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Hormones and their Role in Fertility

How does an ovary know to ripen and release an egg every month? How does the uterus know when the endometrium should thicken . . . or thin? Why does your sex drive rise and fall?

The answer to all of these questions, obviously (given this column’s topic) is hormones. Hormones are the body’s chemical messengers. They’re produced and released by one organ, travel through the blood vessels, and deliver their message to a target organ, causing a change. While the hormones may pass many other organs on their journey, the target organ is the one that recognizes them and can interpret the signal.

The human body produces more than 50 different hormones, including insulin (which lowers blood sugar), antidiuretic hormone (which retains water in the kidneys), and cortisol (the so-called stress hormone). But it’s the reproductive hormones that we’re most concerned with here. The seven main ones are estrogen, progesterone, testosterone, luteinizing hormone (LH), follicle stimulating hormone (FSH), human chorionic gonadotropin (hCG), and prolactin.

Estrogen, Progesterone, and Testosterone
These three hormones are called steroids, in which cholesterol (a lipid) is a key component. Estrogen and progesterone are released by the ovaries and affect two organs: They cause the pituitary gland in the brain to slow down or increase the release of FSH and LH (see below), and they direct the uterus to thicken its lining in preparation for a potential embryo. Estrogen peaks around midcycle, right before ovulation, while progesterone peaks in the second half of the menstrual cycle. (Testosterone, the main male steroid hormone, is also produced in small amounts in the ovaries and is an essential female sex hormone that boosts libido.)

FSH, LH, and hCG
These hormones are referred to as gonadotropins. FSH and LH are produced by the pituitary gland and control the action of the ovaries (and testes in men). These hormones also cause the ovaries to release estrogen, mature eggs, and ovulate. LH, released in a surge, is the hormone that most ovulation predictor kits detect. Another gonadotropin, hCG, is the hormone that most pregnancy tests detect. This hormone is actually produced by the developing embryo when it implants in the uterine lining.

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This hormone is produced by the pituitary gland, and its primary function is to stimulate milk production. But prolactin also affects ovulation and menstruation, and too much prolactin in the blood can cause infertility by inhibiting FSH and GnRH (gonadotropin releasing hormone), and blocking ovulation from being triggered. Prolactin is one reason why women usually don’t ovulate or conceive while breastfeeding.

Each of the body’s hormones has a unique function, and all are necessary—at the right time and in the right amounts—for a successful conception and pregnancy. In fact, many infertility treatments work by adding artificial hormones or blocking natural hormone production to achieve the right balance. For instance, injections of human menopausal gonadotropin (hMG, e.g., Pergonal) and hCG can increase the body’s levels of FSH and LH.  The commonly prescribed fertility drug clomiphene citrate (Clomid, Serophene) also spurs the pituitary gland to produce more FSH and LH; it “fools the brain to cause women to ovulate,” explains Lee P. Shulman, M.D., professor of obstetrics and gynecology at Northwestern University Feinberg School of Medicine   in Chicago.

Fertility problems can also occur when the target organ—the ovaries or uterus—doesn’t properly recognize or respond to the hormones’ messages. Sometimes this happens because of a physical interference, such as with endometriosis, ovarian cysts, scars, or tumors. In this case, surgically removing the obstruction may restore fertility.

And sometimes fertility problems can occur when there are disruptions involving other parts of the body or the production of other hormones. For instance, too much or too little cholesterol may affect steroid hormone production and conception. Also, when the thyroid gland doesn’t produce enough thyroid hormones (T3 and T4), it causes an increase in another hormone, called thyroid stimulating hormone (TSH), but it inhibits production of FSH and LH. Taking thyroid hormone supplements can correct this problem.

Sex hormones can be affected by weight gain and obesity. “Fat leads to increased levels of circulating estrogens, which can alter a woman’s ability to ovulate,” says Dr. Shulman, explaining that the result can be reduced or irregular ovulation. On the other hand, drastic dieting may lower estrogen levels to the point that there isn’t enough estrogen for ovulation to occur.

Understanding the interaction of the reproductive hormones is as essential to understanding reproduction as knowing the basics of male and female anatomy. So next time you’re annoyed by pre-period bloating, feeling generally irritable around your period, or wondering why you’re so interested in sex, blame—or credit—your hormones. And after you’re pregnant, you can credit them for your success, too.

A version of this article originally appeared in the Winter 2007 issue of Conceive Magazine.

Related Topics: Fertility BasicsFertility HealthFertility Hormones; FSHWhen Am I Fertile?