Progestagens



Progestagens (also spelled progestogens or gestagens) are a group of hormones including progesterone. The progestagens are one of the five major classes of steroid hormones, in addition to the estrogens, androgens, mineralocorticoids, and glucocorticoids. All progestagens are characterized by their basic 21 carbon skeleton, called a pregnane skeleton (C21). Similarly, the estrogens possess an estrane skeleton (C18) and androgens, an andrane skeleton (C19).

Progestagens are named for their function in maintaining pregnancy (pro gestational), although they are also present at other phases of the estrous and menstrual cycles. Although the progestagen class of hormones includes all steroids with a pregnane skeleton, exogenous or synthetic hormones are usually referred to as progestins.

Progestagens as precursors to other steroids
In the first step in the steroidogenic pathway, a cholesterol molecule is converted into pregnenolone (P5). P5 and other members of the progestagen class of steroids serve as precursors to all other steroids, including the estrogens, androgens, mineralocorticoids, and glucocorticoids. P5, P4, 17α-hydroxypregnenolone, and 17α-hydroxyprogesterone are all endogenously produced intermediates in the pathway. Thus, all tissues producing steroids, such as the adrenals, ovaries, and testes must be capable of producing progestagens.

In some tissues, the enzymes required for the final product are not all located in a single cell; for example, in ovarian follicles, cholesterol is converted to androstenedione, an androgen, in the theca cells, which is further converted into estrogen in the granulosa cells. Fetal adrenal glands also produce P5 in some species, which is converted into P4 and estrogens by the placenta (see below). In the human, the fetal adrenals produce dihydroepiandrosterone via the P5 pathway.

Progestagen production by the ovary
Progesterone (P4) is the major progestagen produced by the corpus luteum in all mammalian species. Luteal cells possess the necessary enzymes to convert cholesterol to pregnenolone (P5), which is subsequently converted into P4. P4 is highest in the diestrus phase of the estrous cycle.

Progestagen production by the placenta
The role of the placenta in progestagen production varies by species. In the sheep, horse, and human, the placenta takes over the majority of progestagen production, whereas in other species the corpus luteum remains the primary source of progestagen. In the sheep and human, P4 is the major placental progestagen.

The equine placenta produces a variety of progestagens, primarily 5αDHP and 20α5P, beginning on day 60. A complete luteo-placental shift occurs by day 120-150.

Birth control

 * Progestogens are used alone in progestogen only pills, or with an estrogen in combined oral contraceptive pills, a contraceptive patch, and a contraceptive vaginal ring.
 * Medroxyprogesterone acetate (Depo-Provera) and norethindrone enantate (Noristerat) are used by depot injection.
 * Etonogestrol is released by subcutaneous implants (Implanon). Norplant and Jadelle implants release levonorgestrel.
 * Levonorgestrel is released by the intrauterine system IUS (Mirena).

Antiandrogen
Progestinic compounds decrease luteinizing hormone (LH) levels and as such, will have antiandrogenic properties in trans-women and cisgender males alike, due to decreased LH stimulation of the testes. Cyproterone is a common example of a progestinic medication, and is an effective antiandrogen, which has the added benefit of blocking androgen receptors in addition to the progestinic feedback to decrease LH levels.

Progestogen withdrawal bleeding
In a normal menstrual cycle, a sudden rise in progesterone levels triggers menstruation. Norethindrone acetate (brand name Aygestin) and medroxyprogesterone acetate (brand name Provera) may be used to artificially induce progestogen withdrawal bleeding.