Reproductive Physiology

The hormonal choreography of the hypothalamic–pituitary–gonadal (HPG) axis underlies puberty, the menstrual cycle, spermatogenesis and fertility. NEET PG tests the feedback switches (negative → positive oestrogen feedback), the cyclical hormone graphs, and clinical decoding of PCOS and male hypogonadism profiles. Master the axis logic and the rest follows.

The HPG axis — the master circuit

The axis runs Hypothalamus → Anterior pituitary → Gonads, with feedback loops closing the circuit.

• Hypothalamus secretes GnRH (gonadotropin-releasing hormone) in a pulsatile fashion from the arcuate nucleus. Pulsatility is non-negotiable: continuous GnRH down-regulates pituitary receptors (the basis of GnRH-agonist therapy in prostate cancer/endometriosis).
• Anterior pituitary gonadotrophs release FSH and LH (glycoprotein hormones sharing a common α-subunit with TSH and hCG; the β-subunit confers specificity).
• Gonads (ovary/testis) produce sex steroids (oestrogen, progesterone, testosterone) and peptides (inhibin, activin) that feed back.

High-yield: GnRH must be pulsatile to stimulate; continuous/constant GnRH suppresses gonadotropins. This single fact explains both pubertal onset (pulse generator "switching on") and the action of leuprolide/goserelin.

Kisspeptin (KISS1 neurons) is the upstream driver of GnRH pulsatility and the true gatekeeper of puberty. Loss-of-function → hypogonadotropic hypogonadism.

Hormone	Source	Major target/action
GnRH	Hypothalamus (arcuate)	Stimulates FSH + LH (pulsatile)
FSH	Pituitary gonadotroph	Ovary: granulosa cell growth, aromatase. Testis: Sertoli cells, spermatogenesis
LH	Pituitary gonadotroph	Ovary: theca androgen synthesis, ovulation, corpus luteum. Testis: Leydig testosterone
Inhibin B	Granulosa / Sertoli	Selectively suppresses FSH
Oestradiol	Granulosa (aromatase)	Feedback (− then +), endometrial proliferation
Progesterone	Corpus luteum	Secretory endometrium, thermogenesis, negative feedback

The menstrual cycle — phases and hormones

The cycle is described in parallel along two organs: the ovary (follicular → ovulation → luteal) and the endometrium (menstrual → proliferative → secretory). A 28-day cycle is the textbook reference; the luteal phase is fixed at ~14 days, while the follicular phase varies — so cycle-length variation is essentially follicular-phase variation.

Day 1 = first day of bleeding. Ovulation ≈ Day 14.

Follicular phase (Day 1–14)

1. Falling progesterone/oestrogen at the end of the previous cycle releases FSH from negative feedback → FSH rises early.
2. FSH recruits a cohort of antral follicles; one becomes dominant (highest aromatase/FSH-receptor density, lowest threshold).
3. Two-cell, two-gonadotropin model: LH drives theca interna to make androgens (androstenedione) → diffuse to granulosa, where FSH-induced aromatase converts them to oestradiol.
4. Rising oestradiol exerts negative feedback on FSH/LH (keeping them low) — and inhibin B selectively suppresses FSH, helping select a single dominant follicle.

High-yield (two-cell theory): LH → Theca → Androgen; FSH → Granulosa → Aromatase → Oestrogen. Granulosa cells have no 17α-hydroxylase, so they cannot make androgens de novo — they depend on theca-derived precursors.

The feedback switch & LH surge (Day ~12–14)

This is the most tested concept. Oestradiol's effect on the pituitary depends on its concentration and duration:

• Low–moderate, fluctuating oestradiol → negative feedback (early/mid-follicular).
• High, sustained oestradiol (>200 pg/mL for ≥ ~48 h) → positive feedback → massive LH surge (and smaller FSH surge).

Sequence: Dominant follicle ↑↑ oestradiol → threshold crossed → positive feedback → GnRH/LH surge → ovulation ~24–36 h after LH surge onset (and ~10–12 h after the LH peak).

High-yield: Ovulation occurs roughly 24–36 h after the onset of the LH surge and about 10–12 h after the LH peak. Progesterone begins to rise just before ovulation (luteinisation of granulosa) and contributes to the mid-cycle FSH surge.

The LH surge triggers: resumption of meiosis I (extrusion of the first polar body, arrest at metaphase II), follicular wall rupture, and conversion of the follicle into the corpus luteum.

