At present, peripheral measurements of BDNF, NGF, IGF-1, EGF, FGF-2, and HGF are best regarded as research biomarkers, with potential roles in stratifying patients within specific syndromic NDDs—such as Rett syndrome and Phelan–McDermid syndrome—or serving as pharmacodynamic readouts rather than diagnostic tools or general severity markers in ASD, ADHD, or Tourette syndrome [175,176,183,185]. Here, IGF1 is linked to Rett syndrome.