Background: The effect of weight loss by bariatric surgery on gonadal hormones in morbidly obese males is not entirely known. The main objective of the study was to analyze gonadal hormonal changes after weight loss.

Methods: An observational study was conducted before and after 12 months of weight loss at a clinical research center. Thirty-three men [age 40.5 ± 9.9, body mass index (BMI) 50.3 ± 6.1 kg/m(2)] undergoing bariatric surgery were included. The main outcome measures were as follows: changes in total (TT) and free testosterone (FT), estradiol (E2), sex hormone binding globulin (SHBG), luteinizing hormone (LH), follicle-stimulating hormone (FSH), anti-Müllerian hormone (AMH), inhibin B, and prolactin (PRL).

Results: Baseline prevalence of hypogonadism (defined by TT < 300 ng/dl or FT < 65 pg/ml) was 78.8 and 51.5%, respectively. Hypogonadal patients were older and showed inhibin B and AMH significantly lower than those with normal TT. BMI correlated negatively with TT, LH, and SHBG. Regression analyses showed a significant and independent association of hypogonadism with age (OR = 1.2, p = 0.01), BMI (OR = 1.3, p = 0.03), and AMH (OR = 0.4, p = 0.03) after adjustments. After 1 year, percentage of weight loss (%WL) was 18.8 ± 5.2%, and there was a significant increase of TT, FT, SHBG, and FSH and a decrease of E2 and PRL. Prevalence of persistent hypogonadism after surgery was 6% (low TT) and 15% (low FT). %WL was significantly associated with percent changes in SHBG (r = -0.4, p = 0.04), inhibin B (r = -0.4, p = 0.03), and AMH (r = -0.4, p = 0.01). Age and %WL were the only significant and independent parameters associated with %TT change.

Conclusions: Obesity-associated hypogonadism is very prevalent in males with morbid obesity and is mostly reversed after sustained weight loss by bariatric surgery.