Testosterone is the primary male hormone responsible for regulating sex differentiation, producing male sex characteristics, spermatogenesis, and fertility. Testosterone affects almost every tissue in the body. It is vitally important to a man’s health and well-being. It increases metabolism. It produces insulin sensitivity. Testosterone is responsible for the development of primary sexual development, which includes testicular. Testosterone is also involved in regulating secondary male characteristics, which are those responsible for masculinity. These secondary sex characteristics include male hair patterns, vocal changes, and voice deepening, anabolic effects, which include growth spurts in puberty (testosterone increases tissue growth at the epiphyseal plate early on and eventual closure of plate later in puberty) and skeletal muscle growth (testosterone stimulates protein synthesis). Testosterone also stimulates erythropoiesis, which results in a higher hematocrit in males versus females.