Testosterone is the chief male sex hormone, but is also present in much smaller amounts in women. In both sexes, the chemical substance is secreted by the adrenal glands, and in men, it is also produced by the interstitial cells of Leydig in the testes. Along with the other steroid hormones commonly described as androgens, testosterone is centrally involved in spermatogenesis and the development of secondary male sexual characteristics. It is, for instance, the increased production of testosterone during puberty that facilitates the deepening of the male voice as well as the growth of facial hair. Testosterone is also associated with protein synthesis stimulation and amplified muscle development, a fact that has greatly contributed to use of synthetic substances similar to the hormone for athletic performance enhancement.