Progesterone: Functions & Testing

Progesterone is a steroid hormone best known for preparing the body for pregnancy and balancing the effects of estrogen. It rises in the second half of the menstrual cycle and plays roles beyond reproduction in both women and men.

What progesterone is

Progesterone is a steroid hormone made from cholesterol. It belongs to a class called progestogens and works closely with estradiol to coordinate the menstrual cycle and support early pregnancy. The name itself reflects its role — it is the "pro-gestational" hormone, meaning it favors the establishment and maintenance of a pregnancy. Like other steroid hormones, progesterone is fat-soluble, travels in the blood partly bound to carrier proteins, and acts by entering cells and binding to specific progesterone receptors that switch certain genes on or off.

Progesterone also sits at an important junction in the body's steroid pathway. It is made from pregnenolone, and it in turn serves as a building block the body can use to make other steroid hormones, including some adrenal hormones such as cortisol and aldosterone, as well as androgens. This precursor role means progesterone is present and useful in everyone, not only in people who menstruate.

Where it is produced

In people who menstruate, the main source is the corpus luteum — the structure that remains in the ovary after an egg is released each cycle. The corpus luteum is essentially a temporary, progesterone-secreting gland that forms from the follicle after ovulation. During pregnancy, the placenta gradually takes over as the major source, producing progesterone in much larger amounts to sustain the pregnancy. The adrenal glands produce smaller amounts in everyone as part of normal steroid synthesis, and the testes contribute modest amounts in men.

What it does across body systems

Progesterone's effects are most visible in the reproductive system, but its receptors appear in several tissues, so its influence is broader than reproduction alone.

• Prepares the uterus: After ovulation, progesterone transforms the uterine lining (endometrium) from a proliferating tissue into a secretory, nutrient-rich surface that can receive and support a fertilized egg.
• Supports pregnancy: It helps maintain the uterine lining, quiets uterine muscle to reduce contractions in early pregnancy, and prepares breast tissue for later milk production.
• Balances estrogen: It counteracts some of estrogen's growth-promoting effects on the uterine lining, which is one reason the two hormones are considered as a pair.
• Body temperature: The rise in progesterone after ovulation produces a small increase in resting body temperature, the basis of temperature-based cycle tracking.
• Brain and mood: Progesterone is converted in the brain into compounds that interact with calming neural pathways, which may relate to its effects on sleep and mood for some people.
• Precursor role: It serves as a step in making other steroid hormones throughout the body.

Regulation and the feedback loop

Progesterone is tied to the menstrual cycle through the hypothalamic–pituitary–gonadal (HPG) axis. The hypothalamus releases gonadotropin-releasing hormone, which prompts the pituitary to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH). After the mid-cycle LH surge triggers ovulation, the corpus luteum forms and secretes progesterone, causing levels to rise sharply in the luteal (second) phase. If pregnancy does not occur, the corpus luteum breaks down after roughly two weeks, progesterone falls, and the drop triggers menstruation. If pregnancy occurs, signals from the developing embryo (human chorionic gonadotropin) and later the placenta keep progesterone elevated. Because of this pattern, progesterone is low in the first (follicular) half of the cycle and higher in the second half.

Progesterone across the lifespan

The pattern of progesterone changes markedly with life stage. Before puberty, ovulation has not begun, so the cyclic luteal-phase rise is absent. Once regular ovulatory cycles are established, progesterone follows the familiar low-then-high pattern each month. During pregnancy, the placenta produces progesterone in large and rising amounts to sustain the uterine lining and quiet uterine muscle. As the reproductive years end and ovulation becomes less regular, the luteal rise becomes less reliable, and after menopause — when the ovaries no longer release eggs — cyclic ovarian progesterone production ceases and levels remain low. Understanding which life stage a person is in is therefore essential to making sense of any single measurement, because the same value can be entirely expected in one context and unusual in another.

What high or low levels can be associated with

Lower progesterone in the luteal phase may be associated with cycles in which ovulation did not occur, since the corpus luteum that produces it forms only after an egg is released. Elevated levels naturally occur in the luteal phase and during pregnancy, when the placenta produces large amounts. Outside those settings, unusually high progesterone is uncommon and may prompt a clinician to look at the adrenal glands or other sources. Because progesterone partners with estradiol, clinicians often consider the two together along with estradiol. These associations are qualitative and require professional interpretation; see the conditions index for related topics.

How it relates to other hormones

Progesterone rarely acts alone. Its closest partner is estradiol: estrogen builds up the uterine lining in the first half of the cycle, and progesterone then matures and stabilizes it in the second half. The pituitary hormone LH governs the formation of the corpus luteum that makes progesterone, linking it to FSH and LH. As a steroid precursor it also connects to the adrenal pathway shared with cortisol. For a broader map of how these hormones fit together, see the hormones index, and for clinical context the treatments and glossary sections.

How it is measured in blood

Progesterone is measured from a blood sample. For people who menstruate, it is often drawn in the luteal phase (roughly a week before the expected period) because that is when a meaningful rise should be visible; a single value early in the cycle would be expected to be low and is less informative. Because progesterone can be released in small pulses and is timed to the cycle, the day of testing matters more than the time of day. It may be tested alongside other reproductive hormones and is interpreted against the laboratory's own reference intervals. See the blood tests overview for context.

The categories above are illustrative only and are not diagnostic cutoffs. Reference ranges differ between laboratories and assays and depend on cycle phase, pregnancy status, age, and sex.