A 32-year-old woman from Mumbai presents with secondary infertility (unable to conceive for 2 years after a normal first child 5 years ago). She reports amenorrhea for 8 months. On examination, she is thin (BMI 18 kg/m²), with no hirsutism or acne. Serum FSH is 2.2 mIU/mL, LH is 1.8 mIU/mL, prolactin is 18 ng/mL (normal <25), and TSH is normal. Pelvic ultrasound shows a thin endometrium (3 mm) and small ovaries. A hysterosalpingography performed 6 months ago showed a normal cavity and patent tubes. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Hypothalamic Amenorrhea ### Key Clinical Features **Key Point:** The combination of secondary infertility, amenorrhea, **low gonadotropins (FSH 2.2 mIU/mL, LH 1.8 mIU/mL)**, low BMI (18 kg/m²), small ovaries, thin endometrium, normal prolactin, and normal TSH is the classic presentation of **hypothalamic amenorrhea (HA)** — a functional suppression of the hypothalamic-pituitary-ovarian (HPO) axis. ### Diagnostic Criteria for Hypothalamic Amenorrhea 1. **Amenorrhea (≥3 months)** — present (8 months) 2. **Low or low-normal FSH and LH** — present (FSH 2.2, LH 1.8 mIU/mL); reflects reduced GnRH pulsatility 3. **Low BMI / nutritional deficit / stress** — present (BMI 18 kg/m²) 4. **Exclusion of other causes** — prolactin normal, TSH normal, HSG normal (no Asherman's) 5. **Small ovaries on ultrasound** — consistent with hypogonadotropic hypogonadism (understimulated ovaries) ### Why NOT the Other Options? | Feature | Hypothalamic Amenorrhea | POF/POI | Asherman's Syndrome | Hyperprolactinemia | |---|---|---|---|---| | **FSH/LH** | **Low** (↓ GnRH pulsatility) | **Elevated >40 mIU/mL** (loss of negative feedback) | Normal | Normal or low | | **Prolactin** | Normal | Normal | Normal | **Elevated (>25 ng/mL)** | | **Ovarian size** | Small (understimulated) | Small (atrophic/depleted) | Normal | Normal | | **Endometrium** | Thin (low estrogen) | Thin (low estrogen) | Thin/absent (adhesions) | Thin (low estrogen) | | **HSG** | Normal | Normal | **Abnormal (filling defects)** | Normal | | **BMI** | Often low | Variable | Variable | Variable | - **Premature Ovarian Failure (POI):** Requires FSH **>40 mIU/mL on two occasions ≥4 weeks apart** (ASRM criteria). FSH of 2.2 mIU/mL definitively excludes POF — the hallmark of POF is *elevated* FSH due to loss of ovarian negative feedback. (Williams Gynecology, 4e; ASRM 2015 Guideline) - **Hyperprolactinemia:** Prolactin is 18 ng/mL (normal). Excluded. - **Asherman's Syndrome:** HSG showed normal cavity and patent tubes 6 months ago. Excluded. ### Pathophysiology of Hypothalamic Amenorrhea 1. **Energy deficit / low BMI** → reduced leptin → decreased kisspeptin signaling → reduced GnRH pulsatility 2. **Reduced GnRH pulses** → reduced FSH and LH secretion → anovulation and amenorrhea 3. **Low estrogen** → thin endometrium, small understimulated ovaries 4. **Functional (reversible)** — unlike POF, ovarian reserve is intact; restoration of energy balance can restore cycles **Clinical Pearl:** Hypothalamic amenorrhea is the most common cause of secondary amenorrhea in women with low BMI. It is a **diagnosis of exclusion** after ruling out thyroid disease, hyperprolactinemia, and outflow tract obstruction. Per *Williams Gynecology* and *Berek & Novak's Gynecology*, the FSH/LH pattern in HA is characteristically low-normal, distinguishing it from the elevated FSH seen in POF/POI. ### Management - **Address underlying cause:** Nutritional rehabilitation, weight gain, stress reduction - **Fertility:** Ovulation induction with pulsatile GnRH or gonadotropins (FSH/LH) — ovarian reserve is intact - **Bone health:** Calcium, Vitamin D; consider HRT if prolonged amenorrhea [cite: Williams Gynecology 4e Ch 16; Berek & Novak's Gynecology 16e Ch 11; ASRM Practice Committee Opinion on POI 2015]