For the msTBI group with pre-morbid depression, a similarly strong, negative and statistically significant correlation between IGF-1 Z-score and PHQ-9 scores was observed (τ = −0.63 p < 0.0001) Kendall’s Tau scores comparing IGF-1 Z-scores with PHQ-9 scores for mTBI and msTBI groups without preinjury depression were similar to their counterparts with a history of depression (mTBI: τ = −0.52, p < 0.0001; msTBI: τ = −0.63, p < 0.0001). Here, IGF1 is linked to major depressive disorder.