43.4 Hormonal Control of Human Reproduction

Reproductive hormones control how the body develops sex cells, builds secondary sexual characteristics, and runs the menstrual cycle. The signaling chain runs from the brain (hypothalamus and pituitary) down to the gonads, with feedback loops running back up. This section covers each major hormone, how the ovarian and menstrual cycles coordinate, and what happens hormonally during menopause.

Reproductive Hormones and Cycles

Functions of reproductive hormones

The reproductive hormone system works as a chain of command. The hypothalamus signals the pituitary, the pituitary signals the gonads, and the gonads produce hormones that feed back to the brain to fine-tune the whole process.

Gonadotropin-releasing hormone (GnRH) is released by the hypothalamus and acts on the anterior pituitary gland. Its job is to stimulate the pituitary to secrete two gonadotropins: follicle-stimulating hormone (FSH) and luteinizing hormone (LH). GnRH is released in pulses, and the frequency of those pulses helps determine whether FSH or LH is preferentially released.

Follicle-stimulating hormone (FSH) has different targets depending on sex:

• In females: stimulates the growth of ovarian follicles (the structures that house immature eggs, or oocytes) and promotes estrogen production by those follicles
• In males: stimulates spermatogenesis, the production of sperm cells, in the seminiferous tubules of the testes

Luteinizing hormone (LH) also differs by sex:

• In females: triggers ovulation (the release of a mature egg from the ovary), stimulates formation of the corpus luteum from the remnants of the ovulated follicle, and promotes progesterone production by the corpus luteum
• In males: stimulates Leydig cells in the testes to produce testosterone

Estrogen is produced primarily by developing follicles in the ovaries. It has wide-ranging effects:

• Stimulates growth and maintenance of female reproductive organs (uterus, vagina)
• Drives development of secondary sexual characteristics such as breast development and female fat distribution patterns
• Regulates the menstrual cycle through negative feedback on FSH and LH secretion from the pituitary
• At very high concentrations near mid-cycle, estrogen switches to positive feedback, helping trigger the LH surge that causes ovulation

Progesterone is produced by the corpus luteum after ovulation:

• Prepares the uterine lining (endometrium) for implantation by increasing blood supply and secretory activity
• Maintains the endometrium during early pregnancy
• Provides negative feedback on FSH and LH, preventing new follicle development during the luteal phase

Testosterone is produced by Leydig cells in the testes:

• Stimulates development and maintenance of male reproductive organs (prostate gland, seminal vesicles)
• Drives secondary sexual characteristics such as increased muscle mass, voice deepening, and male hair distribution
• Supports spermatogenesis by maintaining the right environment in the seminiferous tubules
• Classified as an androgen, the broader category of hormones that influence male characteristics

Coordination of reproductive cycles

The ovarian cycle (what's happening in the ovary) and the menstrual cycle (what's happening in the uterus) run in parallel, coordinated by the same hormonal changes. A typical cycle lasts about 28 days, though anywhere from 21 to 35 days is normal.

Ovarian cycle:

1. Follicular phase — FSH stimulates growth of several ovarian follicles. As they mature, estrogen levels rise. One follicle becomes dominant while the rest degenerate (a process called atresia).
2. Ovulation — A sharp surge in LH (triggered by high estrogen via positive feedback) causes the dominant follicle to release its mature oocyte, typically around day 14.
3. Luteal phase — The empty follicle transforms into the corpus luteum, which secretes progesterone (and some estrogen) to prepare the uterus for possible implantation.

Menstrual (uterine) cycle:

1. Menstrual phase (days 1–5) — Low estrogen and progesterone levels cause the endometrium to shed. This is menstruation, and day 1 of bleeding marks day 1 of the cycle.
2. Proliferative phase (days 6–14) — Rising estrogen from the growing follicles stimulates the endometrium to thicken and develop new blood vessels.
3. Secretory phase (days 15–28) — Progesterone from the corpus luteum makes the endometrium rich with nutrients and blood supply, ready for a fertilized egg.

If fertilization and implantation don't occur, the corpus luteum degenerates (around day 26–28). The resulting drop in progesterone and estrogen removes support for the endometrium, triggering menstruation and starting the cycle over.

Physiological changes in menopause

Menopause is the permanent end of menstrual cycles, typically occurring between ages 45 and 55. It's officially diagnosed after 12 consecutive months without a menstrual period (amenorrhea).

Perimenopause is the transitional period leading up to menopause, lasting roughly 2 to 8 years. During this time, menstrual cycles become irregular and hormone levels fluctuate as the supply of ovarian follicles dwindles.

Hormonal changes during and after menopause:

• Estrogen and progesterone production decline as the ovaries run out of responsive follicles
• FSH and LH levels increase because there's less estrogen and progesterone to provide negative feedback to the pituitary. Elevated FSH is actually one of the lab markers used to confirm menopause.

Physiological effects of declining estrogen:

• Hot flashes and night sweats from vasomotor instability (the body's temperature regulation becomes less stable)
• Vaginal dryness and tissue thinning (atrophy), which can increase susceptibility to urinary tract infections
• Bone density loss and increased risk of osteoporosis, since estrogen normally promotes bone maintenance
• Changes in lipid metabolism: LDL ("bad") cholesterol tends to rise while HDL ("good") cholesterol falls, increasing cardiovascular disease risk

Post-menopause refers to the years after the final menstrual period. Hormone levels stabilize at consistently low estrogen and progesterone with elevated FSH and LH, reflecting the depleted follicle pool.

Endocrine System and Hormone Signaling

All of the reproductive processes above depend on basic endocrine principles:

• Hormones are chemical messengers secreted into the bloodstream that act on distant target cells.
• Target cells respond because they have hormone receptors, specialized proteins on or within the cell that bind a specific hormone. Cells without the right receptor won't respond, even if the hormone is present in the blood.
• Negative feedback is the most common control mechanism: when hormone levels rise high enough, they suppress further production. For example, high testosterone inhibits GnRH and LH release, keeping testosterone levels in a stable range.
• Positive feedback is less common but critical in reproduction. The best example is the mid-cycle estrogen spike: as estrogen climbs past a threshold, it stimulates (rather than inhibits) more LH release, producing the LH surge that triggers ovulation. Once ovulation occurs, the positive feedback loop ends.
• Gametes (sperm and eggs) are produced and mature under the influence of these reproductive hormones, connecting the endocrine system directly to fertility.