Hyponatremia hypovolemic vs Hyponatremia euvolemic (SIADH)
How to tell Hyponatremia hypovolemic and Hyponatremia euvolemic (SIADH) apart when a board question makes them look alike.
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Volume exam showing hypovolemia versus euvolemia.
Side by side
Hyponatremia with orthostasis, dry mucosa, low effective volume, and sodium improvement after isotonic saline indicates hypovolemic hyponatremia.
Hyponatremia with euvolemic exam, urine osmolality above 100 mOsm/kg, and inappropriately high urine sodium indicates SIADH.
Total body sodium loss exceeds water loss, triggering ADH and secondary free-water retention.
Persistent ADH activity causes water retention despite normal or mildly expanded volume state.
Serum osmolality under 275 mOsm/kg, urine sodium often under 20 mEq/L in extrarenal losses, BUN:Cr elevation, and correction with 0.9% saline support hypovolemia.
Serum osmolality under 275 mOsm/kg with urine osmolality above 100 mOsm/kg and urine sodium above 30 mEq/L in a clinically euvolemic patient supports SIADH.
The trap
Treating SIADH with isotonic saline can worsen sodium when urine is concentrated. In severe symptomatic hyponatremia, use controlled 3% saline correction and avoid overcorrection above 8 mEq/L in 24 hours.
The repair move
Next best step: assess symptom severity, give 0.9% saline for hypovolemic hyponatremia, use 3% saline 100 mL boluses for severe neurologic symptoms, and monitor sodium every 2 to 4 hours to prevent overcorrection.
Try a free question
CoreA 58-year-old woman has sodium 119 mEq/L and serum osmolality 262 mOsm/kg. She is euvolemic on exam, urine osmolality is 520 mOsm/kg, and urine sodium is 46 mEq/L. What is the most likely diagnosis?
Show answer and explanation
Answer: B. SIADH. Euvolemia with inappropriately concentrated urine and elevated urine sodium supports SIADH.
Why the other choices are tempting
- A. Incorrect because hypovolemia usually has volume-depletion signs and often lower urine sodium in extrarenal losses.
- C. Incorrect because pseudohyponatremia from hyperglycemia has high effective osmolality rather than hypotonic pattern.
- D. Incorrect because primary polydipsia usually has very dilute urine, often under 100 mOsm/kg.
- E. Incorrect because adrenal crisis generally presents with hypotension and broader endocrine abnormalities.
Flashcards
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Which bedside clue best supports hypovolemic hyponatremia over SIADH?
Orthostasis and dry mucosa indicating true volume depletion.
What urine profile supports SIADH in hypotonic hyponatremia?
Urine osmolality above 100 mOsm/kg with urine sodium above 30 mEq/L in a euvolemic patient.
Quick answers
What is the fastest way to tell Hyponatremia hypovolemic from Hyponatremia euvolemic (SIADH)?
Volume exam showing hypovolemia versus euvolemia.
What is the key clue for Hyponatremia hypovolemic?
Hyponatremia with orthostasis, dry mucosa, low effective volume, and sodium improvement after isotonic saline indicates hypovolemic hyponatremia.
What is the key clue for Hyponatremia euvolemic (SIADH)?
Hyponatremia with euvolemic exam, urine osmolality above 100 mOsm/kg, and inappropriately high urine sodium indicates SIADH.
What is the common trap with Hyponatremia hypovolemic vs Hyponatremia euvolemic (SIADH)?
Treating SIADH with isotonic saline can worsen sodium when urine is concentrated. In severe symptomatic hyponatremia, use controlled 3% saline correction and avoid overcorrection above 8 mEq/L in 24 hours.