Many of the symptoms that older men experience, such as loss of lean muscle mass, weight gain, and loss of sexual desire, are thought to be due to decreasing testosterone levels. This small study suggests that some of those symptoms may actually be due to low estrogen levels. Because men metabolise estrogen from testosterone, men who have low testosterone levels also have low estrogen levels. As a result, it was not clear which hormone was actually responsible for these symptoms. Either way, it might be time to go visit your doc if symptoms persist.