TH and phenylketonuria: A limited number (up to 2%) of PKU cases are caused by a deficiency of tetrahydrobiopterin (BH4), a co-factor of PAH and other members of aromatic amino acid hydroxylases (AAAHs), such as tyrosine hydroxylase (TH) and tryptophan hydroxylase (TPH), which are required for the biosynthesis of dopamine and serotonin, respectively (Blau, 2016).