In addition to testosterone, the researchers measured estrogen and sex hormone binding globulin, a protein that binds these two hormones. Although testosterone was linked to better scores on the tests, estrogen had essentially no effect on performance, Yaffe said. Previous studies of women have shown that higher estrogen levels can reduce their risk of cognitive decline.
Other research has shown that men have higher levels of both estrogen and testosterone than women, and that women have a 30 percent greater risk of developing Alzheimers Disease, Yaffe said. Some researchers hypothesize that womens increased Alzheimers risk is related to lower hormone levels.
This study doesnt explain how testosterone acts on the brain, Yaffe said, but other studies of mice have shown that the parts of the brain that handle learning and memory tasks are replete with receptors for testosterone.
In comparing scores, the researchers focused on bioavailable testosterone, which is not bound to the proteins, Yaffe said, because when the hormone is bound to protein it has essentially no effect on the brain.