Reproductive Hormones and Feedback

The same toolbox, two programs

Reproductive hormones control puberty, fertility, pregnancy and much more. They use the same toolbox you have seen all unit: the hypothalamus controls the anterior pituitary, which releases tropic hormones that act on a gland, and that gland's hormones feed back. Here the glands are the gonads: the testes in the male reproductive system and the ovaries in the female reproductive system.

The hypothalamus releases GnRH, which causes the anterior pituitary to release two gonadotropins: FSH and LH. These names come from their roles in the ovary (follicle-stimulating and luteinizing), but the same two hormones act on the testes too.

This lesson uses the scientific terms in the Ontario curriculum. It is about how hormones work as a feedback system; it is not about personal decisions, which are for families and health care providers.

Male reproductive hormones: steady negative feedback

In the testes:

• LH stimulates the interstitial (Leydig) cells, found between the seminiferous tubules, to make testosterone.
• FSH, together with testosterone, stimulates the Sertoli cells inside the seminiferous tubules, which nourish developing sperm (spermatogenesis).

Testosterone also causes the secondary sex characteristics that appear at puberty: growth of the larynx (deeper voice), facial and body hair, increased muscle and bone mass, and growth of the reproductive organs.

Feedback: when testosterone rises, it inhibits the hypothalamus (less GnRH) and the pituitary (less LH). Sertoli cells also release a protein hormone, inhibin, that specifically inhibits FSH when sperm production is high. The result is fairly steady hormone levels and continuous sperm production from puberty onward, with a gradual decline in testosterone in older age.

Female reproductive hormones: a monthly cycle

In the female reproductive system, hormone levels change in a repeating cycle, on average about 28 days, though anything from about 21 to 35 days is common in adults, and cycles are often irregular in the first few years after periods begin. Two cycles run together: the ovarian cycle (what happens in the ovary) and the uterine or menstrual cycle (what happens to the lining of the uterus, the endometrium).

1. Follicular phase (about days 1 to 13). Day 1 is the first day of menstruation, when the endometrium from the previous cycle is shed (about days 1 to 5). FSH stimulates several follicles to develop; usually one becomes dominant. The follicle cells make estrogen, which rises steadily and rebuilds the endometrium (the proliferative phase). At first, moderate estrogen has negative feedback on FSH and LH.

2. Ovulation (about day 14). When estrogen has been high for a day or two, the feedback switches to positive: high estrogen makes the hypothalamus and pituitary release a large surge of LH (and a smaller rise in FSH). The LH surge causes the dominant follicle to burst and release its egg (secondary oocyte) about a day later: ovulation.

3. Luteal phase (about days 15 to 28). LH turns the empty follicle into the corpus luteum ("yellow body"), which makes progesterone and estrogen. Progesterone keeps the endometrium thick, glandular and full of blood vessels, ready for an embryo. Progesterone and estrogen together now exert strong negative feedback on GnRH, FSH and LH, so no new follicles develop.

If no pregnancy occurs, the corpus luteum breaks down after about 10 to 14 days. Progesterone and estrogen fall, the endometrium can no longer be maintained and is shed (menstruation), and with the feedback removed, FSH rises again, starting a new cycle.

Pregnancy, birth and menopause

If an egg is fertilized, the early embryo implants in the endometrium about a week later and begins to make human chorionic gonadotropin (hCG). hCG acts like LH: it keeps the corpus luteum alive, so progesterone stays high and the endometrium is not shed. Home pregnancy tests detect hCG in urine. After about the first three months, the placenta makes enough progesterone and estrogen itself.

At birth, the positive feedback loop of oxytocin (Lesson 1) drives labour contractions. After birth, prolactin (anterior pituitary) stimulates milk production and oxytocin (posterior pituitary) causes milk release.

Menopause, typically around ages 45 to 55, happens when the ovaries run out of responsive follicles. Estrogen and progesterone fall and menstrual cycles stop. With little negative feedback, FSH and LH rise to high levels, which is a way doctors confirm menopause.

Medical treatments that act on these feedback loops

Understanding feedback explains how many treatments work:

• Combined hormonal contraceptives (such as the pill, patch or ring) contain synthetic estrogen and a progestin (a progesterone-like hormone). Keeping these hormones at a steady level produces negative feedback that suppresses GnRH, FSH and LH. Without the LH surge, ovulation does not occur. Progestins also thicken cervical mucus. These are prescription medicines chosen with a health care provider.
• Fertility treatments can do the opposite: injections of FSH stimulate several follicles to mature, and an injection of hCG or a similar hormone can trigger ovulation at a planned time, for example as part of in vitro fertilization (IVF).
• Hormone therapy at menopause can relieve symptoms such as hot flashes by replacing some estrogen (with progestin for people who have a uterus).
• Testosterone replacement treats men whose testes make too little testosterone.
• Anabolic steroids misused to build muscle are synthetic testosterone-like hormones. By negative feedback they shut down the body's own LH, FSH and testosterone production, which can reduce sperm production. You will study them further in the lesson on chemical substances.