What Are Gonadotropic Hormones and Their Function?

Gonadotropic hormones are chemical messengers that control the reproductive system in both sexes. The three main ones are follicle-stimulating hormone (FSH), luteinizing hormone (LH), and human chorionic gonadotropin (hCG). FSH and LH are made by the pituitary gland in the brain, while hCG comes from the placenta during pregnancy. Together, they govern everything from sperm and egg production to ovulation, sex hormone levels, and the maintenance of early pregnancy. Their reach, though, extends well beyond the gonads, and the system that regulates them is one of the more elegant feedback loops in human biology.

The Basics of FSH, LH, and hCG

All three gonadotropic hormones belong to the glycoprotein family, meaning they are proteins with sugar molecules attached. They share a common structural feature: each is made of two linked protein chains, a shared alpha subunit that is identical across all three hormones, and a unique beta subunit that gives each hormone its specific biological activity.¹ This is why pregnancy tests can detect hCG specifically even though it is structurally similar to LH: the beta subunit is different enough for an antibody to tell them apart.

FSH does what its name suggests. In women, it stimulates the growth and maturation of ovarian follicles, the fluid-filled sacs that contain developing eggs. In men, it supports Sertoli cells in the testes, which are essential for nurturing developing sperm. LH, meanwhile, triggers ovulation in women and drives the production of testosterone in men. hCG appears only during pregnancy and acts like a supercharged version of LH, keeping the corpus luteum alive so it can continue producing progesterone in the early weeks after conception.

How One Signal Controls Two Hormones

FSH and LH are both released by the same cells in the anterior pituitary, called gonadotropes, and both are triggered by the same signal from the brain: gonadotropin-releasing hormone, or GnRH. That raises an obvious question. If one signal drives both hormones, how does the body produce them in different amounts at different times? The answer lies in the rhythm of GnRH release.

GnRH is not secreted in a steady stream. It pulses out of the hypothalamus in bursts, and the frequency of those bursts determines which hormone the pituitary favors. Faster pulses tend to promote LH production, while slower pulses favor FSH.² Varying pulse speeds activate different signaling pathways inside the gonadotrope cells, effectively letting one hormone act as a switch between two outputs.³ Additional fine-tuning comes from molecules called activin and inhibin, which act directly on the pituitary. Activin boosts FSH production, while inhibin suppresses it, without affecting LH.⁴ The interplay of pulse speed, activin, inhibin, and sex steroids lets the body independently dial FSH and LH up or down across the menstrual cycle or across a man’s day.

What Gonadotropins Do in Female Reproduction

The menstrual cycle is essentially a carefully choreographed dance of gonadotropin levels. In the early follicular phase, FSH rises and stimulates a cohort of ovarian follicles to grow. As those follicles develop, their granulosa cells begin producing estrogen, driven in part by FSH activating the aromatase enzyme that converts androgens into estradiol.⁵ Rising estrogen feeds back to the hypothalamus and pituitary, gradually suppressing FSH so that only the most responsive follicle survives. This is the dominant follicle.

What happens next is one of the more counterintuitive events in human physiology. For most of the cycle, estrogen acts as a brake on GnRH release, keeping gonadotropin levels in check. But when estradiol climbs high enough and stays elevated long enough, around mid-cycle, it flips from suppressing GnRH to stimulating it. This “positive feedback” triggers a massive surge of GnRH, which in turn causes a sharp spike in LH.⁶ That LH surge is the direct trigger for ovulation, rupturing the dominant follicle and releasing the egg.⁷

After ovulation, the remnants of the follicle transform into the corpus luteum, which produces progesterone to prepare the uterine lining for a possible pregnancy. If no embryo implants, the corpus luteum degrades, progesterone drops, and a new cycle begins. If an embryo does implant, the developing placenta starts producing hCG, which rescues the corpus luteum and keeps progesterone flowing during the critical first weeks.⁸

The Male Side

In men, gonadotropin levels do not cycle the way they do in women, but they are no less important. LH acts on Leydig cells in the testes, stimulating them to produce testosterone. That testosterone is needed not only for secondary sex characteristics and libido but also, critically, for sperm production inside the testes, where local testosterone concentrations are many times higher than in the bloodstream.

FSH, meanwhile, targets Sertoli cells, which physically cradle developing sperm cells and supply them with nutrients and signaling molecules. FSH drives Sertoli cell proliferation and maturation, particularly during puberty, and continues to support their function in adulthood. Both hormones working together are needed for normal sperm output. Men with very low FSH, even if their testosterone is adequate, often have reduced sperm counts.

