Biopterin defect in a normal-appearing child affected by a transient phenylketonuria

A child diagnosed as having transient phenylketonuria was found to have reduced synthesis of tetrahydrobiopterin and an abnormal clearance of phenylalanine, but he remained clinically normal when on a normal diet. A small amount of 7,8-dihydrobiopterin was found in his serum; this distinguishes the case from that of malignant hyperphenylalaninaemia.