In this study, we utilized MR analysis to: (1) inspect whether the effect of obesity-related traits (BMI, waist-to-hip ratio [WHR], and waist-to-hip ratio adjusted for body mass index [WHRadjBMI]) on PCa remained consistent with previous MR studies; (2) estimate the causal effect of serum testosterone levels (BT, TT, and sex hormone-binding globulin [SHBG]) on PCa; (3) explore the potential bidirectional relationship between obesity-related traits and serum testosterone levels; and (4) assess whether BMI’s effect on PCa risk is mediated or confounded by serum testosterone levels. Here, SHBG is linked to obesity due to melanocortin 4 receptor deficiency.