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