Progesterone

What it is

Progesterone is a steroid hormone produced primarily by the ovarian corpus luteum after ovulation, with serum levels rising from under 1 nmol/L in the follicular phase to 15–90 nmol/L at the mid-luteal peak. Also called P4 or the "pregnancy hormone," progesterone is synthesized from cholesterol and serves as a biochemical precursor to estrogens, androgens, and cortisol. In Canada, micronized progesterone (brand name Prometrium) is the most commonly prescribed form and is listed on several provincial formularies, including Ontario's ODB and BC PharmaCare, making it accessible to most patients with a valid prescription.

Beyond reproduction, progesterone acts on the brain, cardiovascular system, bone, and immune tissue — a scope that has expanded clinical interest well beyond obstetrics.

Causes and mechanism

Progesterone is synthesized when luteinizing hormone (LH) triggers the corpus luteum — the follicular remnant left after ovulation — to convert cholesterol into progesterone via a series of enzymatic steps. During the first 10 weeks of pregnancy, the corpus luteum is the primary source; the placenta takes over thereafter. The adrenal cortex and, in smaller amounts, the testes also produce progesterone throughout life.

The hormone exerts its effects through two main pathways:

1. Nuclear progesterone receptors (PR-A and PR-B) — bind progesterone inside cells and regulate gene transcription, governing endometrial maturation, myometrial relaxation, and breast tissue differentiation.
2. Membrane-bound receptors and GABA-A modulation — progesterone metabolizes into allopregnanolone, a neurosteroid that potentiates GABA-A receptors, producing sedative and anxiolytic effects. This pathway underlies the sleep-promoting quality of oral micronized progesterone and is the basis for brexanolone (a synthetic allopregnanolone analogue) in postpartum depression.

Symptoms and diagnosis

Low progesterone is not a diagnosis in itself but a laboratory finding associated with several clinical pictures:

Clinical context	Typical serum finding	Common symptoms
Luteal phase defect / infertility	Mid-luteal P4 < 16 nmol/L	Irregular cycles, implantation failure
Recurrent pregnancy loss	P4 < 25 nmol/L in early pregnancy	Spotting, cramping
Perimenopause	Erratic or absent luteal rise	Heavy or irregular bleeding, poor sleep, mood changes
Postmenopause (no HRT)	< 1 nmol/L (expected)	Unopposed estrogen effects if exogenous estrogen is used

In Canada, serum progesterone is measured at LifeLabs or Dynacare, typically ordered on day 21 of a 28-day cycle (or 7 days before expected menstruation) to confirm ovulation. A result above 16–30 nmol/L is generally considered evidence of adequate ovulation, though reference ranges vary by laboratory.

Symptoms of low progesterone overlap significantly with other hormonal conditions — irregular or heavy periods, premenstrual mood changes, difficulty sleeping, and, in pregnancy, spotting or cramping. Because these symptoms are non-specific, serum testing is necessary before attributing them to progesterone deficiency.

Treatment options

Micronized progesterone (bioidentical) — Prometrium 100 mg or 200 mg capsules are structurally identical to endogenous progesterone. Oral administration produces allopregnanolone metabolites that improve sleep; vaginal use bypasses first-pass metabolism and delivers higher uterine concentrations with fewer systemic effects, which is preferred in fertility and early pregnancy support.

Synthetic progestogens (progestins) — Medroxyprogesterone acetate (MPA), norethindrone, and dydrogesterone are synthetic analogues with varying receptor profiles. They do not convert to allopregnanolone and carry a different risk-benefit profile from micronized progesterone, particularly regarding breast tissue and mood.

Clinical indications in Canada include:

• Endometrial protection in menopausal hormone therapy (MHT) for women with a uterus — the 2021 SOGC Menopause Guideline recommends progesterone or a progestin alongside estrogen to prevent endometrial hyperplasia
• Luteal phase support in assisted reproductive technology (ART) cycles
• Treatment of secondary amenorrhea and dysfunctional uterine bleeding
• Threatened or recurrent miscarriage (vaginal micronized progesterone, supported by the PRISM trial, 2019)

Canadian patients can access progesterone prescriptions through in-person gynecologists or family physicians, or through virtual platforms such as Felix, Maple, Cleo, or Phoenix, depending on provincial prescribing rules.

When to see a clinician in Canada

Consult a physician or nurse practitioner if you experience cycles shorter than 21 days or longer than 35 days, mid-cycle spotting, two or more consecutive pregnancy losses, or perimenopausal bleeding changes. If you are already taking estrogen therapy without a uterus, progesterone is generally not required — but confirm this with your prescriber. Patients on menopausal hormone therapy who have a uterus and are not taking a progestogen should discuss endometrial surveillance with their clinician, as unopposed estrogen raises endometrial cancer risk approximately 2–12-fold depending on duration of use.

Limitations and open questions

Research is still emerging on several fronts. The relative breast cancer risk of micronized progesterone versus synthetic progestins remains debated: observational data (including the French E3N cohort) suggest micronized progesterone may carry a lower breast cancer risk than MPA, but no large randomized controlled trial has directly compared the two over long durations. Health Canada has not issued a separate safety communication distinguishing micronized progesterone from progestins on breast cancer risk, and the product monographs for both carry similar class warnings.

The neuroprotective and cardioprotective roles of progesterone — suggested by preclinical and some observational data — have not been confirmed in adequately powered human trials. The optimal route, dose, and duration of progesterone for recurrent pregnancy loss outside of ART settings also remains an area of active study. Clinicians and patients should weigh current evidence with the understanding that guidelines in this area are likely to evolve.