Mean FGF21 level for MDD participants at Baseline was significantly higher compared to that for HC participants (7.3 ± 1.3 pg/mL vs. 6.3 ± 1.5 pg/mL; p < 0.0001), even after adjusting for baseline age, sex, race, Hispanic ethnicity, BMI, and site (β-coefficient = 1.20, p < 0.0001, Cohen’s d = 0.60). This evidence concerns the gene FGF21 and major depressive disorder.