Almost all circulating testosterone and oestradiol are reversibly bound to testosterone-binding globulin (TEBG; also known as sex hormone-binding globulin: SHBG); however, only the free sex steroid is active, and TEBG appears to serve as a reservoir for storing and inactivating sex steroids. At puberty, levels of TEBG decrease slightly from prepubertal levels in girls, and a greater decrease occurs in boys. Thus, in adults, the plasma concentration of total testosterone is almost twenty times greater in men than in women, but the free testosterone is forty times greater in men.
Although prolactin levels rise in girls during puberty, they do not effectively change in boys; this is due to the increased oestradiol secretion that occurs in females compared to levels in males.
