Transient hyperphenylalaninaemia with a high neopterin to biopterin ratio in urine

A case of transient hyperphenylalaninaemia with a maturational delay of dihydropteridine synthesis is described. With the Guthrie test, the patient showed a blood phenylalanine level of 38 mg dl^–1, which had fallen to a normal value without a phenylalanine restricted diet by 3 months of age. The neopterin level and the neopterin to biopterin ratio in the patient's urine were very high at 19 days of age. The blood phenylalanine level did not decrease when tetrahydrobiopterin (2.5 mg kg^–1) was administered at 19 days of age, while administration of tetrahydrobiopterin (7.5 mg kg^–1) at 20 days of age had decreased the blood phenylalanine level to 50% of the preloading level after 24 h. The oral phenylalanine loading test showed the pattern of classic phenylketonuria (PKU) at 15 days of age, but it showed the normal pattern at 1 year 8 months of age.