 receptor agonist. No vasopressor effect (unlike native ADH). Risk: hyponatremia from overcorrection. |
| Hydrochlorothiazide (Microzide) | 25 mg daily | PO | Nephrogenic DI. Paradoxical antidiuresis via proximal volume depletion. Monitor K⁺. |
| Amiloride | 5-10 mg daily | PO | Lithium-induced NDI specifically. Blocks ENaC → blocks lithium entry into principal cells. |
| Indomethacin | 25-50 mg TID | PO | Adjunct for NDI. Reduces prostaglandin antagonism of ADH. GI/renal side effects. |
| D5W | Per free water deficit | IV | Free water replacement. Replace 50% of deficit in first 24h. Limit Na correction ≤ 10-12 mEq/24h. |
🏥 Rounds
Pimp Questions
❓ How do you distinguish central from nephrogenic DI?
DDAVP challenge: give DDAVP 2 mcg IV and measure urine osm. Central DI: Uosm rises > 50% (kidneys respond to exogenous ADH because the problem is lack of production). Nephrogenic DI: Uosm does NOT rise (< 10% increase -kidneys are resistant to ADH). Clinical clues: post-pituitary surgery → central. On lithium → nephrogenic.
❓ What is the triphasic response after pituitary surgery?
Phase 1 (days 1-5): DI -surgical damage → decreased ADH release → polyuria. Phase 2 (days 5-10): SIADH -dying posterior pituitary neurons release stored ADH → water retention → hyponatremia. Phase 3 (day 10+): Permanent DI if > 80% of ADH neurons destroyed. The SIADH phase is dangerous -if you're giving DDAVP from phase 1, continuing it into phase 2 → severe hyponatremia. Must stop DDAVP when UOP drops.
❓ Why do thiazides help nephrogenic DI? Isn't that paradoxical?
Thiazides block NaCl reabsorption in the distal tubule → mild volume depletion → activates proximal tubule Na⁺/H₂O reabsorption → less water delivered to the collecting duct → reduced total urine volume. The collecting duct (where ADH acts) receives less filtrate to work with. It's a volume-depletion-mediated effect, not a direct ADH effect. Reduces UOP by 30-50%.
❓ Why is amiloride specifically useful in lithium-induced nephrogenic DI?
Lithium enters principal cells of the collecting duct via the epithelial sodium channel (ENaC). Once inside, lithium interferes with aquaporin-2 insertion (the water channel that ADH activates). Amiloride blocks ENaC → blocks lithium entry → prevents lithium from impairing the ADH signaling cascade. This is specific to lithium-induced NDI -amiloride does not help other causes of NDI.
❓ What is the free water deficit formula?
FWD = TBW × [(Na/140) − 1]. TBW = 0.6 × weight (kg) for men, 0.5 × weight for women. Example: 70 kg man, Na 160 → FWD = 42 × [(160/140) − 1] = 42 × 0.14 = 6 liters. Replace 50% in first 24h, rest over next 24-48h. Limit Na correction ≤ 10-12 mEq/24h to avoid cerebral edema (brain adapted to hyperosmolality with idiogenic osmoles).
Sample Presentation
Mr. Ahmed is a 45-year-old man, post-op day 2 from transsphenoidal resection of pituitary macroadenoma. Overnight UOP 4.2L in 8 hours (525 mL/hr). Clear, dilute urine. Na 152 (was 139 pre-op), serum osm 312, urine osm 89, urine SG 1.002. Patient reports extreme thirst.
Key Points: Post-surgical central DI -massive polyuria with dilute urine and rising Na. Start DDAVP 1 mcg IV q12h. Replace free water deficit with D5W. Monitor Na q4h. Watch for triphasic response -stop DDAVP immediately if UOP drops and Na starts falling (SIADH phase).
- Urine output hourly -in ICU/acute DI. UOP > 250 mL/hr = uncontrolled DI → needs DDAVP or more free water.
