A 32-year-old woman with a 10-year history of regular 28-day menstrual cycles presents with secondary amenorrhoea of 4 months' duration. She denies weight loss, stress, or recent illness. Pregnancy test is negative. Pelvic examination is normal. Serum TSH, prolactin, and androgen levels are all normal. Pelvic ultrasound shows a normal endometrial stripe and no structural abnormality. What is the most appropriate next step in management?

Explanation

## Clinical Context This patient has **secondary amenorrhoea** with: - Normal TSH, prolactin, androgens → rules out thyroid, prolactin, and androgen excess - Normal pelvic ultrasound → rules out structural pathology - Negative pregnancy test - Previously regular cycles → suggests acquired anovulation or outflow obstruction The next step is to **determine the site of pathology** using the **progestin challenge test**. ## Progestin Challenge Test: Mechanism & Interpretation **Key Point:** The progestin challenge test (PCT) is the **gold standard first-line investigation** for secondary amenorrhoea after pregnancy, thyroid, and prolactin disorders are excluded. ### How It Works ```mermaid flowchart TD A[Administer medroxyprogesterone acetate 10 mg daily × 5 days]:::action --> B{Withdrawal bleeding occurs?}:::decision B -->|Yes| C[Adequate oestrogen + patent outflow tract]:::outcome C --> D[Diagnosis: Anovulation]:::outcome D --> E[Check FSH to assess ovarian reserve]:::action B -->|No| F{Endometrial pathology or outflow obstruction?}:::decision F -->|Suspect outflow obstruction| G[Hysteroscopy]:::action F -->|Suspect low oestrogen| H[Check FSH/LH/oestradiol]:::action ``` ### Interpretation Table | Progestin Challenge Result | Oestrogen Status | Outflow Tract | Diagnosis | |---|---|---|---| | **Withdrawal bleeding** | Adequate | Patent | Anovulation (PCOS, idiopathic, mild hypothalamic) | | **No withdrawal bleeding** | Adequate | Obstructed | Asherman syndrome, cervical stenosis | | **No withdrawal bleeding** | Low | Patent | Hypogonadotropic hypogonadism or ovarian failure | ## Why This Patient Needs PCT Since her TSH, prolactin, and androgens are normal, the amenorrhoea is either: 1. **Anovulation** (most likely) → PCT will show withdrawal bleeding 2. **Outflow obstruction** (less likely but possible) → PCT will show no bleeding 3. **Hypogonadism** (unlikely with normal androgens) → PCT will show no bleeding The PCT **rapidly triages** between these three categories with a simple, non-invasive test. ## Why NOT the Other Options? | Option | Rationale | |--------|----------| | **Serum oestradiol & FSH** | Second-line; order only if PCT shows no withdrawal bleeding. Measuring hormones before PCT wastes time and money. | | **GnRH agonist challenge** | Not a standard diagnostic test. GnRH agonists are used therapeutically, not diagnostically, in amenorrhoea workup. | | **Diagnostic hysteroscopy** | Invasive and premature. Indicated only if PCT shows no withdrawal bleeding (suggesting outflow obstruction). | **High-Yield:** The **diagnostic algorithm for secondary amenorrhoea** is: 1. Exclude pregnancy, thyroid, prolactin disorders 2. **Progestin challenge test** → differentiates anovulation from outflow obstruction/hypogonadism 3. If no withdrawal bleeding → measure FSH/LH/oestradiol or perform hysteroscopy **Mnemonic:** **PCOS** (most common cause of anovulatory amenorrhoea) will show **positive PCT** (withdrawal bleeding). [cite:Harrison 21e Ch 412] **Clinical Pearl:** A positive PCT (withdrawal bleeding) confirms adequate oestrogen and patent outflow tract, making anovulation the diagnosis. This is the most common cause of secondary amenorrhoea in reproductive-age women.