Puberty and the Reawakening of the System

Through most of childhood, the gonadotropin system is essentially idling. GnRH pulses are infrequent and weak, FSH and LH levels are low, and the gonads remain quiescent. Puberty begins when GnRH pulsatility increases, a change now understood to be orchestrated largely by kisspeptin neurons in the hypothalamus. These neurons in a region called the arcuate nucleus are considered an integral part of the GnRH pulse-generating mechanism, and an increase in their activity is obligatory for puberty to start.⁹

What wakes up kisspeptin neurons is still not fully resolved. Genetics, body fat, nutrition, and even light exposure play roles. But once they activate, GnRH pulses accelerate, gonadotropin levels climb, and the gonads begin producing sex steroids. The physical changes of puberty follow in a predictable sequence from there. Delayed or absent puberty can often be traced to problems at one or more levels of this chain, from kisspeptin signaling to the pituitary to the gonads themselves.

When Gonadotropin Levels Go Wrong

Clinicians divide gonadal failure into two broad categories based on gonadotropin levels, and the distinction matters enormously for treatment. In hypergonadotropic hypogonadism, the gonads themselves are failing, so the pituitary cranks out high levels of FSH and LH in a futile attempt to stimulate them. Classic examples include Turner syndrome in women and Klinefelter syndrome in men, as well as premature ovarian insufficiency.¹⁰

In hypogonadotropic hypogonadism, the problem is upstream. The hypothalamus or pituitary is not producing enough GnRH, FSH, or LH, so the gonads never receive the signal to work.¹¹ This can be genetic, as in Kallmann syndrome, or acquired through tumors, head trauma, or extreme weight loss. The practical difference is that people with hypogonadotropic hypogonadism can often be treated with gonadotropin injections to restore fertility, because their gonads are capable of responding if they receive the right signals. People with hypergonadotropic hypogonadism, by contrast, have gonads that cannot respond no matter how much stimulation they get.

A simple blood test measuring FSH and LH levels, combined with sex steroid levels, is usually enough to tell these categories apart. High FSH with low estrogen or testosterone points to gonadal failure. Low or inappropriately normal FSH and LH with low sex steroids points to a brain or pituitary problem.

Aging, Menopause, and the Shift in Gonadotropin Patterns

Menopause is, in gonadotropin terms, the ultimate example of hypergonadotropic hypogonadism. As the ovaries run out of responsive follicles, estrogen and inhibin production drops, and the pituitary responds by pushing FSH and LH to very high levels. FSH tends to rise first, which is why an elevated FSH level in a woman in her late 40s or 50s is often used as a biochemical marker of the menopausal transition.

Interestingly, the picture does not stay static with further aging. Research comparing early postmenopausal women to much older women found that both LH and FSH levels were lower in the older group, and the amplitude of their hormone pulses was reduced. The frequency of pulses stayed about the same, but the coordinated co-release of LH and FSH that was present in early postmenopausal women was lost in older women.¹² This suggests that the pituitary’s capacity to respond to GnRH gradually diminishes with age, even after the ovaries are long out of the picture.

How Stress and Nutrition Tune the System

The reproductive axis does not operate in isolation. It is tightly linked to the body’s stress and energy-sensing systems, and for good biological reason: pregnancy and child-rearing require substantial resources, so the body has mechanisms to suppress reproduction when conditions are unfavorable.

Chronic or severe stress inhibits the reproductive axis at multiple levels. The stress hormone system can suppress GnRH pulsatility, lower gonadotropin release, and reduce sex steroid production.¹³ The cross-talk runs both ways: the reproductive axis can also modulate stress signaling.¹⁴ This explains why prolonged psychological stress, overtraining in athletes, or severe illness can cause periods to stop or sperm counts to drop.

