SIADH vs Diabetes Insipidus: Too Much Water Retained vs Too Much Water Lost
Both disorders involve ADH gone wrong, but every lab value and intervention runs in the opposite direction. Pick the wrong fluid strategy — restricting fluids in a dehydrated DI patient or pushing fluids in a waterlogged SIADH patient — and you accelerate the crisis. The NCLEX loves asking which condition produces dilute urine versus concentrated urine, and students reverse them under pressure.
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- Excess ADH: free water retained
- Small-cell lung CA, CNS injury, SSRIs
- Central: ↓ ADH from pituitary
- Nephrogenic: kidneys resist ADH
- Pituitary surgery/trauma
- Lithium (nephrogenic)
- Weight gain, no edema
- Fluid volume excess
- Low, dark, concentrated urine
- ★Polyuria 4–20 L/day + polydipsia
- Pale dilute urine
- Dehydration, weight loss
- ↓ Na⁺ (< 135)
- ↓ serum osm (< 275)
- Urine SG > 1.030 (concentrated)
- ↑ Na⁺ (> 145)
- ↑ serum osm (> 295)
- ★Urine SG < 1.005 (very dilute)
- ★Fluid restriction 500–1000 mL/day
- Seizure precautions
- Strict I&O, daily weights
- Replace fluids
- Ensure free water access
- Monitor for hypovolemia
- Strict I&O, daily weights
- Vaptans (tolvaptan), demeclocycline
- 3% saline if severe/symptomatic
- Desmopressin (DDAVP) for central DI
- Thiazide + low-Na diet for nephrogenic
- Adhere to fluid restriction
- Weigh daily
- Report confusion/headache (hyponatremia)
- DDAVP long-term (central DI)
- Carry water at all times
- Report return of polyuria
- Weigh daily
- ★Hyponatremic seizures / cerebral edema
- Correct Na⁺ slowly — ODS risk
- Hypovolemic shock
- Severe hypernatremia
- Weak thready pulse, circulatory collapse
- Osmotic demyelination if corrected fast
- Hypernatremic neuro injury, dehydration
SIADH
- Excess ADH: free water retained
- Small-cell lung CA, CNS injury, SSRIs
Diabetes Insipidus
- Central: ↓ ADH from pituitary
- Nephrogenic: kidneys resist ADH
- Pituitary surgery/trauma
- Lithium (nephrogenic)
SIADH
- Weight gain, no edema
- Fluid volume excess
- Low, dark, concentrated urine
Diabetes Insipidus
- ★Polyuria 4–20 L/day + polydipsia
- Pale dilute urine
- Dehydration, weight loss
SIADH
- ↓ Na⁺ (< 135)
- ↓ serum osm (< 275)
- Urine SG > 1.030 (concentrated)
Diabetes Insipidus
- ↑ Na⁺ (> 145)
- ↑ serum osm (> 295)
- ★Urine SG < 1.005 (very dilute)
SIADH
- ★Fluid restriction 500–1000 mL/day
- Seizure precautions
- Strict I&O, daily weights
Diabetes Insipidus
- Replace fluids
- Ensure free water access
- Monitor for hypovolemia
- Strict I&O, daily weights
SIADH
- Vaptans (tolvaptan), demeclocycline
- 3% saline if severe/symptomatic
Diabetes Insipidus
- Desmopressin (DDAVP) for central DI
- Thiazide + low-Na diet for nephrogenic
SIADH
- Adhere to fluid restriction
- Weigh daily
- Report confusion/headache (hyponatremia)
Diabetes Insipidus
- DDAVP long-term (central DI)
- Carry water at all times
- Report return of polyuria
- Weigh daily
SIADH
- ★Hyponatremic seizures / cerebral edema
- Correct Na⁺ slowly — ODS risk
Diabetes Insipidus
- Hypovolemic shock
- Severe hypernatremia
- Weak thready pulse, circulatory collapse
SIADH
- Osmotic demyelination if corrected fast
Diabetes Insipidus
- Hypernatremic neuro injury, dehydration
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SIADH soaks the blood dilute (low Na+, concentrated urine); DI dries the blood concentrated (high Na+, dilute urine).
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Go further
- Commonly confusedSIADH vs Diabetes InsipidusWater retained vs water lost — opposite urine, opposite sodium, constantly mixed up.
- TopicDiabetes InsipidusYour patient is putting out 8 liters of urine a day and still can't quench their thirst — but their blood sugar is perfectly normal. The problem isn't glucose; it's a missing…
- TopicSIADHThe patient's sodium is 118 mEq/L but they're not dehydrated — they're drowning from the inside. SIADH flips everything you'd expect about fluid balance.
- SimulatorFluid & Electrolyte SimulatorChange one lab. Watch the whole patient respond. Sodium, potassium, calcium, magnesium, pH and volume, all linked — with a live ECG and an IV pole where every bag runs.