Revision aid, not clinical advice. This page is educational material written for exam preparation. It is not a substitute for the current guideline or for clinical judgement, and must not be relied on for the care of any individual. Always check up-to-date national guidance and verify independently before acting on anything here.

Core Revision · Physiology

The HPO Axis Explorer

The hypothalamic–pituitary–ovarian axis, drawn out: what each hormone does across the cycle, why the curves move as they do, and what changes when the axis is interrupted. Built for the DRCOG, the DCSRH, the MRCGP AKT, the MSRA, the UKMLA and PLAB — the same physiology is examined in all of them.

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What it looks like

The cycle chart with FSH, LH, oestradiol and progesterone drawn across 28 days, the fertile window shaded, and a card explaining what happens on the day the marker sits on.
Move through the cycle day by day. Each day explains itself in short steps, and the fertile window can be shaded on.
The feedback loop diagram, showing the hypothalamus, pituitary and ovary with the signals between them.
The feedback loops, one piece at a time: negative feedback, the switch to positive feedback, and where prolactin cuts in.
A condition card showing hormone chips, the four curves against the normal cycle, and the explanation beneath.
Twenty-eight conditions and drugs, each with what breaks, what you would measure, and the curves against the normal cycle.
A table of contraceptive methods with the main action of each and the other effects, in the words of the CoSRH guidelines.
Where each method acts, emergency contraception against the LH surge, and when contraception can stop.

What is inside

  • The cycle, day by day. Four curves, the ovary and the endometrium in pictures, and the mechanism for each day in short steps.
  • The feedback loops. The diagram built one piece at a time, with every signal explained.
  • Thirteen hormone cards and the steroid pathway. What each one does, what it looks like in the blood, how it is measured, and what it means when it goes wrong.
  • Twenty-eight conditions and drugs. From PMOS and premature ovarian insufficiency to the contraceptives, HRT, pregnancy and lactation.
  • Contraception on the axis. Where each method acts, emergency contraception and the LH surge, and when contraception can stop after 40.
  • Two levels. Core Lens for the AKT and the MSRA; Deep Dive adds the mechanism, the curves, the cell pictures and the rarer causes for the diplomas.

Take this further

This tool is free. If you are preparing for the DCSRH, the full course puts the same reasoning into exam-style questions: a question bank with Core Lens and Deep Dive explanations, a Core Calendar revision plan built around your exam date, and Core Compass to show you which topics need you next.

Preparing for the MRCGP AKT instead?

Written for revision and not a substitute for the guidelines themselves. Checked against source and clinically reviewed, September 2026.