Why is progestin used for irregular endometrial shedding?

Irregular shedding of the endometrium is due to dysfunction of the hypothalamic-pituitary-ovarian axis. Progesterone both regulates the hypothalamic-pituitary-ovarian axis and directly affects the endometrium to make it shed intact, thus achieving a therapeutic effect.
Short-acting oral contraceptives can also be used to treat irregular endometrial shedding in those without fertility requirements.