puberty is triggered by the brain, which starts the hormones flowing through the bodies of both boys and girls. The first stage of hormonal reactions starts when the hypothalamus releases a hormone called gonadotropin-releasing hormone (GnRH). This hormone, in turn, signals the pituitary gland, an endocrine gland the size of a pea at the base of the brain, to release luteinizing hormone (LH) and follicle stimulating hormone (FSH). These are the hormones that start the process of sexual development.
oestrogen