is a C-21 steroid hormone
involved in the female menstrual cycle
) and embryogenesis
of humans and other species. Progesterone belongs to a class of hormones called progestogens
, and is the major naturally occurring human progestogen.
Progesterone should not be confused with progestins, which are synthetically produced progestogens.
Progesterone was independently discovered by four research groups.
Willard Myron Allen co-discovered progesterone with his anatomy professor George Washington Corner at the University of Rochester Medical School in 1933. Allen first determined its melting point, molecular weight, and partial molecular structure. He also gave it the name Progesterone derived from Progestational Steroidal ketone.
Like other steroids, progesterone consists of four interconnected cyclic hydrocarbons. Progesterone contains ketone and oxygenated functional groups, as well as two methyl branches. Like all steroid hormones, it is hydrophobic.
Progesterone, * like all other steroid hormones
, is synthesized from pregnenolone
, a derivative of cholesterol
. This conversion takes place in two steps. The 3-hydroxyl
group is converted to a keto
group and the double bond
is moved to C-4, from C-5.
Progesterone is the precursor of the mineralocorticoid aldosterone, and after conversion to 17-hydroxyprogesterone (another natural progestogen) of cortisol and androstenedione. Androstenedione can be converted to testosterone, estrone and estradiol.
Progesterone is produced in the adrenal glands
, the gonads (specifically after ovulation in the corpus luteum
), the brain, and, during pregnancy, in the placenta
In humans, increasing amounts of progesterone are produced during pregnancy:
- Initially, the source is the corpus luteum that has been "rescued" by the presence of human chorionic gonadotropins (hCG) from the conceptus.
- However, after the 8th week production of progesterone shifts to the placenta. The placenta utilizes maternal cholesterol as the initial substrate, and most of the produced progesterone enters the maternal circulation, but some is picked up by the fetal circulation and used as substrate for fetal corticosteroids. At term the placenta produces about 250 mg progesterone per day.
- An additional source of progesterone is milk products. They contain much progesterone because on dairy farms cows are milked during pregnancy, when the progesterone content of the milk is high. After consumption of milk products the level of bioavailable progesterone goes up. This observation has resulted in concern that diets high in dairy products might induce pet and human diseases.
In women, progesterone levels are relatively low during the preovulatory phase of the menstrual cycle, rise after ovulation, and are elevated during the luteal phase. Progesterone levels tend to be < 2 ng/ml prior to ovulation, and > 5 ng/ml after ovulation. If pregnancy occurs, progesterone levels are initially maintained at luteal levels. With the onset of the luteal-placental shift in progesterone support of the pregnancy, levels start to rise further and may reach 100-200 ng/ml at term. Whether a decrease in progesterone levels is critical for the initiation of labor has been argued and may be species-specific. After delivery of the placenta and during lactation, progesterone levels are very low.
Progesterone levels are relatively low in children and postmenopausal women. Adult males have levels similar to those in women during the follicular phase of the menstrual cycle.
Progesterone exerts its action primarily through the intracellular progesterone receptor
although a distinct, membrane bound progesterone receptor has also been postulated. Progesterone has a number of physiological effects which are amplified in the presence of estrogen
. Estrogen through estrogen receptors upregulates
of progesterone receptors.
Progesterone is sometimes called the "hormone of pregnancy", and it has many roles relating to the development of the fetus:
- Progesterone converts the endometrium to its secretory stage to prepare the uterus for implantation. At the same time progesterone affects the vaginal epithelium and cervical mucus, making the mucus thick and impermeable to sperm. If pregnancy does not occur, progesterone levels will decrease, leading, in the human, to menstruation. Normal menstrual bleeding is progesterone withdrawal bleeding.
- During implantation and gestation, progesterone appears to decrease the maternal immune response to allow for the acceptance of the pregnancy.
- Progesterone decreases contractility of the uterine smooth muscle.
- In addition progesterone inhibits lactation during pregnancy. The fall in progesterone levels following delivery is one of the triggers for milk production.
- A drop in progesterone levels is possibly one step that facilitates the onset of labor.
The fetus metabolizes placental progesterone in the production of adrenal mineralo- and glucosteroids.
Progesterone, like pregnenolone
, belongs to the group of neurosteroids
that are found in high concentrations in certain areas in the brain and are synthesized there.
Neurosteroids affect synaptic functioning, are neuroprotective, and affect myelination. They are investigated for their potential to improve memory and cognitive ability.
Progesterone as neuroprotectant affects regulation of apoptotic genes.
Its effect as a neurosteroid works predominantly through the GSK-3 beta pathway, as an inhibitor. (Other GSK-3 beta inhibitors include bipolar mood stabilizers, lithium and valproic acid.)
- It raises epidermal growth factor-1 levels, a factor often used to induce proliferation, and used to sustain cultures, of stem cells.
- It increases core temperature (thermogenic function) during ovulation.
- It reduces spasm and relaxes smooth muscle. Bronchi are widened and mucus regulated. (Progesterone receptors are widely present in submucosal tissue.)
- It acts as an antiinflammatory agent and regulates the immune response.
- It reduces gall-bladder activity.
