What is testosterone's effect on the growth spurt? I don't quite understand.
"During puberty, an increase in testosterone levels in boys causes enlargement of the reproductive organs such as the penis and testes, increased libido, increased frequency of erection, and the growth of facial, chest, nipple and pubic hair. Some maturing boys may experience a loss of hair on the scalp, referred to as alopecia.
The fat content beneath the skin also decreases, while muscle mass increases, as do height and rib cage size. The voice also deepens, and the jaws, chin and nose start to take on a more adult appearance. The Adam's apple in the throat also becomes more prominent." [Dr Ananya Mandal, MD, Testosterone Physiological Effects].
In other words the presence of testosterone is what triggers the development of the characteristics we associate with males. Growth in height is controlled by Human Growth Hormone [HGH], something that is shared between both men and women.