GDNF and major depressive disorder: For FE + MDD, the multiple logistic regression method showed that out of the seven tested variables, GDNF in LF and BDNF and TNF-α in BS had a statistically significant effect on the development of FE + MDD, p < 0.05: GDNF in LF (β0 = 14.09, β1 = −0.01, p = 0.01; ORβ0 = 1.32*106 (95%CI: 421.14–3.31*1011), ORβ1 = 0.99 (95%CI: 0.97–0.99)); BDNF (β0 = 14.09, β2 = −0.53, p = 0.003; ORβ0 = 1.32*106 (95%CI: 421.14–3.31*1011), ORβ2 = 0.59 (95%CI: 0.38–0.79)); and TNF-α (β0 = 14.09, β3 = 0.57, p = 0.004; ORβ0 = 1.32*106 (95%CI: 421.14–3.31*1011), ORβ3 = 1.77 (95%CI: 1.27–2.84)).