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.
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).
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