If you’re a healthy guy in your 30s and 40s, your testosterone will be declining—but that doesn’t mean you actually need treatment. “If you go in and say, ‘Well, you know, in the past 10 years I’ve gotten more tired, I’m having trouble keeping weight off…’ that’s simply not enough—it’s a natural phenomenon!” Jacques Baillargeon, ., an epidemiologist at the University of Texas Medical Branch at Galveston, told Men’s Fitness. However, if you’re a man north of 50, and you’re having difficulty getting it up, you’re feeling depressed, and you’re generally unhappy, you should seek out TRT.
But I'm not more aggressive—a behavior change often tied to testosterone. That's not surprising to Robert Sapolsky, ., a neuroendocrinologist at Stanford University and a leading researcher on stress and behavior. "It's really not the case that testosterone 'causes' aggressive behavior," he says. "Instead, it makes the brain more sensitive to social cues that trigger aggression. And in support of that, a guy's testosterone level isn't a very good predictor of how likely he is to be aggressive."
Other significant adverse effects of testosterone supplementation include acceleration of pre-existing prostate cancer growth in individuals who have undergone androgen deprivation; increased hematocrit , which can require venipuncture in order to treat; and, exacerbation of sleep apnea .  Adverse effects may also include minor side-effects such as acne and oily skin, as well as, significant hair loss and/or thinning of the hair, which may be prevented with 5-alpha reductase inhibitors ordinarily used for the treatment of benign prostatic hyperplasia , such as finasteride .  Exogenous testosterone may also cause suppression of spermatogenesis , leading to, in some cases, infertility.  It is recommended that physicians screen for prostate cancer with a digital rectal exam and prostate-specific antigen (PSA) level before starting therapy, and monitor PSA and hematocrit levels closely during therapy.