Tetrahydrobiopterin-unresponsive phenylketonuria

A phenylketonuria in which blood phenylalanine concentrations do not fall substantially (conventionally defined as a reduction of more than 30%) in response to an oral tetrahydrobiopterin (BH4; sapropterin dihydrochloride) loading test, so that treatment with BH4 is not effective and phenylalanine-restricted dietary management remains necessary.