Biomarker 101: Progesterone

Progesterone is only interpretable if you know where in the cycle the sample was taken. Without that, the number is close to meaningless.

What progesterone is

Progesterone is a steroid hormone, produced mainly in the ovaries and, during pregnancy, by the placenta. The adrenal glands contribute small amounts in both sexes, and in men it is produced mostly by the adrenals.

Its central role is preparing and maintaining the lining of the uterus for pregnancy. It also contributes to regulating the menstrual cycle, supports pregnancy as it progresses, and has effects on sleep and immune regulation.

Why cycle timing dominates

Progesterone follows a pronounced pattern across the menstrual cycle. It sits very low before ovulation, rises after it, and peaks in the second half before falling again if pregnancy does not occur.

That means the same person can produce a very low result and a high one within a fortnight, both entirely normal. A result without a cycle day attached cannot be interpreted.

Confirming ovulation

The most common use of a progesterone test is confirming that ovulation has occurred, since the rise happens only afterwards.

For that purpose the sample is taken about seven days before the next period is due — day 21 in a regular 28-day cycle, but genuinely seven days before the expected period rather than day 21 regardless of cycle length. Laboratories interpret the result against thresholds indicating whether ovulation is likely to have taken place.

Other reasons it's measured

Progesterone is also measured in early pregnancy assessment, in investigating abnormal uterine bleeding, and occasionally in men where an adrenal cause is suspected.

What moves progesterone

Lower levels may reflect cycles where ovulation has not occurred, and are associated with polycystic ovary syndrome, an underactive thyroid, raised prolactin, perimenopause, sustained stress and heavy training loads. Progesterone-based contraception also affects results.

Higher levels occur naturally in the second half of the cycle and in pregnancy. Persistently high levels outside those situations may relate to ovarian or adrenal conditions and warrant investigation.

What progesterone won't tell you

• It doesn't work without a cycle day. This is the single most important thing about testing it.
• Confirming ovulation isn't predicting fertility. Ovulating is necessary for conception, not sufficient.
• One cycle isn't a pattern. Cycles vary, and a single result reflects one of them.
• It doesn't identify a cause. A low result indicates where to look next rather than what is happening.

The bottom line

Progesterone rises and falls dramatically by design. Measured at the right point it confirms ovulation reliably; measured at the wrong point it produces a number that looks alarming and means nothing at all.