The LH surge that triggers ovulation is paradoxically preceded by elevated estradiol acting on the hypothalamus/pituitary. What is this mechanism termed and at what estradiol level/duration is it triggered?

• A Inhibin B surge from the dominant follicle directly triggers pituitary LH release
• B Progesterone from the pre-ovulatory follicle provides positive feedback triggering the LH surge
• C Positive feedback of estradiol: sustained high estradiol (>200 pg/mL for >36-48 hours) switches estradiol from negative to positive feedback on GnRH/LH release, triggering the midcycle LH surge ✓
• D Estradiol acutely blocks GnRH pulse frequency, causing a rebound LH hypersecretion after 36 hours

Explanation

Throughout most of the cycle, estradiol exerts negative feedback on the hypothalamo-pituitary axis. However, when the dominant follicle produces sustained high estradiol (>200 pg/mL, maintained for ≥36-48 hours), this paradoxically switches to positive feedback: the pituitary LH gonadotrophs become sensitized to GnRH (upregulation of GnRH receptors), and hypothalamic kisspeptin neurons (ARC and AVPV) increase GnRH pulse frequency and amplitude. The resulting dramatic LH surge (10-fold increase) triggers resumption of meiosis, oocyte maturation, and ovulation ~36 hours later. Progesterone small rise from the pre-ovulatory follicle amplifies but does not initiate the surge.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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