Hypothalamic-Pituitary-Ovarian Axis
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Direct answer
The hypothalamic-pituitary-ovarian axis is a three-tier feedback loop in which pulsatile GnRH from the hypothalamus drives LH and FSH from the anterior pituitary, which in turn control oestradiol, progesterone and inhibin from the ovary. The examinable core is the frequency code: fast GnRH pulses favour LH, slow pulses favour FSH, and oestradiol above about 200 pg/mL for roughly two days flips feedback to positive, triggering the LH surge.
What you must remember
- Hypothalamic source: GnRH leaves the hypothalamus in pulses through the hypophyseal portal circulation; continuous exposure desensitises pituitary receptors and suppresses gonadotrophins, which is why pulsatile delivery is the restorative regimen in hypothalamic failure.
- Pulse frequency code: roughly hourly pulses favour LH synthesis, while slower pulses favour FSH; progesterone in the luteal phase slows pulses and tilts the pituitary towards FSH.
- Kisspeptin: a hypothalamic neuropeptide acting on GnRH neurons through the KISS1R receptor, and the main relay for oestradiol feedback onto GnRH release.
- Negative feedback: low to moderate oestradiol and inhibin suppress LH and FSH, so FSH falls through the follicular phase as one follicle is selected.
- Positive feedback: oestradiol above about 200 pg/mL, sustained for roughly two days in the late follicular phase, converts feedback to positive and produces the LH surge.
- Inhibin B: secreted by granulosa cells of small antral follicles, it suppresses FSH selectively, whereas inhibin A, produced mainly by the corpus luteum, rises in the luteal phase.
- Activin and follistatin: activin stimulates FSH release, and follistatin binds and neutralises it, so the local balance fine-tunes FSH output.
Feedback loops in one regulatory map
A student who can place each signal on the axis by its source, its target and the sign of its feedback can answer most vignette questions about gonadotrophin levels without memorising numbers. The table below is the working map for the examination.
| Signal | Made by | Main action | Feedback effect |
|---|---|---|---|
| GnRH | Hypothalamic neurons | Releases LH and FSH in pulses | Desensitised by continuous exposure |
| Kisspeptin | Hypothalamic neurons | Drives GnRH release | Carries oestradiol feedback |
| Oestradiol | Granulosa cells, from theca androgen | Builds endometrium, triggers LH surge | Negative at low levels, positive when sustained high |
| Progesterone | Corpus luteum | Slows LH pulse frequency | Negative with oestradiol in luteal phase |
| Inhibin B | Small antral granulosa cells | Suppresses FSH selectively | Falls with reduced follicle count and age |
| Activin | Pituitary and ovary | Stimulates FSH | Neutralised by follistatin |
Read the table as a pair of loops: the first two rows set the pulse, the middle rows set ovarian output, and the last two rows trim FSH. Vignettes usually test the sign of feedback at a given stage of the cycle, not the absolute hormone values.
Where the exam tries to catch you
Most errors on this axis come from locating the lesion at the wrong level.
- Trap: Reading raised FSH with low oestradiol as a hypothalamic problem Correct: Raised FSH points to the ovary, as in premature ovarian insufficiency; hypothalamic failure gives low or normal FSH and LH.
- Trap: Expecting continuous GnRH to boost gonadotrophins Correct: Continuous exposure desensitises pituitary receptors and suppresses LH and FSH; pulsatile delivery is the treatment.
- Trap: Blaming falling oestradiol for the mid-cycle LH surge Correct: The surge follows sustained high oestradiol, which switches feedback from negative to positive at the hypothalamus and pituitary.
- Trap: Crediting oestradiol alone for FSH suppression in the follicular phase Correct: Inhibin B from growing follicles adds selective FSH suppression, contributing to the decline that lets one follicle become dominant.
Frequently asked questions
Why does FSH fall during the follicular phase while follicles are still growing?
Rising oestradiol and inhibin B from the granulosa cells of growing follicles suppress FSH, so the cohort loses its gonadotrophin support. The follicle with the most FSH receptors and aromatase activity keeps growing and becomes dominant.
What oestradiol level is needed to trigger the LH surge?
The LH surge needs oestradiol above about 200 pg/mL, sustained for roughly two days in the late follicular phase. A brief high peak does not trigger the surge, because the hypothalamus responds to the duration of the high level as well as its size.
Why must GnRH be delivered in pulses rather than as a continuous infusion?
Pulsatile GnRH keeps pituitary GnRH receptors responsive, whereas continuous exposure desensitises them and suppresses LH and FSH. This is why a pump delivering GnRH in pulses is used for women with hypothalamic hypogonadism who need ovulation induction.
What does progesterone do to the axis in the luteal phase?
Progesterone from the corpus luteum slows GnRH and LH pulse frequency, so LH output falls relative to FSH during the luteal phase. This effect lasts only while the corpus luteum functions.
What role does kisspeptin play in the axis?
Kisspeptin is a hypothalamic neuropeptide that acts on GnRH neurons through the KISS1R receptor, also called GPR54. It carries the oestradiol signal to GnRH neurons, which is why kisspeptin is central to the onset of puberty and to amenorrhoea caused by altered energy balance.