When the GnRH analog binds to its receptor, there is a massive release of the hormones FSH (follicle-stimulating hormone) and LH (luteinizing hormone). This is called the flare up effect.
This release of gonadotropins occurs endogenously, that is, the pituitary gland, located in the brain, synthesizes and releases these hormones into the bloodstream.
The flare-up effect occurs approximately in the first two days after the administration of a GnRH agonist. After this time, the pituitary gland becomes desensitized to the initial effect of the agonist, and gonadotropin synthesis is blocked.