Gonadotropins are the two hormones that directly control fertility in both men and women: follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Without them, eggs don’t mature, ovulation doesn’t happen, and sperm production stalls. They act as the chemical messengers between your brain and your reproductive organs, and they’re also the basis for many injectable fertility treatments.
How Your Brain Controls Reproduction
The whole system starts in a small region of the brain called the hypothalamus. It releases a signaling hormone in pulses, and those pulses tell the pituitary gland (a pea-sized structure at the base of the brain) to secrete FSH and LH into the bloodstream. The speed of those pulses matters: faster pulses favor LH release, while slower pulses favor FSH. This pulsing pattern is what gives the menstrual cycle its phases and keeps male hormone production steady.
FSH and LH then travel to the ovaries or testes, where they do their work. The sex hormones produced there, primarily estrogen and testosterone, feed back to the brain to dial production up or down. This feedback loop is how the body keeps reproductive hormones in balance from month to month.
What FSH and LH Do in the Ovaries
Each month, a group of small fluid-filled sacs called follicles begins growing in the ovaries. FSH is the hormone responsible for recruiting these follicles and pushing them to develop. It stimulates the cells lining each follicle to divide, form the fluid-filled cavity, and produce estrogen. Specifically, FSH activates an enzyme system that converts androgens into estrogen, which is why estrogen levels rise steadily during the first half of the menstrual cycle.
LH works alongside FSH but plays a distinct role. It stimulates the outer layer of each follicle to produce the androgen building blocks that get converted into estrogen. As the cycle progresses, LH drives the continued remodeling and growth of the developing follicle. Eventually, one follicle becomes dominant and the others regress.
The dramatic spike in LH, often called the “LH surge,” is what triggers ovulation. This surge causes the dominant follicle to rupture and release a mature egg. After ovulation, LH supports the leftover follicle structure as it transforms into a temporary hormone-producing body that secretes progesterone to prepare the uterine lining for a potential pregnancy.
What FSH and LH Do in the Testes
In men, the two gonadotropins divide the work between two different cell types. LH acts on Leydig cells, which sit in the tissue between the sperm-producing tubes. These cells respond to LH by making testosterone, the hormone essential for sperm production, sex drive, and muscle mass. FSH, meanwhile, acts on Sertoli cells, which line the inside of the sperm-producing tubes. Sertoli cells create the chemical environment that germ cells need to develop into mature sperm.
Both hormones are required. Testosterone alone isn’t enough to produce healthy sperm in normal quantities. FSH ensures the Sertoli cells produce the signaling molecules and nutrients that guide immature germ cells through the weeks-long process of becoming functional sperm, which are then transported to the epididymis for storage and final maturation.
Gonadotropin Levels as a Fertility Marker
Measuring FSH and LH through a blood test gives clinicians a window into how well the reproductive system is functioning. In women, FSH is typically measured on day 2 or 3 of the menstrual cycle. During the follicular phase, normal FSH generally falls between 1 and 9 IU/L, and LH between 1 and 12 IU/L. An FSH level that’s elevated above the normal range at this point in the cycle suggests the ovaries are working harder to develop follicles, which can indicate diminished ovarian reserve, meaning fewer eggs remain.
In men, persistently low levels of both FSH and LH point to a condition called hypogonadotropic hypogonadism, where the brain isn’t sending enough signal to the testes. This is one of the most treatable causes of male infertility because the testes themselves are often healthy and just need the right hormonal cues. Elevated FSH in men, on the other hand, can indicate that the testes aren’t responding well, since the brain compensates by producing more signal.
Gonadotropin Injections in Fertility Treatment
When the body doesn’t produce enough FSH or LH on its own, or when fertility treatments require more follicles than a natural cycle produces, gonadotropin injections step in. These are subcutaneous shots, usually self-administered daily, containing lab-made or urine-derived versions of FSH, LH, or both. They’re used in IUI (intrauterine insemination) cycles to stimulate the ovaries to produce one or a few mature eggs, and in IVF to stimulate multiple follicles at once.
Most patients with a normal expected response start at 150 to 200 IU per day of FSH. Those expected to respond poorly, such as older patients or those with diminished ovarian reserve, often begin at 300 IU per day, with some protocols going up to 450 IU per day. Doses are adjusted every few days based on ultrasound monitoring and blood work.
For men with hypogonadotropic hypogonadism, treatment typically begins with hCG (a hormone that mimics LH) given several times per week for three to six months to stimulate testosterone production. If sperm production hasn’t resumed by then, FSH injections are added. One multi-center study found that men who were previously producing no sperm or very little were able to reach an average concentration of 22 million sperm per milliliter after about four months of combined treatment.
Types of Gonadotropin Medications
There are two broad categories: recombinant gonadotropins, which are manufactured using genetic engineering and contain pure FSH or LH, and urinary-derived gonadotropins, which are extracted from the urine of postmenopausal women and contain a mix of FSH and LH activity. Both are effective, but they aren’t identical in outcomes.
A Cochrane review covering 11 trials and over 3,000 women found that urinary-derived gonadotropins containing LH activity produced slightly higher live birth rates than pure recombinant FSH. For every 1,000 women treated, roughly 255 achieved a live birth with the urinary product compared to about 223 with recombinant FSH. Clinical pregnancy rates showed a similar pattern. The difference is modest, and many clinics use both types depending on patient characteristics and protocol preferences.
Risks of Gonadotropin Therapy
The most significant risk is ovarian hyperstimulation syndrome (OHSS), a condition where the ovaries overrespond to stimulation and swell, sometimes causing fluid to leak into the abdomen. Moderate OHSS occurs in roughly 3% to 7% of stimulated cycles, while severe OHSS, which can require hospitalization, occurs in 0.1% to 2%. Modern protocols have reduced these rates significantly through careful monitoring and the use of alternative trigger medications that avoid hCG when risk is high.
Other common side effects include injection site soreness, bloating, mood swings, and headaches. Multiple pregnancies (twins or higher) are also a well-known risk, particularly in IUI cycles where the number of mature follicles is harder to control than in IVF, where the number of embryos transferred is a deliberate decision.