As I’ve heard it explained, the HGH is intended to speed up growth before elevated testosterone levels cause one’s long-bones to close, because once they close you can’t grow taller.

But I’ve also heard of a more elegant solution to the problem; that bones don’t close because of the testosterone, they close because the body converts the testosterone into estrone by a process called aromatization, and the estrone causes the bones to close. And there are anti-aromatase drugs available to stop that conversion, one aromatase blocker is called "Arimidex". One or two of the parents here have said that they used it with their boys, with fewer side-effects than HGH.