Transient Neonatal Hyperphenylalaninemia due to Immature Development of 7, 8‐Dihydrobiopterin Synthesis

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Summary We describe a case with transient neonatal hyperphenylalaninemia, possibly due to immature biosynthesis of 7, 8‐dihydrobiopterin (BH 2 ). In the neonatal period his blood phenylalanine levels were high and the patterns of urinary pterins were similar to those found in defective synthesis of BH 2 , which results in a type of hyperphenylalaninemia. But the laboratory data became almost normal after three months. Transient hyperphenylalaninemia has generally been thought to be caused by the delayed maturation of phenylalanine hydroxylase. But it is suggested that some cases with transient neonatal hyperphenylalaninemia are due to immaturity of BH 2 cofactor biosynthesis rather than that of phenylalanine hydroxylase.