Because patients with urinary hypouricemia tend to show very low serum uric acid levels (lower than 2 mg/dl), candidate causes in this range of serum uric acid levels may be the decreased production of uric acid caused by certain conditions, such as xanthinuria, purine nucleoside phosphorylase deficiency, xanthine oxidase inhibitors, and liver diseases. This evidence concerns the gene PNP and hyperinsulinemic hypoglycemia, familial, 4.