Let’s face it, there’s no one thing that makes you a “real man,” but your testosterone levels play a pretty important role. Testosterone is the hormone that makes a man’s body develop sperm, and is responsible for increased muscle mass, all that body hair (whether you want it or not), and giving males a deeper voice. Beyond the physical, low testosterone can cause significant health problems for men, including fatigue, sleeplessness, depression, and increased fat storage.

It’s important to understand this misunderstood hormone. No, it doesn’t make you more aggressive—in fact, there’s evidence that low testosterone causes irritability and anger. Knowing the truth about testosterone, what it does to your body, and how to properly monitor it are keys to keeping your mental and physical health running optimally.