Protection against type 2 diabetes -- or susceptibility to it -- appears to be one of them.
People with the lowest SHBG levels have a tenfold higher risk of type 2 diabetes than those with the highest SHBG levels.
"Sex hormone-binding globulin may have a causal role in the risk of type 2 diabetes," conclude Eric L. Ding, ScD, of the Harvard School of Public Health, and colleagues.
Ding and colleagues looked at data collected during the Women's Health Study from healthy, postmenopausal female health professionals who did not receive hormone therapy. Over the course of the study, 359 of these women developed type 2 diabetes. They were compared to 359 study participants who did not develop diabetes.
After accounting for diabetes risk factors -- including weight and waist size -- Ding's team found that SHBG levels were strongly linked to diabetes risk. Women with the lowest SHBG levels were much more likely to have diabetes than those with higher SHBG levels.
To validate the finding, the researchers repeated the study in men enrolled in the Physicians' Health Study, comparing 170 men who developed diabetes during the study to 170 matched men who did not.
The results in men were virtually the same as those in women.
Moreover, Ding and colleagues found that men and women with a gene variant called rs6259 had 10% higher SHBG levels than those without it. And those with a gene variant called rs6257 had 10% lower SHBG levels.
People with the SHBG-raising gene were less likely to have diabetes; people with the SHBG-lowering gene were more likely to have diabetes.
The researchers suggest that a person's SHBG level may be an important factor in determining type 2 diabetes risk and the need for early treatment.
The findings appear in the Sept. 17 issue of the New England Journal of Medicine.