Insulin resistance and sex hormone-binding globulin are independently correlated with low free testosterone levels in obese males

Male obesity is associated with decreased testosterone levels but the pathophysiological mechanisms behind this association are not completely understood. This study aimed to investigate the impact of hyperglycaemia/insulin resistance and sex hormone-binding globulin (SHBG) levels on testosterone levels in a population of obese men. We investigated the impact of several clinical, anthropometric and analytic measures on testosterone levels in 150 obese males. Testosterone deficiency was present in 52.0% of the enrolled patients. This percentage dropped to 17.6% when only calculated free testosterone (FT) was accounted, as SHBG levels were correlated negatively with body mass index (r=-.20; p<.05). Older age (p<.05) and higher homoeostasis model assessment of insulin resistance (HOMA-IR) (p<.01) and lower SHBG levels (p<.05) were independently correlated with lower FT. Weight and fasting plasma glucose lost their statistical significance after multivariate adjustment. Patients with type 2 diabetes mellitus and pre-diabetes had lower FT than those with normal glucose tolerance (p<.05 and p<.01 respectively). Insulin resistance, and not hyperglycaemia and weight per se, seems to be the main determinant of low testosterone levels in obese males. Low SHBG levels are correlated with low FT even after HOMA-IR adjustment. This suggests that SHBG can be associated with testosterone deficiency beyond the influence of insulin resistance unlike previously reported.