After controlling for age, body mass index, and medications, concentrations of IL-6 were elevated in participants with high comorbidity (TBI + PTSD + depression) compared to those with no more than one service-related disorder, and TBI alone was not associated with increased IL-6 (d = 0.88, 95% CI: 0.15, 1.61). Here, IL6 is linked to major depressive disorder.