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]

Overview

In hyponatremia, depending on the cause, there can be different laboratory abnormalities.

To see the causes of hyponatremia, click here.

To see the differential diagnosis of hyponatremia, click here.

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