As men age, it is natural for them to gradually lose testosterone. When testosterone levels are low, men can experience fatigue, loss of lean muscle and trouble gaining muscle mass, decreased sex drive, high blood pressure, premature aging, and sexual dysfunction. Low T is not just a symptom of aging, however, and can often arise from underlying health issues. These underlying health conditions include obesity, diabetes, infections, hormonal disorders, cancer, and kidney and liver disease. Some medications have also been known to lower testosterone levels.