Testosterone levels peak by early adulthood and drop as you age—about 1% to 2% a year beginning in the 40s. As men reach their 50s and beyond, this may lead to signs and symptoms, such as impotence or changes in sexual desire, depression or anxiety, reduced muscle mass, less energy, weight gain, anemia, and hot flashes. While falling testosterone levels are a normal part of aging, certain conditions can hasten the decline. These include:

The potential downside of this positive feedback loop, Coates argues, is that testosterone levels can eventually surge past optimal levels and have the opposite effect – leading to overconfidence and poor decision-making. When this happens to animals, Coates, observed, they “go out in the open, pick too many fights [and] patrol areas that are too large…Risk taking becomes risky behaviour.”
Some of these signs and symptoms can be caused by various underlying factors, including medication side effects, obstructive sleep apnea, thyroid problems, diabetes and depression. It's also possible that these conditions may be the cause of low testosterone levels, and treatment of these problems may cause testosterone levels to rise. A blood test is the only way to diagnose a low testosterone level.
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