- It normalizes blood clotting and vascular tone, zinc and copper levels, cell oxygen levels, and use of fat stores for energy.
- It assists in thyroid function, in bone building by osteoblasts, in bone, teeth, gums, joint, tendon, ligament and skin resilience and in some cases healing by regulating various types of collagen, and in nerve function and healing by regulating myelin.
- It appears to prevent endometrial cancer (involving the uterine lining) by regulating the effects of estrogen.
The use of progesterone and its analogues have many medical applications -- both to address acute situations, and to address the long-term decline of natural progesterone levels. Because of the poor bioavailability of progesterone when taken orally, many synthetic progestins have been designed. However, the roles of progesterone may not be fulfilled by the synthetic progestins which in some cases were designed solely to mimic progesterone's uterine effects.
Progesterone is poorly absorbed by oral ingestion unless micronised
and in oil, or with fatty foods; it does not dissolve in water. Products such as Prometrium, Utrogestan, Minagest and Microgest are therefore capsules containing micronised progesterone in oil - in all three mentioned the oil is peanut
oil, which may cause serious allergic reactions
in some people, but compounding pharmacies
, which have the facilities and licenses to make their own products, can use alternatives. Vaginal and rectal application is also effective, with products such as CRINONE and PROCHIEVE bioadhesive progesterone vaginal gels (the only progesterone products FDA-approved for use in infertility and during pregnancy) and Cyclogest, which is progesterone in cocoa butter
in the form of pessaries
. Progesterone can be given by injection
, but because it has a short half-life
they need to be daily. Implants, for a longer period, are also available. Marketing of progesterone phamaceutical products, country to country, varies considerably, with many countries having no oral progesterone products marketed, but they can usually be specially imported by pharmacies through international wholesalers
"Natural progesterone" products derived from yams, do not require a prescription. Wild yams contain a plant steroid called diosgenin, however there is no evidence that the human body can metabolize diosgenin into progesterone. Diosgenin can however be chemically converted into progesterone in the lab.
- Progesterone is used to support pregnancy in Assisted Reproductive Technology (ART) cycles such as In-vitro Fertilization (IVF). While daily intramuscular injections of progesterone have been the standard route of administration, a recent meta-analysis showed that the intravaginal route with an appropriate dose and dosing frequency is equivalent to daily intramuscular injections.
- Progesterone is used to control anovulatory bleeding. It is also used to prepare uterine lining in infertility therapy and to support early pregnancy. Patients with recurrent pregnancy loss due to inadequate progesterone production may receive progesterone.
- Progesterone is being investigated as potentially beneficial in treating multiple sclerosis, since the characteristic deterioration of nerve myelin insulation halts during pregnancy, when progesterone levels are raised; deterioration commences again when the levels drop.
- Vaginally dosed progesterone is being investigated as potentially beneficial in preventing preterm birth in women at risk for preterm birth. The initial study by Fonseca suggested that vaginal progesterone could prevent preterm birth in women with a history of preterm birth.
A subsequent and larger study showed that vaginal progesterone was no better than placebo in preventing recurrent preterm birth in women with a history of a previous preterm birth, but a planned secondary analysis of the data in this trial showed that women with a short cervix at baseline in the trial had benefit in two ways: a reduction in births less than 32 weeks and a reduction in both the frequency and the time their babies were in intensive care. In another trial, vaginal progesterone was shown to be better than placebo in reducing preterm birth prior to 34 weeks in women with an extremely short cervix at baseline. An editorial by Roberto Romero discusses the role of sonographic cervical length in identifying patients who may benefit from progesterone treatment.
Note that methods of hormonal contraception do not contain progesterone but a progestin.
Progesterone may affect male behavior.
Lawn chemicals and no-till agricultual practices may disturb both estrogen and progesterone metabolism.
Since most progesterone in males is created during testicular production of testosterone, and most in females by the ovaries, the shutting down (whether by natural or chemical means), or removal, of those inevitably causes a considerable reduction in progesterone levels. Previous concentration upon the role of progestagens (progesterone and molecules with similar effects) in female reproduction, when progesterone was simply considered a "female hormone", obscured the significance of progesterone elsewhere in both sexes.
The tendency for progesterone to have a regulatory effect, the presence of progesterone receptors in many types of body tissue, and the pattern of deterioration (or tumor formation) in many of those increasing in later years when progesterone levels have dropped, is prompting widespread research into the potential value of maintaining progesterone levels in both males and females.
It has been observed in animal models that females have reduced susceptibility to traumatic brain injury and this protective effect has been hypothesized to be caused by increased circulating levels of estrogen and progesterone in females. A number of additional animal studies have confirmed that progesterone has neuroprotective effects when administered shortly after traumatic brain injury. Encouraging results have also been reported in human clinical trials.
The mechanism of progesterone protective effects may be the reduction of inflammation which follows brain trauma.
- Kimball JW Progesterone. Kimball's Biology Pages. Retrieved on 2008-06-18..
- Progesterone Resource Center. PMS, Menopause, and Progesterone Resource Center. Oasis Advanced Wellness, Inc. Retrieved on 2008-06-18..