Low Serum or Plasma Sodium

Differential Diagnosis

Increase in extracellular fluid volume as produced by congestive heart failure, cirrhosis and ascites, producing dilutional hyponatremia

GI loss of sodium from vomiting, diarrhea, or malabsorption, especially with replacement of fluid and not electrolytes

Renal sodium loss from diuretic use or chronic renal insufficiency

Addison's disease producing adrenal insufficiency

Syndrome of inappropriate antidiuretic hormone secretion (SIADH), as caused by a CNS lesion, pulmonary disease, carcinoma, porphyria

Suggested Additional Lab Testing

Tests for the presence of congestive heart failure and cirrhosis

Indicators of renal function, such as creatinine and BUN

Serum aldosterone level

Serum or plasma BUN, and urine sodium and osmolarity, are valuable in demonstrating hyponatremia and differentiating among the causes of hyponatremia.

  • In SIADH, both urine sodium and urine osmolarity are increased.

  • In the presence of a diuretic inducing hyponatremia, urine sodium is increased, but urine osmolarity is usually isotonic to plasma.

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