From the article:
The researchers measured several forms of testosterone in almost 3,000 blood samples collected over a 40-year period from 759 men in the Baltimore Longitudinal Study on Aging, of whom 111 were diagnosed with prostate cancer. One form of testosterone, called free testosterone, which is biologically active and can actually be used by the prostate, was found to be associated with increased prostate cancer risk, according to J. Kellogg Parsons, M.D., instructor of urology at the Brady Urological Institute at Johns Hopkins and lead researcher of the study.
“Since testosterone replacement therapy increases the amount of free testosterone in the blood, older men considering or receiving testosterone replacement should be counseled as to the association until data from long-term clinical trials becomes available,” says Parsons.
The association between free testosterone and prostate cancer risk in older men was not affected by height, weight, percent of body fat, or muscle mass. Total testosterone levels and dehydroepiandrosterone sulfate (DHEAS), another androgenic hormone, were also unrelated to prostate cancer risk, while the protein that binds testosterone in blood, called sex hormone-binding globulin (SHBG), was associated with a slightly decreased risk for prostate cancer.