Hyponatremia Algorithm: Volume Status, Urine Studies, SIADH
Renal & Endocrine · 8 min read · 2026-06-15
Introduction
Hyponatremia questions test whether you can run a structured algorithm under time pressure rather than recall isolated facts. The exam rewards students who move through serum osmolality, volume status, and urine studies in order, because each step eliminates large branches of the differential. This article lays out that algorithm and highlights where SIADH fits relative to other causes.
Step 1: Confirm It Is Hypotonic Hyponatremia
Check serum osmolality first, because not all hyponatremia is hypotonic.
- Isotonic (pseudohyponatremia): hyperlipidemia, hyperproteinemia (lab artifact from older assays)
- Hypertonic: hyperglycemia or mannitol pulling water into the vascular space and diluting sodium (correct measured Na by adding 1.6 mEq/L for every 100 mg/dL glucose above 100)
- Hypotonic: the true hyponatremia category requiring the volume-status algorithm below
Step 2: Assess Volume Status
Once hypotonic hyponatremia is confirmed, categorize the patient clinically as hypovolemic, euvolemic, or hypervolemic using exam findings (mucous membranes, jugular venous pressure, edema, orthostatics).
Hypovolemic Hyponatremia
Total body water and sodium are both low, but sodium loss exceeds water loss.
- Check urine sodium
- Urine Na less than 20: extrarenal loss (vomiting, diarrhea, third-spacing)
- Urine Na greater than 20: renal loss (diuretics, mineralocorticoid deficiency, salt-wasting nephropathy)
Hypervolemic Hyponatremia
Total body water and sodium are both increased, but water increases more, diluting sodium. Effective circulating volume is low despite total body overload.
- Heart failure
- Cirrhosis
- Nephrotic syndrome
- Urine Na is typically low (less than 20) because the kidney senses low effective circulating volume and retains sodium avidly
Euvolemic Hyponatremia
Total body sodium is roughly normal, with a pure or near-pure water excess.
- SIADH
- Hypothyroidism
- Adrenal insufficiency
- Primary polydipsia
- Reset osmostat
Step 3: Use Urine Osmolality to Separate SIADH from Primary Polydipsia
In euvolemic hyponatremia, urine osmolality is the key discriminator.
- Urine osmolality less than 100 mOsm/kg: primary polydipsia or low solute intake (the kidney is appropriately diluting urine)
- Urine osmolality greater than 100 mOsm/kg with urine Na greater than 40: SIADH is likely, since ADH is inappropriately concentrating the urine despite serum hypotonicity
Diagnostic Criteria for SIADH
- Hypotonic hyponatremia
- Urine osmolality greater than 100 mOsm/kg (inappropriately concentrated)
- Urine sodium greater than 40 mEq/L
- Euvolemia clinically
- Normal thyroid and adrenal function
- No diuretic use
Common causes include small cell lung cancer, CNS lesions, pulmonary infections, and drugs such as SSRIs, carbamazepine, and cyclophosphamide.
Comparison Table
Common Pitfall
A frequent error is diagnosing SIADH without excluding hypothyroidism and adrenal insufficiency first, both of which can mimic the euvolemic, concentrated-urine picture. Always check TSH and morning cortisol before committing to SIADH on an exam vignette. Another pitfall is correcting hyponatremia too quickly; sodium should generally rise no more than 8 mEq/L in 24 hours to avoid osmotic demyelination syndrome, particularly in chronic hyponatremia or alcoholic and malnourished patients.
Clinical Pearl
Diuretic use, especially thiazides, can produce a picture resembling SIADH with low serum sodium and concentrated urine, but the history of diuretic use itself excludes a formal SIADH diagnosis and should prompt discontinuation as first-line management rather than fluid restriction alone.
Summary
Work through hyponatremia in order: confirm true hypotonic hyponatremia, assess clinical volume status, then use urine sodium and urine osmolality to pinpoint the mechanism. SIADH is a diagnosis of euvolemic hyponatremia with inappropriately concentrated urine, made only after excluding thyroid, adrenal, and diuretic causes. Following this sequence prevents the common exam trap of jumping straight to SIADH.