BDNF and major depressive disorder: Twelve measures, which were the most investigated in the 23 articles, were used in this systematic review and meta-analysis: Beck Depression Inventory (BDI), Hospital Anxiety (HADS-A) and Depression (HADS-D) scales, Rosenberg Self-Esteem Scale (RSS), Symptom Check List-90-Revised (SCL-90-R), Anxiety Sensitivity Index (ASI-3), Brain‐Derived Neurotrophic Factor (BDNF), Perceived Stress Scale (PSS), University of California Los Angeles Loneliness Scale (UCLA-LS), and Liebowitz Social Anxiety Scale (LSAS).