A drop in testosterone levels can lead to a gain in body fat, a loss of muscle mass, and a decreased sex drive. But a new study in the 'New England Journal of Medicine' suggests that there may be another harmful culprit: low estrogen levels. Men and women produce both hormones, but men have far more testosterone, and women more estrogen. So when researchers gave a group of healthy men shots that stopped them from producing testosterone for 16 weeks, as expected, the men had less lean muscle mass. When they blocked estrogen production in another group, the men had increased body fat. A decrease in either hormone caused a diminished sex drive. Until now, "the only thing that's ever been proposed as a role for estrogen deficiency in men is bone loss," says Dr. Joel Finkelstein, an endocrinologist at Massachusetts General Hospital, who led the study. The research hints that estrogen may be controlling a lot more than previously thought, and this may provide insight to diagnosing men who have symptoms of low testosterone but levels that are too high for therapy. These men may, in fact, be suffering from low estrogen and should discuss treatments with their doctor.