Tetrahydrobiopterin (BH4), the cofactor required for the enzymatic conversion of monoamine precursors tryptophan and tyrosine via phenylalanine hydroxylase (PAH), and tryptophan hydroxylase and tyrosine hydroxylase, which are essential for the biosynthesis of neurotransmitters serotonin, dopamine and norepinephrine, all these are dysregulated in patients with depressive syndromes (Sperner-Unterweger et al. 2014; Qiu et al. 2015). Here, PAH is linked to depressive disorder.