A study of 574 men participating in the Baltimore Longitudinal Study of Aging found that low levels of testosterone were connected with a greater risk of developing Alzheimer's disease. The participants were followed for 19 years, and their levels of free testosterone?the amount of testosterone circulating in the blood?were checked periodically. At the beginning of the study, none of the participants had Alzheimer's disease, but during the follow-up period 54 developed the disease.
It is not uncommon for levels of testosterone to decrease with age, but it was noted that levels had dropped dramatically in the men who developed Alzheimer's disease. In addition, for some participants this drop in testosterone was found 10 years before Alzheimer's disease was diagnosed.