And higher levels also increase your risk of getting prostate cancer.

Thomas G,

No, higher levels do not increase your risk of getting prostate cancer: "A review of 25 studies comparing testosterone levels in a total of 2767 controls and 1481 patients with prostate cancer found the mean testosterone levels at diagnosis to be the same in both groups in 15 studies (60%), higher in prostate cancer patients in four (16%) and lower in six (24%)." (http://www.medscape.com/viewarticle/521214_2)

Exercise and fitness are known to increase male hormones including testosterone. Perhaps the lower heart disease rate and higher testosterone levels found are both simply the result of some older men who are more fit.

One sensible explanation could be that since male testosterone levels are known to drop with age, men who have testosterone dropping off faster than average might simply be aging faster, and therefore lower testosterone might simply be one of the signs that they are deteriorating/aging systemically at a faster rate than average.

In other words lower testosterone might not be the cause of more heart attacks. Aging faster might be the cause of both conditions.

In which case increasing testosterone wouldn't really help men's hearts.