FSH & LH: What They Control
| Hormone | Released By | Target | Effect |
|---|---|---|---|
| FSH (follicle-stimulating hormone) | Anterior pituitary | Ovarian follicles / testicular Sertoli cells | Women: follicle growth. Men: sperm production (spermatogenesis) |
| LH (luteinising hormone) | Anterior pituitary | Ovary / testicular Leydig cells | Women: triggers ovulation / corpus luteum. Men: testosterone production |
Clinical Patterns
| Pattern | FSH | LH | Interpretation |
|---|---|---|---|
| Primary ovarian insufficiency / menopause | High (>25 IU/L) | High | Ovaries failing — pituitary compensates |
| PCOS | Normal/Low | Raised (LH:FSH >2:1) | Disordered GnRH pulsatility |
| Secondary hypogonadism (pituitary/hypothalamic) | Low | Low | Central problem — not primary gonadal failure |
| Mid-cycle ovulation (women) | Mid-range | Surge (25–40) | Normal ovulation |
| Male primary hypogonadism | High | High | Testicular failure → pituitary compensation |
| Male secondary hypogonadism | Low/Normal | Low | Pituitary or hypothalamic dysfunction |
Timing Matters in WomenIn pre-menopausal women, FSH and LH must be interpreted in the context of cycle day. Day 2–5 (follicular phase) is the standard timing for ovarian reserve assessment. Mid-cycle values (day 12–14) are used to detect the LH surge.
What FSH level indicates menopause?
FSH ≥25–30 IU/L on two samples taken 4–6 weeks apart in a woman with amenorrhoea for 12 months confirms menopause. In women under 45, premature ovarian insufficiency (POI) is the diagnosis.
Can low FSH cause infertility?
Yes. Low FSH with low LH (hypogonadotrophic hypogonadism) indicates the pituitary is not stimulating the ovaries/testes. Causes include hypothalamic amenorrhoea, prolactinoma, pituitary tumour, or Kallmann syndrome.
How is FSH used for fertility assessment?
Day 3 FSH (early follicular phase) above 10–12 IU/L indicates reduced ovarian reserve. Combined with antral follicle count (AFC) on ultrasound and anti-Müllerian hormone (AMH), it guides IVF planning.
What is the LH:FSH ratio in PCOS?
An LH:FSH ratio >2:1 or >3:1 on a day 2–5 sample is a classic PCOS pattern, though it's not present in all women with PCOS and is no longer required for diagnosis under Rotterdam criteria.
Medical Disclaimer: This page is for general education only and does not replace professional medical advice. Always consult a qualified healthcare provider.