TPH2 and attention deficit-hyperactivity disorder: Considering these findings, a possible hypothesis for serotonergic dysregulation in ADHD could be genetic changes due to polymorphisms in TPH2, DDC, and ASMT that hinder the conversion of tyrosine into serotonin and melatonin or tryptophan in dopamine or epigenetic changes induced by stress in prenatal or early postnatal stages in these pathways in the central nervous system, leaving more available tryptophan at the peripheral level to be metabolized through the kynurenine pathway, favoring an imbalance between neurotoxic and neuroprotective metabolites that can cross the blood–brain barrier.