GDF15 and Parkinson disease: Incidentally, in our previous study, serum GDF15 levels were 1472.22 ± 820.12 pg/mL in patients with PD (72.44 ± 8.84 years) and 1092.83 ± 543.97 pg/mL in healthy individuals (71.93 ± 8.86 years) [10].