Progesterone
Progesterone climbs sharply after ovulation as the corpus luteum forms, sustains the early uterine lining, and modulates mood, sleep, and immune function across the luteal phase. A mid-luteal serum value above 10 ng/mL confirms that ovulation occurred this cycle; a value below 3 ng/mL on day 21 of a typical 28-day cycle is consistent with anovulation. In pregnancy, progesterone climbs into the hundreds; postmenopausally, it falls to near-zero baseline.
Why this biomarker matters
Progesterone is the second of the two ovarian sex hormones and the cleanest serum confirmation that ovulation took place. Anovulatory cycles produce estradiol without the post-ovulatory progesterone surge, which is why mid-luteal progesterone is the standard test for unexplained infertility, irregular cycles, and luteal-phase deficiency. Anovulation is more common than most women appreciate, particularly in late perimenopause, in athletes with low energy availability, in PCOS, and in the year following hormonal contraception cessation. Beyond fertility, progesterone has well-documented effects on GABAergic neurotransmission (via its metabolite allopregnanolone), which is why luteal-phase sleep is often deeper and waking heart rate slightly higher. Premenstrual dysphoric disorder appears to involve abnormal sensitivity to the rapid late-luteal progesterone fall. Bioidentical micronized progesterone is used in postmenopausal hormone therapy to protect the endometrium when systemic estradiol is given; its sleep- and anxiety-modulating effects are part of the therapeutic profile in that context.
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These biomarkers contextualize and unlock a clearer picture than any single value can.
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