- A High circulating levels of the LH hormone itself are sometimes thought to directly inhibit further secretion of estrogen from the developing ovarian follicle in most textbook accounts during normal conditions
- B When estrogen rises above a threshold during follicular phase, it switches from negative to positive feedback on the hypothalamus-pituitary axis, causing massive GnRH and then LH surge, triggering ovulation
- C The mid-cycle LH surge is sometimes thought to be triggered mainly by rising levels of progesterone hormone rather than by estrogen itself as generally observed in typical laboratory settings under usual circumstances
- D FSH hormone itself is sometimes thought to directly cause the mid-cycle LH surge to occur each cycle, largely without any estrogen signal involved according to most researchers in the majority of cases studied
Correct answer: B. When estrogen rises above a threshold during follicular phase, it switches from negative to positive feedback on the hypothalamus-pituitary axis, causing massive GnRH and then LH surge, triggering ovulation
Explanation: Estrogen normally inhibits GnRH/FSH/LH (negative feedback). But when estrogen rises above ~200 pg/mL for 48+ hours (as dominant follicle matures), it switches to positive feedback: stimulates GnRH pulse frequency increase → massive LH surge (10-fold) → ovulation within 36-48 hours. This is a rare example of positive endocrine feedback.
Concept context
Male and female reproductive systems, gametogenesis, fertilization, implantation, pregnancy, and reproductive health.