Low testosterone levels, also known as hypogonadism, is a medical condition where the body produces low levels of testosterone, the male sex hormone. It can cause a variety of symptoms such as decreased sex drive, erectile dysfunction, decreased muscle mass, increased body fat, mood swings, fatigue and decreased bone density. Low testosterone levels can be caused by various factors, including aging, injury to the testicles, genetic conditions, and certain medical treatments. To diagnose low testosterone levels, a doctor will usually order a blood test to measure testosterone levels and may also consider a patient's symptoms and medical history. Treatment for low testosterone levels may include testosterone replacement therapy, which can be in the form of injections, gels, or pellets.