Testosterone
. Testosterone, principal male hormone, or androgen , produced mainly in the Leydig cells in the male testes. The Leydig cells also produce two other androgens of less potency and in much smaller quantities. Testosterone stimulates the development of the male secondary sex characteristics after puberty , causing growth of the beard and pubic hair, development of the penis, and change of voice. The hormone also aids in the growth, muscular development, and masculine body contour of the adult male. See Hormone . If, before puberty, little or no testosterone secretion occurs, secondary sexual characteristics fail to develop. In addition, the long bones continue to grow abnormally and give the patient a characteristic tall but effeminate build. If testicular failure follows puberty, less obvious changes occur, although gradual recession of beard, weakening of muscles, increased deposition of fat, and change in voice may develop slowly, with infertility usually present and decreased libido ...