GnRH agonists/antagonists

Gonadotropin releasing hormone (GnRH) agonists/antagonists lower the production of sex hormones. This includes testosterone in people with testicles, and estrogen in those with ovaries. Since they both achieve the same results, they have similar uses, such as for treating hormone related conditions such as breast and prostate cancer, fibrocystic breasts, endometriosis, and polycystic ovarian disease (PCOS).

Both kinds of compounds work by stopping the production of GnRH hormone by the pituitary gland (though they have different methods of action), which tell the ovaries or testes to produce estrogen or testosterone.

Agonists overstimulate the GnRH receptor, leading to an initial spike in FSH and LH (and thus a spike in estradiol in those with ovaries and testosterone in those with testes), followed by a drop in FSH, LH, estradiol, and testosterone due to receptor downregulation.

Antagonists compete with GnRH for the GnRH receptor, but don't act as it, effectively halting FSH and LH production. They have no hormone surge as with agonists.

Agonists are medications such as leuprolide (Lupron), goserelin (Zoladex), and buserelin. Leuprolide is already used as a hormone blocker to suppress puberty, and is used in endometriosis treatment. Leuprolide is also used as a "puberty blocker" in transgender people, and buserelin is sometimes used in breast cancer treatment is conjunction with other medicines like tamoxifen.

Antagonists include medications like cetrorelix, degarelix, and ganirelix. Both are often used for treatment of fibrocystic breasts and endometriosis, and cancers such as breast cancer and prostate cancer.

Application to transition
Both kinds of GnRH suppressors may be used to reduce levels of unwanted hormones produced by the body, and subsequently agonists could be used with a combination of hormones, like estrogen + a SERM (tamoxifen/raloxifene), testosterone and finasteride, etc.