Testosterone is an essential hormone for both men and women. It affects your bone and muscle growth as well as facial and body hair growth, but testosterone also determines your sex drive, fertility, red blood cell production, and even your mood. Low testosterone—or low T—can affect many areas of your overall health, and testosterone replacement therapy comes with its own set of risks and rewards. If you think that your testosterone levels may be low, a healthcare provider will be able to discern whether you have low T and offer a range of options about what you can do next.