Additionally, we observed an inverse relationship between pre-treatment BDNF serum levels and the severity of anxiety and depression symptoms after treatment, according to the final State and Trait Anxiety Inventory–state anxiety subscale (STAI-S) (r = −0.503, p = 0.001) and HDRS (r = −0.630, p < 0.001) scores. Here, BDNF is linked to major depressive disorder.