MCAT content library / Biology & biochemistry / Reproductive hormone cycles
Read reproductive hormone curves as a feedback system: identify the ovarian event, the uterine response, and whether the dominant loop is negative or positive feedback.
About 18 minutes, including the hormone-curve application and checks.
Useful foundations
On this page
- Start with the hypothalamic–pituitary–gonadal axis
- Concept: the ovarian and uterine cycles are coordinated but not identical
- Worked application: read the curve before naming the phase
- Pattern: negative feedback dominates most of the cycle
- Rule: the LH surge is a special positive-feedback interval
- Check your reasoning
- Continue learning
- Sources
Start with the hypothalamic–pituitary–gonadal axis
Hypothalamic GnRH stimulates the anterior pituitary to release FSH and LH. In the ovary, FSH supports follicular development and estradiol production, while LH contributes to follicular steroid production, ovulation, and corpus-luteum function. Ovarian hormones then feed back to the hypothalamus and pituitary.2
In males, the same axis supports testosterone production and spermatogenesis through coordinated LH and FSH signaling. The details of the female cycle are more visibly cyclic, but the underlying logic is the same: hypothalamic signal → pituitary gonadotropins → gonadal response → feedback.
Concept: the ovarian and uterine cycles are coordinated but not identical
The ovarian cycle tracks follicular development, ovulation, and corpus-luteum activity. The uterine cycle tracks endometrial shedding, proliferation, and secretory preparation. Estradiol produced during follicular development supports endometrial proliferation. After ovulation, progesterone from the corpus luteum helps convert the endometrium into a secretory state suited for implantation.2
If pregnancy does not occur, corpus-luteum function declines, progesterone and estradiol fall, and the functional endometrium is shed. Thus, menstruation follows loss of steroid support rather than serving as the initiating signal for ovarian hormone production.
Worked application: read the curve before naming the phase
An original 28-day model shows relative hormone concentrations and endometrial thickness at four time points. Values are normalized and intended only to illustrate relationships.
| Day | Estradiol | LH | Progesterone | Endometrial thickness |
|---|---|---|---|---|
| 4 | 0.4 | 0.5 | 0.2 | Low |
| 12 | 2.4 | 0.8 | 0.3 | Increasing |
| 14 | 2.8 | 7.5 | 0.5 | Increasing |
| 21 | 1.6 | 0.4 | 4.2 | High/secretory |
Day 12 shows rising estradiol with a growing endometrium, consistent with late follicular/proliferative activity. Day 14 shows the defining LH surge and therefore marks the ovulatory interval. Day 21 is dominated by progesterone and a secretory endometrium, consistent with luteal-phase corpus-luteum activity.
The table is more useful than memorizing “day 14” as a universal ovulation date. Real cycle length varies, particularly in the follicular phase. The mechanistic marker is the hormonal and ovarian pattern, not a fixed calendar date.
Pattern: negative feedback dominates most of the cycle
For much of the cycle, ovarian steroid hormones and inhibin restrain upstream gonadotropin output. This negative feedback prevents continuously maximal FSH and LH secretion. After the corpus luteum regresses, declining ovarian steroid levels reduce negative feedback, allowing FSH to rise and support recruitment of a new follicular cohort.
A receptor-block experiment can use that logic. If ovarian steroid signaling to the hypothalamus and pituitary is lost while the gonads remain capable of responding, upstream GnRH/FSH/LH signaling tends to rise because the negative-feedback signal is weakened.
Rule: the LH surge is a special positive-feedback interval
Sustained high estradiol from the dominant follicle reverses the usual feedback relationship for a limited interval, promoting a large LH surge that triggers ovulation. This is a physiological positive-feedback episode embedded within a system otherwise dominated by negative feedback.2
Do not generalize that “estrogen always increases LH.” At lower or ordinary cycle concentrations, estrogen participates predominantly in negative feedback. The effect depends on concentration, duration, and cycle context.
ACTIVE PRACTICE · REPRODUCTIVE PHYSIOLOGY
Read the Hormone Pattern Before Naming the Phase
Use the original illustrative 28-day model, not a clinical reference range. Select a time point to compare estradiol, LH, progesterone, and the endometrial response. The hormonal pattern, rather than a universal calendar day, supports the phase interpretation.
Day 4: estradiol 0.4; LH 0.5; progesterone 0.2; endometrium low.
Reveal the Feedback Principle
Most of the cycle involves predominantly negative feedback. Sustained high estradiol shortly before ovulation can produce a transient positive-feedback LH surge. The subsequent progesterone-dominant interval follows corpus-luteum development. Exact cycle-day timing varies among individuals.
Check your reasoning
Choose an answer before opening its feedback.
Check 1
A hormone profile shows a sharp LH peak with high estradiol and still-low progesterone. Which event is most directly associated?
A. Ovulation
Correct. The LH surge triggers ovulation.
B. Menstrual shedding caused by high progesterone
Review this choice. Menstruation follows withdrawal of progesterone and estradiol.
C. Long-term maintenance of the corpus luteum without pregnancy
Review this choice. That requires a different hormonal context.
D. Completion of spermatogenesis
Review this choice. The profile describes the ovarian cycle.
Check 2
Progesterone is high and the endometrium is secretory. Which ovarian structure is most likely active?
A. Corpus luteum
Correct. The corpus luteum is the major progesterone source in the luteal phase.
B. An unselected primordial follicle only
Review this choice. The secretory phase follows ovulation and luteinization.
C. Vas deferens
Review this choice. The vas deferens is part of the male reproductive tract.
D. Neural tube
Review this choice. The neural tube is embryologic tissue.
Check 3
Ovarian steroid receptors that mediate negative feedback in the hypothalamus/pituitary are blocked. Which upstream change is most plausible?
A. FSH and LH tend to rise because negative feedback is weakened.
Correct. Removing feedback releases upstream signaling from inhibition.
B. FSH and LH must permanently fall to zero.
Review this choice. Loss of negative feedback predicts the opposite direction.
C. The pituitary becomes a gonad.
Review this choice. Feedback changes signaling, not organ identity.
D. Ovulation becomes independent of LH by definition.
Review this choice. LH remains a key ovulatory signal.
Continue learning
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- Hormone signals and cellular receptors
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