Sodium, 24-Hour Urine

In cases of hyponatremia, urine sodium <10 mmol/L may indicate extrarenal depletion: dehydration (gastrointestinal or sweat loss), congestive heart failure, liver disease or nephrotic syndromes.

Urine sodium >10 mmol/L may indicate diuretics, emesis, intrinsic renal diseases, Addison disease, hypothyroidism, or syndrome of inappropriate antidiuretic hormone (SIADH).² In hypothyroidism and in SIADH, Na⁺ and Cl^- may be >40 mmol/L.³ (Depending on intake, such results also can be found in normal individuals.) In SIADH, urinary sodium is usually >20 mmol/L. Inappropriate secretion of antidiuretic hormone (SIADH) was found in 7% of 250 patients with small cell lung cancer.⁴ Such patients have hyponatremia, often severe, with hypo-osmolar serum, high urinary sodium excretion with urine osmolality greater than that of serum. Acute and subacute diseases of the CNS, TB and other chronic pulmonary diseases may also cause SIADH. SIADH may also be caused by acute intermittent porphyria, LE, occasional malignant neoplasms other than small cell carcinoma of lung, and a number of drugs.⁵

The classification as presented here is overly abbreviated for clinical application. Pitfalls exist (eg, increase of Na⁺ necessary to balance excretion of penicillin).³

Urine Na⁺ >40 mmol/L in oliguria suggests acute tubular necrosis.^3,6 (However, spot urine sodiums without other data have been criticized for their applicability to this diagnosis.)

Low Na⁺ excretion may be found with early obstructive uropathy and with the oliguria of acute glomerulonephritis³ and in some patients with x-ray contrast acute renal failure.

Silver et al recommend measurement of urinary Na⁺ excretion in patients with nephrolithiasis and hypercalciuria.⁷

It is important to know the urinary sodium level in patients with unexplained hyperchloremic metabolic acidosis when the diagnosis of distal renal tubular acidosis is being considered.⁸