1. What Gynaecological Reasoning Demands

First principle: localise the lesion on one axis

Nearly every reproductive complaint is a disturbance of a single control loop:

Whatever the presentation — amenorrhoea, infertility, hirsutism, abnormal bleeding — the discipline is the same: decide which level is failing. A single test does most of the localising, because the pituitary reports on the ovary for you.

The move that localises: read the gonadotropins

When estrogen is low, FSH tells you where the problem is:

The ovary failing means the pituitary shouts louder (FSH up). A silent brain means the pituitary never shouts (FSH down). This one idea recurs in menopause, amenorrhoea and infertility.

Reasoning workflow

1. Anchor in time & safety: could she be pregnant? Is anything an emergency (ectopic, torsion, sepsis)? 🚩
2. Localise the level using history + FSH/estradiol ± prolactin/TSH.
3. Ask about structure only when the endocrine picture doesn't explain it (outflow obstruction, Müllerian anomaly, Asherman).
4. Treat the level, framed by the patient's goal (symptom relief vs fertility vs cancer risk).

Pitfalls

• Reaching for imaging or karyotype before the two free questions — pregnant? and where on the axis?
• Treating a number instead of a patient (an ultrasound finding is not a syndrome).

Pearls

• 🎯 FSH is the localiser. High = ovarian; low = central. Commit this before anything else.
• 🩺 In any woman of reproductive age with pelvic pain or amenorrhoea, the first test is a pregnancy test — reflex, not optional.

Next → Before the axis can run, the body must become female — reasoning from SRY to phenotype (Concept 2).