Testosterone is the major male sex hormone and is produced in the testes. It controls male sexual development and reproduction. Testosterone levels increase during puberty to an adult peak but may decrease in the elderly male. Testosterone is produced by the Leydig cells of the testes. In adult males, testosterone levels show a diurnal variation, with the highest levels detected in the early morning. Levels also increase after exercise and gradually decrease with age.
In women, testosterone levels are 5% to 10% of those in males. Only 2% to 3% of testosterone circulate in the free, biologically active form; the remainder is bound to sex hormone-binding globulin or albumin.