Progesterone is a C-21 steroid produced by the corpus luteum (women), adrenals (both sexes), and CNS.
Mechanism: Primary action is uterine. Prepping the endometrium for implantation. Its metabolite allopregnanolone is a potent positive allosteric modulator of GABA-A, producing calming, sleep-promoting, anxiolytic effects.
Lab range: Women: varies dramatically by cycle phase (luteal peak 5–20 ng/mL). Men: typically 0.1–0.5 ng/mL, less clinically tracked.
Caveats: Synthetic progestins (medroxyprogesterone) are not progesterone. Different receptor activity, worse safety profile. Bioidentical micronized progesterone (oral, transdermal, vaginal) is the modern standard.