Progesterone

Progesterone is a naturally occurring steroid hormone, chemically pregn-4-ene-3,20-dione, and the principal progestogen in humans. It is synthesized from cholesterol by way of pregnenolone, and in women of reproductive age it is produced mainly by the corpus luteum of the ovary, with the placenta becoming the dominant source during pregnancy and the adrenal glands and nervous tissue contributing smaller amounts ^[1].

The hormone has wide-ranging physiological roles. Across the menstrual cycle its rise in the second half prepares the endometrium for a possible pregnancy; during pregnancy it supports implantation, calms the muscle of the uterus and helps maintain gestation; and it participates in the development of the breasts ^[1]. Progesterone is also an active neurosteroid, with effects on the central nervous system that include neuroprotection and support for myelination ^[1].

As a medication, natural progesterone is used in several settings. In menopausal hormone therapy it is combined with estrogen to protect the lining of the womb from estrogen's stimulating effect ^[4]; it is used as luteal support in in vitro fertilisation; and it is given to reduce the risk of preterm birth in selected pregnancies. The evidence for the last of these is strong: an individual-patient-data meta-analysis found that vaginal progesterone lowers the risk of preterm birth and improves newborn outcomes in women with a short cervix ^[2], and the large EPPPIC collaboration confirmed that vaginal progesterone and injectable 17-hydroxyprogesterone caproate reduce early birth in high-risk singleton pregnancies, with the greatest benefit when the cervix is short ^[3]. Progesterone has also been studied, with less convincing results, in areas such as traumatic brain injury ^[1].

Natural progesterone is a prescription medicine available in several forms, including oral micronized capsules, vaginal gels and pessaries, and injections; because its absorption by mouth is limited, micronized and vaginal preparations are commonly used. It is distinct from the many synthetic progestins used in contraception and other therapies. Side effects tend to be mild and can include drowsiness (particularly with oral capsules), breast tenderness, mood changes and irregular bleeding ^[1][4].

Progesterone is an endogenous hormone with a long record of clinical use, and systemic administration is normally prescription-controlled. Oral micronized, vaginal, and intramuscular formulations are used in menopausal hormone therapy, luteal support during fertility treatment, and prevention of preterm birth in selected patients.

The most common effects reflect its neurosteroid metabolite allopregnanolone and include drowsiness, dizziness, and sedation, which is why oral dosing is usually scheduled at night. Other effects include breast tenderness, bloating, headache, and menstrual changes. A subset of individuals report negative mood or dysphoria at intermediate neurosteroid concentrations, a biphasic pattern also seen with related GABAergic steroids.

Intravenous progesterone solutions used in the traumatic brain injury trials were associated with an increased rate of phlebitis and thrombophlebitis. Abrupt discontinuation after sustained exposure can transiently raise anxiety and seizure susceptibility through GABA-A receptor subunit remodeling, the so-called neurosteroid withdrawal phenomenon. Transdermal progesterone creams are widely sold over the counter, but their systemic absorption and clinical value are debated, and they are not equivalent to prescribed systemic progesterone. As with any systemic hormone, use should be individualized and medically supervised.

Progesterone is metabolised largely by CYP3A4, and that single fact drives most of its interactions.

Strong CYP3A4 inhibitors raise exposure; ketoconazole increases the bioavailability of oral micronized progesterone, and ritonavir, clarithromycin, itraconazole and grapefruit juice work by the same route. Inducers do the opposite. Rifampicin, carbamazepine, phenytoin, phenobarbital and St John's wort accelerate its clearance and can leave a progestogenic effect too weak to do its job, which matters most where progesterone is providing endometrial protection alongside estrogen.

Oral micronized progesterone is also converted to allopregnanolone, a positive allosteric modulator at GABA-A receptors, which is why it is sedating. That sedation is additive with benzodiazepines, z-drugs, alcohol, opioids and sedating antihistamines, and the combined effect on alertness is larger than either alone. Vaginal progesterone largely bypasses first-pass metabolism and shows less of both effects.

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Progesterone was isolated and its structure established in 1934 by several competing groups, and the corpus luteum hormone was soon recognized as essential to gestation. In 1941 Hans Selye reported that large parenteral doses of progesterone and related steroids produced reversible surgical anesthesia in rodents, the first clear hint that pregnane steroids act directly on the nervous system. The modern neurosteroid concept emerged from the work of Etienne-Emile Baulieu and colleagues, who showed in the 1980s that certain steroids persist in the brain independently of the adrenals and gonads.

In 1986 Majewska and coworkers demonstrated that reduced progesterone metabolites are barbiturate-like modulators of the GABA-A receptor, and in 1988 Su and colleagues found that progesterone binds sigma receptors, proposing a bridge between the endocrine, nervous, and immune systems. Through the 1990s and 2000s the groups of Baulieu, Schumacher, Koenig, and De Nicola established progesterone's roles in myelination and neuroprotection.

Encouraging animal data and a positive phase II pilot (ProTECT, 2007) led to two large phase III trauma trials, ProTECT III and SyNAPSe, both reported in 2014 as negative. The underlying pathway nevertheless remains clinically vindicated through the approval of allopregnanolone-based antidepressants for postpartum depression.

Progesterone is one of the most important and widely trusted hormones in medicine, indispensable to reproduction and a staple of both fertility treatment and menopausal care. Clinicians regard natural micronized progesterone highly because it is molecularly identical to the body's own hormone, and it is used with confidence to protect the lining of the womb when estrogen is given in hormone therapy.

Its obstetric value is supported by strong evidence: vaginal progesterone can reduce the risk of preterm birth in women with a short cervix and can improve the chance of a live birth in women with early pregnancy bleeding and prior miscarriages. It is honest to note that its benefit is not universal; rigorous reviews find little effect in unselected women, so its advantages are real but specific to the right patients. Backed by decades of use and its place on the WHO essential medicines list, progesterone remains a genuinely essential and well-validated therapy.