Hyponatremia osmolality values
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Hyponatremia serum osmolality values classification hypotonic isotonic hypertonic thresholds mOsm/kg
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Hyponatremia is serum sodium <135 mEq/L and is first classified by serum osmolality:
Serum osmolality ranges in hyponatremia
| Type of hyponatremia | Serum osmolality (mOsm/kg) | Typical meaning |
|---|---|---|
| Hypotonic hyponatremia (true hyponatremia) | <275 | Excess free water relative to sodium |
| Isotonic hyponatremia (pseudo-hyponatremia) | 275-295 | Lab artifact from severe hyperlipidemia/proteinemia |
| Hypertonic hyponatremia (translocational) | >295 | Osmotically active solutes (usually glucose, mannitol) pull water into ECF |
Common companion urine osmolality values (for hypotonic hyponatremia workup)
- Urine Osm <100 mOsm/kg: suggests primary polydipsia or low-solute intake
- Urine Osm >400 mOsm/kg: AVP/ADH effect is strong
- Intermediate values often reflect mixed causes
(Harrison's, p. 1406)
If you want, I can also give you the full stepwise diagnostic algorithm (including urine sodium cutoffs).
Yes
Use this quick algorithm for hyponatremia interpretation.
1) Confirm true hyponatremia
- Serum Na <135 mEq/L
2) Check serum osmolality
- <275 mOsm/kg: hypotonic hyponatremia (true water excess)
- 275-295: isotonic hyponatremia (pseudohyponatremia)
- >295: hypertonic hyponatremia (usually hyperglycemia, mannitol)
3) If hypotonic, check urine osmolality
- <100 mOsm/kg: kidney appropriately suppressing ADH
- Primary polydipsia
- Low solute intake (beer potomania, tea-and-toast)
- >100 mOsm/kg: ADH active (inappropriate or appropriate)
4) Then check urine sodium (spot urine Na)
- Urine Na <20-30 mEq/L: low effective arterial volume
- Hypovolemia (GI loss, third spacing)
- Edematous states (heart failure, cirrhosis, nephrotic syndrome)
- Urine Na >30-40 mEq/L: renal sodium loss or euvolemic causes
- SIADH
- Diuretics (especially thiazides)
- Adrenal insufficiency
- Salt-wasting states
5) Assess volume status clinically
- Hypovolemic hypotonic hyponatremia
- Euvolemic hypotonic hyponatremia (often SIADH, hypothyroid, adrenal insufficiency)
- Hypervolemic hypotonic hyponatremia (HF, cirrhosis, nephrotic syndrome)
6) Exclude endocrine mimics early
- TSH, free T4
- Morning cortisol ± ACTH stimulation test
7) Severity and urgency (for treatment direction)
- Severe symptoms (seizure, coma, severe confusion): urgent hypertonic saline pathway
- Correction limits (general safety targets):
- usually <=8 mEq/L in 24 h (more conservative in high-risk patients)
- avoid overcorrection to prevent osmotic demyelination
If you want, I can convert this into a one-page emergency treatment table (when to use 3% saline, bolus amounts, and monitoring frequency).
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