Sodium

The 24-hour urine sodium test (urinary Na+, natrium) quantifies daily renal sodium excretion to assess overall water and electrolyte balance. In patients with abnormal serum sodium, pairing urine sodium with serum sodium and osmolality supports the differential diagnosis, including distinguishing syndrome of inappropriate antidiuretic hormone secretion from hypovolemia or the pharmacologic effects of diuretics. This measurement assists in evaluating renal sodium handling and effective circulating volume in chronic kidney disease, heart failure, cirrhosis, and nephrotic syndrome. It is used in the assessment of mineralocorticoid excess or deficiency and to monitor adherence to dietary sodium restriction and the impact of diuretic therapy.

Sodium is the dominant cation in the extracellular compartment and a principal driver of extracellular fluid volume and systemic acid–base equilibrium. Together with potassium and chloride, sodium participates in generating transmembrane electrical potentials in nerve and muscle tissue. Whole‑body sodium balance reflects gastrointestinal absorption and renal regulation. Aldosterone and natriuretic peptides modulate tubular sodium transport, antidiuretic hormone governs water reabsorption, and thirst regulates intake. Dietary sodium—primarily as sodium chloride—typically meets physiologic requirements. In health, the kidneys vary sodium excretion to maintain plasma sodium concentration within a narrow range. Accordingly, 24‑hour urinary sodium excretion represents the net effect of dietary intake and renal handling over the collection period.