GDNF and Parkinson disease: In the PD group, the serum GDNF and HVA levels were negatively correlated with TMT-A (rGDNF = −0.304, P < 0.01; rHVA = −0.334, P < 0.01) and TMT-B (rGDNF = −0.329, P < 0.01; rHVA = −0.323, P < 0.01) scores.