Nutritional status matters just as much. The fat-derived hormone leptin acts as a metabolic green light for reproduction. It signals the brain that energy stores are adequate, and through kisspeptin neurons and also directly on pituitary gonadotropes, it permits normal GnRH signaling.¹⁵ Mice that cannot produce leptin are infertile, and deleting leptin receptors specifically on pituitary gonadotrope cells caused infertility in about 70% of female mice in one study.¹⁶ In humans, this is the mechanism behind the loss of menstrual periods in women with very low body fat, whether from anorexia, extreme exercise, or chronic malnutrition. The ghrelin system, a hunger-signaling hormone from the stomach, also feeds into gonadotropin regulation, adding another layer of metabolic input.¹⁷

Environmental Chemicals and Gonadotropin Disruption

A growing body of research has identified synthetic chemicals in the environment that can interfere with gonadotropin signaling. These endocrine disruptors can act at several points along the chain: altering the levels of FSH or its receptor, disrupting the intracellular signaling pathways FSH activates, or changing the gene expression that FSH is supposed to regulate in follicle cells.¹⁸

One recent example involves carbendazim, a fungicide used in agriculture that has been detected in human follicular fluid. In laboratory experiments, carbendazim reduced steroid hormone production that had been stimulated by FSH, LH, or the growth factor IGF-1, and mice exposed to the chemical showed prolonged reproductive cycles, fewer ovulatory structures, and decreased gonadotropin and estradiol levels.¹⁹ Whether typical environmental exposure levels in humans are high enough to cause similar effects is still an active area of investigation, but findings like these explain the growing interest in how everyday chemical exposures might be contributing to declining fertility in industrialized populations.

Gonadotropins in Medicine

The clinical use of gonadotropins is perhaps most visible in fertility treatment. During in vitro fertilization (IVF), injectable FSH is used to stimulate the ovaries to produce multiple eggs in a single cycle, rather than the usual one. LH or hCG is then given as a trigger shot to induce final egg maturation before retrieval. The doses involved are substantial: protocols may use thousands of international units of hCG for the trigger, and some research has explored combining lower doses of hCG with FSH as an alternative approach.²⁰

Beyond fertility, hCG measurement is a critical diagnostic tool. In pregnancy, rising hCG levels confirm viability and can flag problems like ectopic pregnancy or miscarriage when levels do not double as expected. But hCG is also produced by certain cancers. Gestational trophoblastic disease, testicular germ cell tumors, and choriocarcinoma all secrete variants of hCG, and serial monitoring of these levels is used both to diagnose these cancers and to track whether treatment is working.²¹ Improved assays and imaging have made earlier diagnosis possible, reducing complications.²²

hCG Beyond Pregnancy and the Gonads

For decades, hCG was thought of strictly as a pregnancy hormone that acts on the ovaries. Research over the past ten to fifteen years has complicated that picture. Receptors for hCG have been found in a variety of tissues outside the gonads, and multiple structural variants of the molecule exist, including hyperglycosylated hCG and a free beta subunit form. Each variant appears to have somewhat different biological roles. Hyperglycosylated hCG, for instance, has been implicated in both the invasive growth of the early placenta and, when produced abnormally, in the development of certain tumors. Other non-gonadal applications include the use of hCG levels to predict preeclampsia risk, to screen for Down syndrome during pregnancy, and potentially to influence uterine muscle contractility.²³ The expanding list of tissues that respond to hCG suggests it is less a niche pregnancy hormone and more a widespread signaling molecule whose full range of actions researchers are still cataloging.

Circadian Rhythms and Gonadotropin Pulsatility

GnRH pulses do not happen at random. They are influenced by the body’s internal clock, and disrupting that clock has measurable reproductive consequences. In laboratory experiments, cells that produce GnRH were found to contain active circadian clock genes, and when the function of those clock genes was disrupted, the normal pulsatile pattern of GnRH release changed. Interfering with one clock gene decreased GnRH pulse frequency, while overexpressing another increased pulse amplitude.²⁴ Mice with a mutation in a core clock gene were subfertile and had longer reproductive cycles. This finding adds biological weight to the observation that shift workers and people with chronically disrupted sleep patterns often experience menstrual irregularities and reduced fertility. The clock is not just keeping time; it is directly wired into the machinery that controls reproductive hormone release.

An Ancient System Across Species

The GnRH-gonadotropin system is not unique to mammals. It is remarkably old in evolutionary terms, present in some form across virtually all vertebrates. Fish have been particularly informative for understanding how the system evolved: most bony fish carry two or three different forms of GnRH and up to five types of GnRH receptors, each with distinct locations in the brain and different functional roles.²⁵ In mammals, the system has been simplified to one dominant GnRH form and one primary receptor type, but the underlying logic of a hypothalamic peptide controlling pituitary gonadotropins that in turn drive the gonads is conserved across hundreds of millions of years of evolution. The fact that such a wide range of species relies on this same basic architecture speaks to how effective it is at coordinating reproduction with environmental conditions, whether the signal comes from water temperature in a coral reef or from leptin levels in a human adipocyte.