Oestrogen Physiology
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Direct answer
Oestradiol-17β, the most potent natural oestrogen, is made in the ovarian follicle by a two-cell partnership: LH drives theca cells to produce androgens, which diffuse to granulosa cells where FSH-induced aromatase converts them to oestradiol. Levels trace the follicular story — early follicular phase around 30-50 pg/mL, a preovulatory surge of 200-500 pg/mL just before the LH peak, a luteal-phase plateau near 100-200 pg/mL, and after menopause a fall below 20 pg/mL while FSH rises above 30-40 mIU/mL. Oestrogen proliferates the endometrium, builds the secondary sexual characteristics, protects bone by restraining osteoclastogenesis (via RANKL suppression), raises HDL and lowers LDL, increases clotting factors, and induces hepatic SHBG synthesis — the last explaining why combined pills improve acne and hirsutism. Its feedback is biphasic: negative on the pituitary through most of the cycle, switching to positive feedback above a sustained threshold of roughly 200 pg/mL for about 48 hours to trigger the LH surge — the switch oral contraceptives abolish.
What you must remember
- Two-cell, two-gonadotrophin theory: theca (LH, androgens) plus granulosa (FSH, aromatase) equals oestradiol — the standard diagram every examiner asks you to draw.
- Potency order: oestradiol (E2) strongest, oestrone (E1) intermediate, oestriol (E3) weakest and mainly placental — E3 serves as a foetal wellbeing marker.
- Cycle numbers: early follicular 30-50 pg/mL, preovulatory peak 200-500, luteal 100-200 pg/mL; menopausal oestradiol below 20 with FSH above 30-40 mIU/mL and 12 months of amenorrhoea — the diagnostic set.
- Feedback switch: sustained oestradiol near 200 pg/mL for about 48 hours converts negative to positive feedback, generating the mid-cycle LH surge; progestogens from the corpus luteum then hold subsequent FSH recruitment down.
- Bone physiology: oestradiol suppresses osteoclast differentiation (less RANKL, more osteoprotegerin) and limits cytokine-driven resorption — postmenopausal loss is fastest in trabecular bone (spine, wrist) in the first 5-10 years at 2-5 per cent yearly.
- Metabolic and hepatic effects: HDL up, LDL down, but factors II, VII, VIII, X and fibrinogen up (the venous thromboembolism risk of oestrogen-containing contraceptives); SHBG induction lowers free testosterone — the therapeutic logic in polycystic ovary syndrome.
- Puberty sequence: thelarche (breast bud, oestrogen-driven) is the first sign, then adrenarche, then the growth spurt — girls gain about 25 cm, and oestrogen finally fuses the epiphyses, which is why precocious puberty costs final height.
- Placental and male roles: oestriol from the foeto-placental unit; in men, aromatised oestradiol guards bone and epiphyseal timing.
A worked case across the reproductive arc
A 13-year-old develops breast buds — thelarche — with a growth spurt following: follicular oestradiol proliferates the ducts and stroma, the spurt rides oestrogen-growth hormone synergy, and the same hormone will later fuse the epiphyses and cap her height. Mid-cycle, her oestradiol crosses the sustained 200 pg/mL threshold, feedback flips positive, the LH surge fires, and ovulation follows within 12-24 hours — the physiology every calendar method tries to track.
Skip forward forty years: the follicular pool is exhausted, oestradiol sits under 20 pg/mL, and lost negative feedback drives FSH above 40 — hot flushes, accelerated bone loss, urogenital atrophy. Hormone replacement controls symptoms and conserves bone, though long-term use carries breast-cancer and thromboembolism signals — modern prescribing uses the lowest effective dose for the shortest appropriate duration.
Where students slip
Students name the granulosa cell as the sole source of oestradiol; it is a duet — theca under LH makes the androgen substrate, granulosa under FSH aromatises it — and the theca's LH dependence is why luteinising hormone matters even before ovulation. The second slip is forgetting the feedback flip: oestrogen is not a simple negative-feedback hormone, and the sustained-threshold positive feedback (about 48 hours near 200 pg/mL) is the mechanism oral contraceptive pills precisely prevent — suppress the surge and you suppress ovulation. Third, students attribute menopausal bone loss to calcium deficiency; it is oestrogen-deficient osteoclast release, fastest in trabecular bone — which is why spine and wrist fractures present first, and why the answer "alendronate or HRT," not "calcium alone," is the correct exam response.
Frequently asked questions
How is oestradiol synthesised in the ovarian follicle?
By the two-cell theory: LH-stimulated theca cells produce androstenedione, which granulosa cells convert to oestradiol using FSH-induced aromatase — both gonadotrophins are required.
What plasma oestradiol pattern defines a normal menstrual cycle?
Early follicular 30-50 pg/mL, a preovulatory peak of 200-500 pg/mL, a luteal plateau of 100-200 pg/mL, and baseline just before menses — tracked with LH and FSH in infertility work-ups.
How does oestrogen feedback switch from negative to positive?
Sustained oestradiol near 200 pg/mL for about 48 hours in the late follicular phase converts pituitary feedback to positive, generating the mid-cycle LH surge that triggers ovulation.
Why does bone loss accelerate after menopause?
Oestrogen deprivation releases osteoclastogenesis from RANKL suppression — trabecular resorption runs 2-5 per cent yearly in the early postmenopausal years, driving vertebral and wrist fractures.
Which coagulation changes do oestrogen-containing contraceptives cause?
Hepatic induction of factors II, VII, VIII, X and fibrinogen raises venous thromboembolism risk — the reason for screening for thrombophilia history and preferential progestogen-only methods in at-risk women.