SIGNAL // LOADING
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Glossary term
Progesterone
The luteal-phase hormone. Drops first in perimenopause. Essential for endometrial protection on estrogen HRT.
Progesterone is produced by the corpus luteum after ovulation and peaks around cycle day 21. Serum levels above 3-5 ng/mL on day 21 confirm ovulation; below that suggests anovulation or luteal-phase defect. Progesterone drops earlier than estradiol in perimenopause, which is why sleep disruption and mood shifts often precede hot flashes. On HRT, anyone with an intact uterus needs micronized progesterone (typically 100-200 mg nightly) for endometrial protection when systemic estradiol is used. Not interchangeable with synthetic progestins - micronized is closer to endogenous.
- Symbol
- P4
- Unit
- ng/mL
- Typical range
- Luteal phase day 21 typically > 3-5 ng/mL confirms ovulation.
Medically reviewed byDr. Lena Okafor, MD
Medical Director - updated April 2026How we review
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