GAD1 and major depressive disorder: Serotonin levels were significantly lower in both the treated (165.9 ± 4.0 pg/mL, P = 0.030) and untreated (111.0 ± 32.0 pg/mL, P = 0.026) GAD/MDD groups when compared to controls (432.9 ± 103.0 pg/mL) (Table S2).