## Endocrine Profile in PCOS **Key Point:** The hallmark endocrine abnormality in PCOS is an elevated LH:FSH ratio, typically >2:1 or 3:1, due to increased GnRH pulse frequency and amplitude. ### Mechanism 1. Insulin resistance and hyperinsulinemia stimulate ovarian androgen production 2. Excess androgens cause abnormal GnRH secretion patterns 3. Increased GnRH pulse frequency preferentially stimulates LH release over FSH 4. Result: **Elevated LH with relatively normal or low FSH** ### Clinical Correlates | Parameter | PCOS Pattern | Normal Pattern | | --- | --- | --- | | LH level | Elevated (often >10 mIU/mL) | 5–25 mIU/mL (follicular) | | FSH level | Normal or low (3–8 mIU/mL) | 5–30 mIU/mL (follicular) | | LH:FSH ratio | >2:1 or 3:1 | ~1:1 | | Androgen levels | Elevated (free testosterone, androstenedione) | Normal | **High-Yield:** The elevated LH:FSH ratio is one of the Rotterdam criteria for PCOS diagnosis (along with clinical/biochemical hyperandrogenism and polycystic ovarian morphology on ultrasound). **Clinical Pearl:** Elevated LH drives thecal cell androgen production, while low FSH fails to stimulate adequate aromatase in granulosa cells, preventing normal estrogen synthesis and follicle maturation. This explains both the hyperandrogenism and anovulation in PCOS. ### Why This Matters - Explains anovulation and irregular cycles - Drives acne, hirsutism, and male-pattern baldness - Contributes to insulin resistance and metabolic dysfunction - Guides treatment (e.g., GnRH agonists suppress LH; metformin reduces hyperinsulinemia)
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