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Renal
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Hyponatremia Evaluation
Flowchart
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Serum osm → urine osm → volume status.
Decision
Outcome
Decision
Hyponatremia — first test?
✓
Serum osmolality
✗
Fluid restrict immediately
Decision
Serum osmolality?
✓
>290 → consider hyperglycemia or renal failure (translocational/pseudo)
✓
Normal/low (<275) → check urine osmolality
Outcome
Translocational (hyperglycemia, mannitol) or pseudohyponatremia (paraproteins, hyperlipidemia)
Decision
Urine osm <100?
✓
Yes → primary polydipsia or low solute (beer potomania, tea-and-toast)
✓
No → urine sodium <25?
Outcome
Restrict water; address low solute intake
Decision
Urine Na?
✓
<25 → volume depletion (or 3rd-spacing: CHF, cirrhosis)
✓
>25 → SIADH, adrenal insufficiency, hypothyroidism, salt-wasting
Outcome
Isotonic saline (true volume deplete) or treat underlying CHF/cirrhosis with diuresis + restriction
Outcome
SIADH → fluid restriction ± salt tabs ± vaptans; check cortisol, TSH
Correct chronic hyponatremia <8 mEq/L/24 h to avoid osmotic demyelination.
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