- Serum Na⁺ q4-6h -target decline ≤ 10-12 mEq/24h in chronic hypernatremia. Faster correction → cerebral edema.
- Urine osmolality -rising Uosm on DDAVP confirms central DI and response to therapy.
- Daily weights + strict I&Os -fluid balance critical
- Post-pituitary surgery: UOP + Na q4-6h × 72h minimum. Watch for triphasic response -DI phase (days 1-5) may transition to SIADH (days 5-10). Stop DDAVP if UOP drops and Na falls.
- Lithium level -if continuing lithium with amiloride, monitor both
📋 Summary
Summary
The Picture
Polyuria above 3 L/day, polydipsia, dilute urine with osmolality below 300, and a rising serum sodium. Central DI is failure to make ADH; nephrogenic DI is failure to respond to it. A conscious patient with free access to water keeps the sodium nearly normal, so a markedly high sodium means the patient could not drink, which is usually the sicker situation.
Confirm It Is Really DI
Simultaneous serum and urine osmolality: a high or high-normal serum osmolality with inappropriately dilute urine makes the diagnosis. Exclude an osmotic diuresis first, above all hyperglycemia, and also mannitol, high-protein feeds and post-obstructive or post-ATN diuresis, all of which produce large urine volumes for entirely different reasons.
Desmopressin Separates the Two Types
Water deprivation followed by desmopressin: central DI concentrates the urine, nephrogenic DI does not. Copeptin measurement is replacing the classic deprivation test in many centers because it is faster and safer. Never water-deprive an already hypernatremic patient: they have made the diagnosis for you.
Central DI: Give Desmopressin
10 to 40 mcg daily intranasally, 0.1 to 0.4 mg BID orally, or 1 to 4 mcg q12h IV. It is a selective V2 agonist with no vasopressor effect, unlike vasopressin. Causes: post-neurosurgery or pituitary surgery, head injury, tumor, infiltrative disease, and brain death. Get a pituitary MRI and check the other pituitary axes, since isolated DI is uncommon.
Nephrogenic DI: Fix the Cause, Then Use the Paradox
Stop the offending drug where possible, and correct hypercalcemia and hypokalemia, both of which cause reversible resistance. Thiazides work paradoxically: hydrochlorothiazide 25 mg daily causes mild volume depletion, which increases proximal sodium and water reabsorption and so reduces delivery to the distal nephron and cuts urine output. Add a low-sodium diet, which amplifies the same effect, and indomethacin as an adjunct.
Lithium Has Its Own Answer
Amiloride 5 to 10 mg daily blocks ENaC, which is the channel lithium uses to enter the collecting duct cell. That makes it the specific agent for lithium-induced nephrogenic DI, and it can be continued while the lithium is continued if the psychiatric indication requires it. The defect may be irreversible after long exposure, so it is worth catching early.
Correct the Sodium at the Right Speed
Replace with free water, D5W or oral water, and correct chronic hypernatremia by no more than 10 mEq/L per 24 h. Replace about half the calculated deficit in the first 24 h and reassess. Faster correction causes cerebral edema, because the brain has generated osmoles it cannot shed quickly. Add ongoing urinary losses to the calculation, which is what the formulas leave out and why patients fall behind.
The Post-Neurosurgical Triphasic Response
DI for a few days, then a period of SIADH as stored ADH is released from the degenerating posterior pituitary, then permanent DI. The middle phase is where patients get into trouble, since desmopressin continued through it causes profound hyponatremia. Check sodium at least twice daily post-operatively and be prepared to stop the drug rather than simply continuing a standing order.
📄 One Pager
Text version
One-Pager
Diabetes Insipidus
Central vs Nephrogenic
DDAVP Challenge
Give DDAVP 2 mcg IV → measure Uosm. Rises > 50% = central (responds). No change = nephrogenic (resistant). Copeptin: low = central DI.
Treatment
Central: DDAVP + free water. Nephrogenic: remove cause + thiazide + amiloride (lithium) + low-Na diet. ICU: match UOP with D5W mL-for-mL.
Related Topics
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