testosterone
What causes puberty to begin? The hypothalamus in the brain tells the pituitary gland to secrete hormones that target the testes. The main pituitary hormone involved is luteinizing hormone LH . It stimulates the testes to secrete testosterone. Testosterone, in turn, promotes protein synthesis and growth. It brings about most of the physical changes of puberty, some of which are shown in Figure below . In addition to the changes shown below, during puberty male facial hair begins to grow, the shoulders broaden, and the male voice deepens. You can watch an animation of these and other changes that occur in boys during puberty at the Interactive Body link: http://www. bbc. co. uk/science/humanbody/body/interactives/lifecycle/teenagers/ .