As illustrated in Fig. 1, in the unadjusted linear mixed-effect model, we found that patients with bipolar disorder had a 22.0% higher BDNF level compared with healthy controls (B = 1.220, 95% CI 1.104–1.343, P < 0.001), whereas the 8.1% higher level as compared with unaffected first-degree relatives was not statistically significant (B = 1.081, 95% CI 0.955–1.227, P = 0.214). Here, BDNF is linked to bipolar disorder.