label: tetrahydrobiopterin-responsive hyperphenylalaninemia/phenylketonuria
label_lc: tetrahydrobiopterin-responsive hyperphenylalaninemia/phenylketonuria
description: Tetrahydrobiopterin-responsive hyperphenylalaninemia/ phenylketonuria (BH4-responsive hyperphenylalaninemia/ phenylketonuria) is a form of phenylketonuria (PKU), an inborn error of amino acid metabolism, characterized by mild to moderate symptoms of PKU including impaired cognitive function, seizures, and behavioral and developmental disorders, and a marked reduction and normalization of elevated phenylalanine concentrations after oral loading with tetrahydrobiopterin (BH4; sapropterin dihydrochloride), an essential cofactor of phenylalanine hydroxylase.