TNF-α levels indicated statistically significant correlations with tryptophan, kynurenine, 3-hydroxykynurenine/tryptophan, and serotonin levels in patients with MDD without T2DM (β =  − 0.603, adjusted p = 0.032; β = 0.718, adjusted p = 0.002; β = 0.608, adjusted p = 0.003; β = 0.450, adjusted p = 0.049, respectively). Here, TNF is linked to major depressive disorder.