When testosterone levels are too low or too high, the effects can reach many parts of the body. This chapter describes the symptoms of low testosterone, such as fatigue, mood changes and reduced libido, and some possible causes. It explains that high levels are rare in males and can lead to early puberty, while in females high testosterone may cause acne, excess hair and irregular cycles, with PCOS one of the leading causes. The chapter shows how imbalance can create a cycle involving body fat and inflammation, looks at lifestyle factors such as diet, alcohol, sleep and stress, and explains why proper diagnosis matters.