Classifying tetrahydrobiopterin responsiveness in the hyperphenylalaninaemias.

Background A significant percentage of patients with hyperphenylalaninaemia (HPA) due to primary deficiency of the phenylalanine hydroxylase enzyme (PAH) respond to a dose of tetrahydrobiopterin (BH4) with an increased rate of phenylalanine (Phe) disposal. The effect is exploited therapeutically, with some patients on BH4 even tolerating a normal diet.