Which hormone, when at consistently high levels for a sustained period, switches from negative to positive feedback on GnRH and LH, leading to the LH surge?
Progesterone
FSH
Estrogen
Inhibin
The Correct Answer is C
Estrogen, specifically estradiol (E2), plays a dual role in the hypothalamic-pituitary-ovarian axis through both negative and positive feedback mechanisms. During the late follicular phase, when estradiol levels remain consistently high—typically >200 pg/mL for at least 50 hours—the feedback on the hypothalamus and anterior pituitary shifts from negative to positive. This stimulates a GnRH surge, which triggers a massive LH release (LH surge), leading to ovulation. The LH surge also initiates the transformation of the ruptured follicle into the corpus luteum.
Rationale for correct answer
C. Estrogen at sustained high levels switches to positive feedback, stimulating GnRH and causing the LH surge. This is the critical hormonal trigger for ovulation around day 14 of a typical 28-day cycle.
Rationale for incorrect answers
A. Progesterone exerts only negative feedback on GnRH, FSH, and LH throughout the cycle, especially during the luteal phase. It does not cause the LH surge and actually prevents further ovulation.
B. FSH is regulated by negative feedback via estrogen and inhibin. It does not trigger the LH surge and is not part of the positive feedback loop leading to ovulation.
D. Inhibin is secreted by granulosa cells and specifically suppresses FSH release via negative feedback. It plays no role in triggering the LH surge or stimulating GnRH.
Take home points
- High sustained estrogen levels trigger positive feedback on GnRH and LH.
- The LH surge causes ovulation and luteinization of the follicle.
- Progesterone and inhibin exert negative feedback and suppress further ovulation.
- FSH supports follicle growth but is not involved in the LH surge trigger.
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Related Questions
Correct Answer is ["A","B"]
Explanation
The ovaries are the primary source of estrogen and progesterone during the menstrual cycle. These steroid hormones regulate endometrial development, ovulation, and feedback control of the hypothalamic-pituitary axis. Estrogen is mainly secreted by granulosa cells of developing follicles during the follicular phase, reaching peak levels of 200–300 pg/mL pre-ovulation. Progesterone is produced by the corpus luteum in the luteal phase, with mid-luteal levels of 5–20 ng/mL, stabilizing the endometrium for implantation.
Rationale for correct answers
A. Estrogen is produced by ovarian granulosa cells during follicular development. It promotes endometrial proliferation and regulates FSH and LH secretion via feedback.
B. Progesterone is secreted by the corpus luteum after ovulation. It prepares and maintains the endometrium for implantation and inhibits GnRH, FSH, and LH secretion to prevent further ovulation.
Rationale for incorrect answers
C. FSH is secreted by the anterior pituitary, not the ovaries. It stimulates follicular growth in the ovaries and estrogen production.
D. LH is also secreted by the anterior pituitary, not the ovaries. It triggers ovulation and supports the corpus luteum.
E. GnRH is secreted by the hypothalamus, not the ovaries. It regulates the release of FSH and LH from the pituitary.
Take home points
- The ovaries produce estrogen (follicular phase) and progesterone (luteal phase).
- FSH and LH come from the anterior pituitary, not the ovaries.
- GnRH originates in the hypothalamus and regulates pituitary hormones.
- Ovarian hormones regulate endometrial changes and provide feedback control.
Correct Answer is B
Explanation
Corpus luteum
The corpus luteum is a temporary endocrine structure formed from the ruptured Graafian follicle after ovulation. It secretes high levels of progesterone, moderate estrogen, and minimal inhibin A to maintain the luteal phase of the menstrual cycle. Progesterone levels peak around day 21 of a 28-day cycle and range from 5–20 ng/mL in the mid-luteal phase. Its main role is to stabilize the endometrial lining for potential implantation and to inhibit GnRH, FSH, and LH via negative feedback. If fertilization does not occur, it regresses into the corpus albicans, causing a drop in hormones and onset of menstruation.
Rationale for correct answer
B. The corpus luteum secretes progesterone during the luteal phase to maintain the endometrium for possible implantation. It forms after ovulation and is hormonally dependent on LH for function.
Rationale for incorrect answers
A. Estrogen is secreted mainly by the dominant follicle during the follicular phase, not by the corpus luteum. The follicular phase precedes ovulation, and the corpus luteum forms after ovulation.
C. The ovum is released by the ruptured follicle due to the LH surge, not by the corpus luteum. The corpus luteum forms after ovulation and plays no direct role in ovum release.
D. GnRH is secreted by the hypothalamus and regulated by sex steroid feedback. The corpus luteum does not stimulate GnRH; in fact, progesterone from the corpus luteum inhibits it.
Take home points
- The corpus luteum secretes progesterone to maintain the endometrium.
- It forms after ovulation from the ruptured follicle.
- It does not release the ovum; that happens before it forms.
- Its hormones inhibit GnRH, FSH, and LH to prevent further ovulation.
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