Gonadotrophin-releasing hormone (GnRH) is produced and secreted by specialised nerve cells which can be found in the hypothalamus of the brain. These are then released into small blood vessels into order to be transported to the pituitary gland, where they are responsible for stimulating FSH and LH hormones.
During puberty there is a rapid elevation in the levels of GnRH and this triggers the onset of sexual maturation. At this point the production of GnRH, FSH and LH are controlled by the levels of progesterone or oestrogens (in women) and the levels of testosterone (in men). So for example, if the levels of these hormones rises, then this triggers the production of GnRH to be decreases and vice versa.