Luteal phase (Day 14–28)

• The corpus luteum secretes progesterone (dominant) + oestrogen.
• Progesterone → secretory endometrium (glycogen-rich, ready for implantation), thickens cervical mucus, and exerts negative feedback keeping FSH/LH low (preventing new follicle recruitment).
• No fertilisation → corpus luteum regresses (luteolysis) at ~Day 24–26 → progesterone & oestrogen fall → menstruation + FSH disinhibition → next cycle.
• Fertilisation → trophoblast hCG (LH-like) rescues the corpus luteum → continued progesterone until the placenta takes over (~7–9 weeks) — the luteoplacental shift.

Feature	Follicular phase	Luteal phase
Dominant hormone	Oestradiol	Progesterone
Ovarian structure	Maturing (Graafian) follicle	Corpus luteum
Endometrium	Proliferative	Secretory
FSH/LH trend	FSH high early, then low; LH low until surge	Both low (neg. feedback)
Basal body temp	Lower	Higher (↑ ~0.3–0.5 °C)
Duration	Variable	Fixed ~14 days

High-yield (thermogenic effect): Progesterone raises basal body temperature by ~0.3–0.5 °C (acts on the hypothalamic thermoregulatory centre). A biphasic BBT chart with a sustained mid-cycle rise is presumptive evidence of ovulation. Monophasic chart → anovulation.

Confirming ovulation

• Mid-luteal (Day 21) serum progesterone > 3 ng/mL (often > 10 indicates robust ovulation) is the practical confirmatory test.
• LH urinary kits detect the surge (predicts ovulation in next ~24–36 h).
• Biphasic BBT (retrospective).

Oogenesis & follicular dynamics

• Oogonia proliferate in fetal life; peak ~6–7 million at 20 weeks, falling to ~1–2 million at birth and ~400,000 at puberty; only ~400 ovulate in a lifetime.
• Primary oocytes arrest in prophase of meiosis I (diplotene/dictyate) from fetal life until the LH surge.
• Meiosis I completes at ovulation (→ secondary oocyte + first polar body).
• Meiosis II completes only at fertilisation (→ ovum + second polar body); otherwise arrests at metaphase II.

Mnemonic — "Meiosis I = ovuLation, Meiosis II = fertilisation." The two arrests (prophase I → metaphase II) are perennial favourites.

Male reproductive physiology — spermatogenesis & testosterone

The testis has two compartments under separate gonadotropin control:

• LH → Leydig cells (interstitial) → testosterone. Testosterone feeds back negatively on LH (and GnRH).
• FSH → Sertoli cells → support spermatogenesis + secrete inhibin B (negative feedback selectively on FSH) and androgen-binding protein (ABP), which keeps intratesticular testosterone very high (~50–100× serum) — essential for spermatogenesis.

High-yield: LH acts on Leydig cells (testosterone); FSH acts on Sertoli cells (spermatogenesis support + inhibin B). Inhibin B is the marker of Sertoli-cell/spermatogenic function and selectively suppresses FSH. So in primary testicular failure with damaged tubules, inhibin B falls and FSH rises disproportionately.

Spermatogenesis facts:

• Takes ~64–74 days (≈ 2.5 months) + ~12–14 days epididymal transit.
• Sequence: Spermatogonia → primary spermatocyte → secondary spermatocyte → spermatid → spermatozoon (spermiogenesis = maturation of spermatid to sperm).
• Occurs at ~2–3 °C below core temperature (scrotal cooling); cryptorchidism/varicocele impair it.
• Sertoli cells form the blood–testis barrier (tight junctions) — immunological isolation of haploid germ cells.

Testosterone:

• ~95% from Leydig cells; circulates bound to SHBG and albumin.
• Converted by 5α-reductase → dihydrotestosterone (DHT) (external genitalia, prostate, hair) and by aromatase → oestradiol (bone epiphyseal closure, libido, negative feedback).

High-yield (genital embryology link): Testosterone → Wolffian (mesonephric) duct derivatives — SEED (Seminal vesicle, Epididymis, Ejaculatory duct, Ductus deferens). DHT → external genitalia + prostate. AMH (from Sertoli) regresses Müllerian ducts. 5α-reductase deficiency → ambiguous genitalia at birth, virilisation at puberty.

