Question
Explain the hormonal regulation of the menstrual cycle. How do FSH, LH, estrogen, and progesterone interact through feedback mechanisms to control follicular development, ovulation, and endometrial changes?
Solution — Step by Step
Progesterone and estrogen levels are at their lowest. Without hormonal support, the endometrium breaks down — menstrual bleeding begins. The hypothalamus, sensing low estrogen, releases GnRH, which stimulates the anterior pituitary to release FSH (and some LH).
FSH stimulates several ovarian follicles to develop. As follicles grow, they secrete increasing amounts of estrogen. Estrogen does two things: (1) rebuilds the endometrium (proliferative phase), and (2) initially inhibits FSH and LH through negative feedback — ensuring only the dominant follicle survives.
When the dominant Graafian follicle produces very high, sustained estrogen, the feedback switches from negative to positive. This triggers a sharp spike in LH (the LH surge) from the anterior pituitary. The LH surge causes the Graafian follicle to rupture, releasing the secondary oocyte — ovulation.
After ovulation, the ruptured follicle transforms into the corpus luteum, which secretes progesterone (and some estrogen). Progesterone maintains the thickened endometrium (secretory phase), making it ready for implantation. Progesterone also inhibits GnRH/FSH/LH (negative feedback), preventing new follicle development.
If fertilisation does not occur, the corpus luteum degenerates (day 24-28), progesterone drops, the endometrium sheds, and the cycle restarts.
flowchart TD
A[Hypothalamus: GnRH] --> B[Anterior Pituitary: FSH + LH]
B --> C[Ovary: Follicle development]
C --> D[Rising Estrogen]
D -->|Low levels| E[Negative feedback on FSH/LH]
D -->|High sustained levels| F[Positive feedback - LH surge]
F --> G[OVULATION Day 14]
G --> H[Corpus Luteum forms]
H --> I[Progesterone rises]
I --> J[Maintains endometrium]
I --> K[Negative feedback on GnRH]
K -->|No fertilisation| L[Corpus luteum degenerates]
L --> M[Progesterone drops]
M --> N[Menstruation - cycle restarts]Why This Works
The entire cycle is controlled by feedback loops. Low estrogen = negative feedback on LH (keeps it suppressed during follicle growth). High sustained estrogen from a mature follicle = positive feedback (triggers the LH surge). After ovulation, progesterone takes over with negative feedback, suppressing new follicle development until the cycle resets.
Common Mistake
The trickiest concept: estrogen provides both negative and positive feedback on LH — depending on its concentration and duration. Low estrogen = negative feedback. High, sustained estrogen from the mature Graafian follicle = positive feedback → LH surge. Students who say “estrogen always inhibits LH” will get the ovulation mechanism wrong.
If asked “which hormone is responsible for ovulation?” — the answer is LH (specifically, the LH surge). Estrogen triggers the LH surge, but LH directly causes follicle rupture.