For morbidly obese men, levels of sex hormones increased after bariatric surgery, in some cases to higher levels than expected based on the amount of weight loss, researchers found.

In a single-center study, men with a median body mass index of 43.9 kg/m2 who underwent bariatric surgery had a significant increase in the majority of sex hormones, including total testosterone and sex hormone binding globulin (SHBG) at 6 months and 1 year after surgery, according to Edoardo Mannucci, MD, of Azienda Ospedaliero-Universitaria Careggi in Florence, Italy, and colleagues.

And levels of total testosterone and SHBG were significantly higher than expected at both of those time points, the researchers reported online in Fertility & Sterility.

Obesity in men is associated with reduced levels of testosterone and SHBG, but the relationship between sex hormones and weight loss is less clear.

To assess those potential changes in those hormones after weight-loss surgery, Mannucci and colleagues conducted a cross-sectional and a longitudinal study of patients at their bariatric surgery center.

The cross-sectional study involved 161 men who had a median BMI of 29.2 kg/m2, and was used to define the relationship between BMI and hormones including total testosterone, calculated free testosterone (cFT), estradiol (E2), SHBG, follicle-stimulating hormone (FSH), and luteinizing hormone (LH).

The longitudinal study involved 24 morbidly obese patients who had bariatric surgery -- either gastric bypass, gastric banding, or biliopancreatic diversion -- at the Italian hospital.

BMI and hormone levels were assessed at both 6 months and 1 year.

The cross-sectional study showed that, as expected, several hormone levels fell with greater obesity, including total testosterone, free testosterone, SHBG, FSH, and LH, while heavier patients had higher levels of estradiol.

The curve estimation for the relationship between sex hormones and BMI suggested an exponential model for total testosterone, free testosterone, FSH, and LH, while estradiol was best described by a linear model.

For SHBG, the best fit was a power model, they reported.

In the longitudinal study of morbidly obese patients, the median weight loss from baseline was 24.6% at 6 months and 27% at 1 year.

Mannucci and colleagues found significant increases in total testosterone, free testosterone, SHBG, and gonadotropins 6 months after surgery, and these levels were kept stable at 1 year.

They also saw significant decreases in estradiol levels at 6 months that held stable through 12 months.

When they compared the measured levels after weight loss with expected sex hormone levels, they found significantly higher-than-expected levels for total testosterone and SHBG.

But there were no significant differences from expectations for the other hormones, except for FSH at 12 months.

There was a significant correlation with BMI loss and extra gains in total testosterone (P=0.001) and extra gains in SHBG (P=0.025) at 6 months and again at 12 months (P=0.025 and P=0.013, respectively).

Those findings remained significant after adjusting for age, they added.

When discussing a possible explanation for their findings, they pointed out that bariatric surgery has demonstrated an ability to improve other clinical parameters in obese patients, namely hypertension, glucose tolerance, and insulin resistance.

"Considering that blood glucose has been demonstrated to affect SHBG levels, the surgically obtained amelioration of glucose tolerance and insulin resistance may influence SHBG levels and contribute to the difference between observed and predicted SHBG recovery," they said.

Mannucci and colleagues noted that the study was limited by its small sample size, by the inclusion of different surgical procedures, and by a lack of data on erectile function and other symptoms of hypogonadism.

Still, they concluded that the effects of weight loss induced by bariatric surgery on sex hormones are different from those predicted on the basis of adiposity, with higher-than-expected increases in total testosterone and SHBG.

Although the data are preliminary and need to be confirmed in larger samples with longer follow-up, they wrote, the findings indicate that the relationship between adiposity and sex hormones "could be more complex than anticipated."