Puberty — the hormonal sequence

Puberty begins when the kisspeptin–GnRH pulse generator reactivates (nocturnal GnRH pulses first). The HPG axis is active in the neonatal "mini-puberty," then quiescent through childhood, then reactivates.

Girls (Tanner sequence): Thelarche (breast) → Pubarche (pubic hair) → Growth spurt → Menarche.

Mnemonic — "Boobs, Pubes, Grow, Flow." Breast budding (thelarche) is the first sign; menarche is last (~2–2.5 yr after thelarche).

Boys: Testicular enlargement (≥ 4 mL / > 2.5 cm — first sign) → pubic hair → penile growth → growth spurt (later/peak ~Tanner 4).

• Adrenarche (adrenal androgen rise, DHEA-S) precedes and is independent of gonadarche.
• Precocious puberty: < 8 yr (girls), < 9 yr (boys). Central (GnRH-dependent) vs peripheral (GnRH-independent).
• Delayed puberty: no breast by 13 (girls) / no testicular enlargement by 14 (boys).

Axis state	LH/FSH	Sex steroids	Example
Hypogonadotropic hypogonadism	Low/normal	Low	Kallmann (± anosmia), constitutional delay, pituitary lesion
Hypergonadotropic hypogonadism	High	Low	Turner (45,XO), Klinefelter (47,XXY), gonadal failure

PCOS — the classic hormonal profile

PCOS (polycystic ovary syndrome) is the most tested "hormone-decoding" scenario. Core pathophysiology: increased GnRH pulse frequency → ↑ LH (with relatively normal/low FSH) → ↑ LH:FSH ratio → theca hyperandrogenism, compounded by insulin resistance/hyperinsulinaemia that augments ovarian androgen output and lowers SHBG (raising free testosterone).

Rotterdam criteria (≥ 2 of 3):

1. Oligo-/anovulation
2. Clinical/biochemical hyperandrogenism (hirsutism, acne; ↑ free testosterone)
3. Polycystic ovaries on USG (≥ 12 follicles 2–9 mm and/or ovarian volume > 10 mL) — string of pearls.

High-yield (PCOS hormone panel): ↑ LH, normal/low FSH → ↑ LH:FSH ratio (classically ≥ 2–3:1), ↑ free testosterone, ↓ SHBG, often ↑ insulin/IR, mildly ↑ oestrone (peripheral aromatisation of androgens in adipose), and frequently ↑ AMH. Hyperandrogenism halts follicular maturation → anovulation.

• Management: lifestyle/weight loss first-line; combined OCPs for menstrual regulation + hyperandrogenism; letrozole (aromatase inhibitor) is now first-line for ovulation induction/infertility (superior to clomiphene); metformin for insulin resistance/metabolic component.
• Long-term risks: type 2 diabetes, endometrial hyperplasia/carcinoma (unopposed oestrogen), dyslipidaemia, metabolic syndrome.

High-yield: Letrozole > clomiphene for ovulation induction in PCOS (higher live-birth rate). Clomiphene is a selective oestrogen receptor modulator acting at the hypothalamus to block negative feedback → ↑ FSH.

Male hypogonadism — investigation logic

Decode by reading testosterone + LH/FSH together:

Stepwise approach: Confirm low morning total testosterone (×2) → measure LH & FSH → LH/FSH high = primary (testicular); LH/FSH low/normal = secondary (central) → then karyotype / MRI / prolactin / iron studies as indicated.

Type	Testosterone	LH/FSH	Localisation	Examples
Primary (hypergonadotropic)	Low	High	Testis	Klinefelter (47,XXY), mumps orchitis, chemo/radiation, cryptorchidism, trauma
Secondary (hypogonadotropic)	Low	Low/normal	Hypothalamus/pituitary	Kallmann, pituitary tumour, hyperprolactinaemia, haemochromatosis, exogenous steroids/opioids

High-yield (Klinefelter 47,XXY): Tall, small firm testes, gynaecomastia, azoospermia/infertility, ↑ FSH & LH, low testosterone, ↑ oestradiol; ↓ inhibin B. Most common congenital cause of primary hypogonadism/male infertility.

High-yield (Kallmann): Hypogonadotropic hypogonadism + anosmia/hyposmia (failed GnRH-neuron + olfactory placode migration; KAL1/ANOS1). Low LH/FSH/testosterone; responds to pulsatile GnRH or gonadotropins.

