IGF1 and Parkinson disease: Neurotrophic factors, such as brain‐derived neurotrophic factor (BDNF), glial cell‐derived neurotrophic factor (GDNF), nerve growth factor (NGF), transforming growth factor β (TGFβ), insulin‐like growth factor 1 (IGF1), epidermal growth factor (EGF) and fibroblast growth factor 1 (FGF1), are involved in neuronal survival, maintenance and regeneration, and glial activation, malfunctions of which contribute to the pathogenesis of PD.34, 35, 36, 37, 38, 39, 40