FIGURE 3.

Key hormones involved in the initiation of puberty. Ultimately, puberty is initiated by profound increases in gonadotropin releasing hormone (GnRH) that initiate cyclic synthesis and release of gonadotropins, which then regulate reproduction, reproductive behaviors and secondary sexual characteristics, and sex steroid levels. Several factors may promote the increase in GnRH that precedes puberty, including: reduced sensitivity to inhibition of GnRH secretion by the sex steroids; increased leptin, which reduces inhibition of gonadotropins and their releasing hormones by the gonadotropin inhibiting hormone (GnIH); decreases in melatonin, with an associated reduction in GnIH production; and increases in kisspeptin, perhaps triggered by a “somatometer” or “pubertal clock,” which acts to further increase GnRH secretion. Orange boxes denote pubertal triggers. Blue text indicates sites of action or hormone release. Solid lines indicate positive feedback. Dashed lines indicate inhibitory influences.