A 28-year-old woman from Mumbai presents with secondary amenorrhea of 6 months' duration. She reports that her periods were regular until 8 months ago, when she began an intense fitness regimen (exercising 2 hours daily) and adopted a strict vegan diet, reducing her body weight from 58 kg to 48 kg (BMI from 22 to 18.2). She denies galactorrhoea, visual disturbances, or heat intolerance. On examination, she appears lean but otherwise well, with normal vital signs and normal thyroid examination. Serum FSH is 3.2 mIU/mL (low-normal), LH is 2.8 mIU/mL (low-normal), prolactin is 8 ng/mL (normal), and TSH is 2.1 mIU/mL (normal). Pelvic ultrasound shows a normal uterus and ovaries with no follicular activity. A pregnancy test is negative. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Hypothalamic Amenorrhea Secondary to Low Energy Availability ### Key Clinical Features **Key Point:** Secondary amenorrhea triggered by significant weight loss (16.6% body weight reduction) and intense exercise, with low-normal gonadotropins, normal prolactin, and normal thyroid function, is diagnostic of hypothalamic amenorrhea due to low energy availability (LEA). ### Pathophysiology Hypothalamic amenorrhea occurs when the hypothalamic GnRH pulse generator becomes suppressed due to metabolic stress — most commonly from: 1. **Low energy availability** — insufficient caloric intake relative to energy expenditure 2. **Rapid weight loss** — loss of >10–15% body weight 3. **Excessive exercise** — particularly endurance or high-intensity training 4. **Nutritional deficiency** — inadequate micronutrient intake The mechanism involves: - ↓ Leptin (adipokine) → suppression of GnRH secretion - ↑ CRH and cortisol → inhibition of GnRH pulsatility - ↓ IGF-1 (marker of energy availability) → reduced FSH and LH stimulation ### Why This Diagnosis Fits **High-Yield:** The temporal relationship between lifestyle change (exercise + diet) and amenorrhea onset, combined with significant weight loss (16.6%), is the hallmark of hypothalamic amenorrhea. | Feature | Finding | Significance | | --- | --- | --- | | **Trigger** | Intense exercise + restrictive diet | Classic precipitant | | **Weight loss** | 58 → 48 kg (16.6% reduction) | Exceeds threshold for LEA | | **FSH/LH** | Low-normal (3.2 / 2.8) | Hypogonadotropic hypogonadism | | **Prolactin** | Normal (8 ng/mL) | Rules out prolactinoma | | **TSH** | Normal (2.1 mIU/mL) | Rules out hypothyroidism | | **Ovarian ultrasound** | No follicular activity | Reflects suppressed FSH | | **Pregnancy test** | Negative | Rules out pregnancy | ### Clinical Pearl **Clinical Pearl:** Hypothalamic amenorrhea is **reversible** if energy balance is restored. Weight gain of 2–3 kg and reduction in exercise intensity often restore menses within 3–6 months. This is NOT a permanent endocrine disorder but rather a functional suppression of the HPO axis. ### Diagnostic Algorithm ```mermaid flowchart TD A[Secondary Amenorrhea]:::outcome --> B{Pregnancy?}:::decision B -->|Yes| C[Pregnancy]:::outcome B -->|No| D{TSH, Prolactin normal?}:::decision D -->|No| E[Thyroid disease / Hyperprolactinaemia]:::outcome D -->|Yes| F{FSH/LH levels?}:::decision F -->|High| G[Premature ovarian insufficiency]:::outcome F -->|Low-normal| H{Weight loss / Exercise history?}:::decision H -->|Yes| I[Hypothalamic Amenorrhea - LEA]:::action H -->|No| J{Hyperandrogenism?}:::decision J -->|Yes| K[PCOS]:::outcome J -->|No| L[Other hypothalamic cause]:::outcome ``` ### Management 1. **Nutritional counselling** — increase caloric intake to achieve positive energy balance 2. **Reduce exercise intensity** — modify training regimen 3. **Weight restoration** — target BMI 20–22 (normal range) 4. **Micronutrient supplementation** — iron, B12, vitamin D if deficient 5. **Monitoring** — menses typically resume 3–6 months after energy balance restoration [cite:Yen and Jaffe's Reproductive Endocrinology 8e Ch 12]