Hypouricemia in neonates with syndrome of inappropriate secretion of antidiuretic hormone.

A prospective study of serum levels of uric acid in 23 hyponatremic neonates was performed. Infants on diuretic medications or with renal failure were excluded. The infants were separated into two groups: group I consisted of 11 neonates with clinical evidence of syndrome of inappropriate secretion of antidiuretic hormone (SIADH), (mean +/- SD serum sodium 127 +/- 1.36 mEq/liter). Group II included 12 infants with hyponatremia (mean serum sodium 128 +/- 1.10 mEq/liter) associated with decreased effective vascular volume manifest by a fractional sodium excretion less than 1%. The groups were similar for gestational and postnatal ages, birth weight, clinical conditions, and concurrent use of drugs. The serum urate concentration in neonates with SIADH was 2.46 +/- 0.54 mg/dl; serum urate concentration in group II infants was 8.49 +/- 2.45 mg/dL (p less than 0.001). Water restriction in the group I infants with SIADH resulted in a rise in mean serum urate concentration (p less than 0.001). Fractional excretion of urate was elevated during hyponatremia in the group I infants (to 78 +/- 0.13%) and fell to 51 +/- 0.08% after correction (p less than 0.001). In group I infants, a direct correlation was found between fractional excretion of urate and sodium (r = 0.7667, p less than 0.001). These results indicate that hypouricemia is common in infants with suspected SIADH and seems to be due to increased urate clearance secondary to volume expansion.