IFNG and attention deficit-hyperactivity disorder: Imbalances in TRP metabolism66 and a lower serum levels of 3-HK30 have been observed in children with ADHD, and children with ADHD have higher levels of cytokines, including IL-6 and IFN-γ, which are known to activate KP and correlate with clinical parameters of attention, impulsivity, and hyperactivity31.