You've probably heard of testosterone and, while it certainly plays an important role in men's health, it's only one of several hormones at work. Male-produced hormones each fuel a complex endocrine system that sends signals to organs throughout the body, from the brain to the testes. These hormone levels change from hour to hour [source: WebMD]. And, if hormones get out of balance or begin to decline (which happens at about age 40, sometimes sooner), the body begins to store too much fat and prompts a person to eat when he is not really hungry. This slippery slope leads to metabolic syndromes, such as diabetes. A hormone imbalance also hampers a man's ability to fight stress, while making him feel exhausted, anxious, irritable and less interested in sex [source: Hurlock]. If that doesn't sound like a cocktail for disaster, we don't know what does. So, what role does testosterone play in this cast of hormones? Maybe not the one you think. Find out why, on the next page.