IGF1 and major depressive disorder: In the CENC study involving veterans, correlations were observed between the PHQ-9 and both exosomal and plasma NfL.97 In a separate investigation of 331 patients with mTBI, serum insulin-like growth factor-1 (IGF-1) within the first week displayed a significant negative correlation with depression and anxiety scores at both the initial and sixth week post-injury.