Testosterone is a male sex hormone, also known as androgen, that is primarily produced in the testes. Proper levels of testosterone are fundamental for the normal sexual development of males, as well as for sexual functions. Testosterone levels are particularly high during puberty when boys undergo biological and physiological changes as they transition into adulthood, such as the development of facial hair, increased muscle mass, and a deeper voice. Low-T, or low testosterone, is a condition characterized by lower than normal levels of testosterone, and may also be referred to as a testosterone deficiency, or Testosterone Deficiency Syndrome (TD). Testosterone levels are considered lower than normal when the body has less than 300 nanograms of testosterone per deciliter (ng/dL). Symptoms associated with Low-T include: • Low sex drive  • Fatigue • Reduced lean muscle mass • Irritability • Erectile dysfunction • Depression Because there are many other possible reasons for these symptoms, such as injury to the testicles, diabetes, overweight and obesity, substance abuse, and certain congenital (present since birth) conditions, it is important to receive a hormone test to determine if low levels of testosterone is the cause.