## Hormonal Control of Ovulation **Key Point:** The LH surge is the critical trigger for ovulation, occurring approximately 24–36 hours before the release of the oocyte from the ovary. ### Mechanism of LH Surge 1. **Rising estradiol levels** during the late follicular phase reach a critical threshold (>200 pg/mL for 24–48 hours) 2. **Positive feedback** on the anterior pituitary and hypothalamus 3. **Sudden release** of GnRH from the hypothalamus 4. **Massive LH secretion** (10–20 fold increase) within hours 5. **Ovulation** follows 24–36 hours after the LH surge onset ### Comparison of Hormones During Midcycle | Hormone | Midcycle Pattern | Role | | --- | --- | --- | | **LH** | Sharp surge (peak 24–36 hrs before ovulation) | **Triggers ovulation** | | **FSH** | Mild surge (smaller than LH) | Supports follicle growth | | **Estradiol** | Peak just before LH surge | Positive feedback trigger | | **Progesterone** | Minimal until after ovulation | Corpus luteum hormone | **High-Yield:** The LH surge is the **single most reliable marker** of impending ovulation and is used in ovulation prediction kits (LH urine tests). **Clinical Pearl:** In assisted reproductive technology (ART), exogenous hCG is used to mimic the LH surge and trigger final oocyte maturation before egg retrieval. [cite:Ganong's Review of Medical Physiology 26e Ch 23] 
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