For MDD, the multiple logistic regression method showed that out of the seven tested variables, GDNF in LF, cortisol, and TNF-α had a statistically significant effect on the occurrence of MDD, p < 0.05: GDNF in LF (β0 = −3.83, β1 = −0.02, p = 0.01; ORβ0 = 0.02 (95%CI: 3*10-5-5), ORβ1 = 0.98 (95%CI: 0.97–0.99)); cortisol (β0 = −3.83, β2 = 0.02, p = 0.01; ORβ0 = 0.02 (95%CI: 3*10-5-5), ORβ2 = 1.02 (95%CI: 1–1.03)); and TNF-α (β0 = −3.83, β3 = 0.56, p = 0.01; ORβ0 = 0.02 (95%CI: 3*10-5-5), ORβ3 = 1.8 (95%CI: 1.22–3.12)). Here, GDNF is linked to major depressive disorder.