With regard to the subgroup analysis stratified by BMI and study quality, significantly lower IL-6 levels were associated with BMIs of <25 kg/m2 and ≥25 kg/m2 in the controls (SMD = 0.99, 95% CI = 0.33–1.65, P = 0.003 and SMD = 0.64, 95%CI = 0.18–1.10, P = 0.006, respectively; S3 Fig) but not in the PCOS patients with low and high BMIs using a random-effects model (I2 = 93%, P<0.001 and I2 = 90%, P<0.001, respectively). Here, IL6 is linked to polycystic ovary syndrome.