SIADH vs Diabetes Insipidus
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.
Side-by-side comparison
- Excess ADH → retains free water (overload)
- Small-cell lung CA, CNS injury, SSRIs
- ↓ ADH (central) or kidney resists ADH (nephrogenic)
- Pituitary surgery/trauma; lithium (nephrogenic)
- Weight gain, no edema; fluid overload
- Low, dark, concentrated urine
- ★Polyuria 4–20 L/day + polydipsia
- Pale dilute urine; dehydration, weight loss
- ↓ Na⁺ (< 135); ↓ serum osm (< 275)
- ↑ urine specific gravity (> 1.030)
- ↑ 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 hypovolemia; strict I&O, weights
- Vaptans (tolvaptan), demeclocycline
- 3% saline only if severe (Na < 120)
- Desmopressin (DDAVP) for central DI
- Thiazide + low-Na diet for nephrogenic
- Adhere to fluid restriction; daily weights
- Report confusion/headache (hyponatremia)
- Lifelong DDAVP; carry water
- 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 from rapid correction
- Hypernatremic neuro injury, dehydration
SIADH (Excess ADH)
- Excess ADH → retains free water (overload)
- Small-cell lung CA, CNS injury, SSRIs
Diabetes Insipidus (Deficient ADH)
- ↓ ADH (central) or kidney resists ADH (nephrogenic)
- Pituitary surgery/trauma; lithium (nephrogenic)
SIADH (Excess ADH)
- Weight gain, no edema; fluid overload
- Low, dark, concentrated urine
Diabetes Insipidus (Deficient ADH)
- ★Polyuria 4–20 L/day + polydipsia
- Pale dilute urine; dehydration, weight loss
SIADH (Excess ADH)
- ↓ Na⁺ (< 135); ↓ serum osm (< 275)
- ↑ urine specific gravity (> 1.030)
Diabetes Insipidus (Deficient ADH)
- ↑ Na⁺ (> 145); ↑ serum osm (> 295)
- ★Urine SG < 1.005 (very dilute)
SIADH (Excess ADH)
- ★Fluid restriction 500–1000 mL/day
- Seizure precautions; strict I&O, daily weights
Diabetes Insipidus (Deficient ADH)
- Replace fluids; ensure free water access
- Monitor hypovolemia; strict I&O, weights
SIADH (Excess ADH)
- Vaptans (tolvaptan), demeclocycline
- 3% saline only if severe (Na < 120)
Diabetes Insipidus (Deficient ADH)
- Desmopressin (DDAVP) for central DI
- Thiazide + low-Na diet for nephrogenic
SIADH (Excess ADH)
- Adhere to fluid restriction; daily weights
- Report confusion/headache (hyponatremia)
Diabetes Insipidus (Deficient ADH)
- Lifelong DDAVP; carry water
- Report return of polyuria; weigh daily
SIADH (Excess ADH)
- ★Hyponatremic seizures / cerebral edema
- Correct Na⁺ slowly — ODS risk
Diabetes Insipidus (Deficient ADH)
- Hypovolemic shock; severe hypernatremia
- Weak thready pulse, circulatory collapse
SIADH (Excess ADH)
- Osmotic demyelination from rapid correction
Diabetes Insipidus (Deficient ADH)
- Hypernatremic neuro injury, dehydration
★ marks the fact that sets a column apart.
Clinical Pearl
SIADH soaks the blood dilute (low Na+, concentrated urine); DI dries the blood concentrated (high Na+, dilute urine).
Play this as a game — free
Drill SIADH vs Diabetes Insipidus in Swipe Right & Speed Sort. 14-day free trial, no card required.