• Isolated ↑ FSH with normal LH/testosterone → suggests selective seminiferous tubule/spermatogenic failure (Sertoli dysfunction, low inhibin B) — e.g., post-chemo, Sertoli-cell-only syndrome.
• Always check prolactin (prolactinoma suppresses GnRH) and screen for haemochromatosis in unexplained secondary hypogonadism.

Key differentials & decode table

Scenario	LH	FSH	Oestrogen/Testosterone	Most likely
↑ LH:FSH, ↑ androgen, anovulation, obese female	↑	nl/↓	↑ free T	PCOS
↑ FSH & LH, amenorrhoea, hot flushes, < 40 yr	↑↑	↑↑	↓ oestrogen	Premature ovarian insufficiency
↓ LH & FSH, low weight/athlete, amenorrhoea	↓	↓	↓ oestrogen	Functional hypothalamic amenorrhoea
↑ LH & FSH male, small testes, tall, XXY	↑	↑	↓ T	Klinefelter
↓ LH & FSH male + anosmia	↓	↓	↓ T	Kallmann
↑ prolactin, ↓ LH/FSH, galactorrhoea	↓	↓	↓	Prolactinoma

Complications / clinical correlates worth remembering

• Unopposed oestrogen (chronic anovulation, PCOS, oestrogen-only HRT) → endometrial hyperplasia → carcinoma. Progesterone is protective.
• OHSS (ovarian hyperstimulation syndrome) — iatrogenic from gonadotropin/hCG ovulation induction; VEGF-driven capillary leak.
• Hyperprolactinaemia (drugs, prolactinoma, hypothyroidism) → suppressed GnRH → secondary hypogonadism, galactorrhoea, amenorrhoea.
• Hypothyroidism raises TRH → ↑ prolactin → menstrual irregularity (link tested often).

Recently asked / exam angle

• Positive vs negative oestrogen feedback — which phase, which threshold triggers the LH surge (answer: high sustained oestradiol, late follicular).
• Two-cell two-gonadotropin theory — pair theca/LH/androgen with granulosa/FSH/aromatase; granulosa lacks 17α-hydroxylase.
• Timing of ovulation relative to LH surge (24–36 h after surge onset; 10–12 h after peak).
• Progesterone thermogenic effect / biphasic BBT as ovulation evidence; Day-21 progesterone.
• Hormone identification graphs — labelling FSH/LH/oestrogen/progesterone peaks on the cycle curve.
• PCOS panel (↑ LH:FSH, ↑ free T, ↓ SHBG, ↑ AMH) and letrozole as ovulation-induction DOC.
• Klinefelter vs Kallmann — primary (↑ gonadotropins) vs secondary (↓ gonadotropins, anosmia).
• Inhibin B as the selective FSH-suppressing marker of Sertoli/spermatogenic function.
• Meiotic arrest points of the oocyte (prophase I → metaphase II).
• hCG rescue of corpus luteum and the luteoplacental shift timing (~7–9 weeks).

Rapid revision

1. GnRH must be pulsatile to stimulate; continuous GnRH suppresses gonadotropins (basis of leuprolide therapy).
2. LH → theca → androgens; FSH → granulosa → aromatase → oestradiol (two-cell theory).
3. High sustained oestradiol → positive feedback → LH surge; low/fluctuating oestradiol → negative feedback.
4. Ovulation ~24–36 h after LH surge onset, ~10–12 h after the peak.
5. Luteal phase is fixed (~14 days); follicular phase is variable.
6. Progesterone raises BBT ~0.3–0.5 °C → biphasic chart = ovulation; Day-21 progesterone confirms it.
7. Inhibin selectively suppresses FSH; inhibin B marks Sertoli/spermatogenic function.
8. Oocyte arrests in prophase I until ovulation (→ metaphase II until fertilisation).
9. PCOS: ↑ LH:FSH (~≥2–3:1), ↑ free testosterone, ↓ SHBG, ↑ AMH, insulin resistance; letrozole = first-line ovulation induction.
10. Klinefelter (47,XXY): ↑ LH & FSH, ↓ testosterone = primary (hypergonadotropic) hypogonadism.
11. Kallmann: ↓ LH/FSH/testosterone + anosmia = secondary (hypogonadotropic) hypogonadism.
12. hCG rescues the corpus luteum; placenta takes over progesterone at ~7–9 weeks (luteoplacental shift).