Hyponatremia laboratory findings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Saeedeh Kowsarnia M.D.[2]

Laboratory Findings

Biochemical evaluation for finding the etiologies of hyponatremia :

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2

For different causes of hyponatremia, click here.

For approach to differential diagnosis, click here.

For the clinical approach to diagnosis, click here.

Laboratory findings in different causes of hyponatremia :

Conditions	Sodium status	Plasma osmolality	Urine osmolality	Differentiation of causes
Hypervolemic hyponatremia	TBW ↑↑ Serum Na ↑	< 275 mOsm/kg	> 100 mOsm/kg	UNa > 20 mEq/L: Acute or chronic renal failure Fluid overload Psychogenic polydipsia Diuretic use in: Heart failure , Liver disease, Nephrotic syndrome UNa < 20 mEq/L: Heart failure Primary polydipsia Cirrhosis Nephritic/nephrotic syndrome
Euvolemic hyponatremia	TBW ↑ Serum Na ↔	< 275 mOsm/kg	> 100 mOsm/kg	Variable UNa : SIAD/ SIADH with fluid restriction Hypothyroidism UNa > 40 mEq/L: Cortisol deficiency ACTH deficiency Hypothyroidism Drugs SIAD/ SIADH
Hypovolemic hyponatremia	TBW ↔ Serum Na ↓↓	< 275 mOsm/kg	> 500 mOsm/kg	UNa > 30 mEq/L: Renal loss: Osmotic diuresis (glucose, urea, bicarbonaturia) Salt-Iosing nephropathy Addison disease CSW Diuretics UNa < 30 mEq/L: Diuretics Extra renal loss: Vomiting (Ucl ↓) Diarrhea Pancreatitis Sweating Small bowel obstruction