[Serum concentration of uric acid, a diagnostic 'must' in patients with hyponatremia]

The renal handling of urate involves complete glomerular filtration followed by reabsorption and secretion mainly in the proximal renal tubule. The amount of urate excreted depends on the extracellular fluid volume. Expansion of extracellular fluid volume increases and contraction of extracellular fluid volume decreases the clearance of urate. In the syndrome of inappropriate secretion of antidiuretic hormone, there is volume expansion associated with low uric acid. A comparable situation is observed whenever exaggerated amounts of free water are consumed. Similarly, hypouricaemia and a tendency to low serum sodium concentrations are observed in normal pregnancy, a state with volume expansion, a physiological situation reversed when eclampsia occurs. On the other hand, when hyponatraemia is associated with contraction of extracellular fluid volume as it is often the case when thiazides or loop-diuretics are administered, hyponatraemia is paralleled by increased serum uric acid concentrations.