What effect does oestrogen have on the hypothalamus when produced by developing follicles?

• A. Negative feedback to reduce GnRH and FSH
• B. No effect
• C. Positive feedback to increase GnRH and FSH
• D. Inhibits LH

Answer: (C)

As follicles mature, they produce more estrogen. When estrogen levels rise to a high threshold just before ovulation, they switch the signal on the hypothalamus from negative to positive feedback. This positive feedback increases GnRH pulses, which stimulates the pituitary to release more gonadotropins, including FSH (and especially LH). The rise in GnRH and the subsequent gonadotropin surge drive ovulation. So the correct idea is that estrogen provides positive feedback to the hypothalamus, increasing GnRH (and thus gonadotropins). The other options don’t fit this mid-cycle shift: estrogen does not consistently suppress GnRH during this phase (that would be negative feedback), it does not produce no effect, and it does not inhibit LH.