Why is understanding the relationship of testosterone to cardiovascular risk so important?

Epidemiological studies hint at a beneficial influence of endogenous circulating testosterone (T), or its metabolite dihydrotestosterone (DHT), such that men with lower concentrations of T or DHT appear to have poorer health outcomes including frailty, diabetes, cardiovascular disease, and mortality. Small interventional studies of T have shown favorable effects on surrogate outcome measures, but a large randomized controlled trial (RCT) with the prespecified outcome of cardiovascular events has not been performed and would be logistically demanding. In the absence of such a definitive RCT, there is a controversy about the cardiovascular risks of T-therapy fuelled by contradictory findings from retrospective analyses of insurance databases of men prescribed T. The US Testosterone Trials (T-Trials) are the largest published RCTs of T-therapy in older men with symptoms or signs of hypogonadism and circulating T <9.54 nmol l−1 at baseline. The T-Trials showed a modest benefit of T-therapy over a 12-month period on sexual function, a significant benefit in bone density and for anemia and neutral effect on cognition. The T-Trials cardiovascular sub-study was designed to determine the effects of T in these older men, and there was a statistically significant difference in the increase in noncalcified plaque volume in the T-treated group compared to placebo, but it is difficult to interpret these results due to differences in baseline coronary plaque burden (>50% difference) between the treatment and placebo arms of the subset involved. Therefore, there continues to be ongoing uncertainty over the effect of T-therapy on the cardiovascular system in men.