Through their studies, they found out that Serum levels of TNF-α, IL-1β, IL-6, Insulin Growth Factor-1 (IGF-1), malondialdehyde (MDA), and C-reactive protein (CRP) have been reported to be elevated, and the serum levels of BDNF and serotonin were lower in patients with MDD [20]. Here, TNF is linked to major depressive disorder.