IGF1 and neurodegenerative disease: The levels of IGF-1 gradually decrease in the middle/late stages of PD, and we speculate that the possible reasons are as follows: on the one hand, the activity of the growth hormone GH-IGF-1 axis gradually decreases with increasing age, which may promote the development of neurodegenerative diseases such as PD in the aging process; on the other hand, during the course of the disease, due to the depletion of the neuroprotective effects of IGF-1, the levels of IGF-1 decrease in the middle/late stages[22].