FSH and LH: The Gonadotropin Peptide Hormones

Pituitary Peptide Hormones

2 glycoprotein hormones

FSH and LH are heterodimeric glycoprotein hormones sharing an identical 92-amino acid alpha subunit but differing in their beta subunits, which determine receptor specificity and reproductive function.

Casati et al., Biochemical Pharmacology, 2023

Casati et al., Biochemical Pharmacology, 2023

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Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are the two gonadotropin peptide hormones produced by the anterior pituitary gland. They are the direct hormonal signals that drive reproductive function in both sexes: FSH stimulates egg development in ovaries and sperm production in testes, while LH triggers ovulation and testosterone synthesis. Both are glycoproteins, meaning they are peptide chains with attached sugar molecules that are essential for their biological activity. They share an identical alpha subunit but have unique beta subunits that determine which receptor each hormone activates. The entire system is controlled by a single hypothalamic peptide, gonadotropin-releasing hormone (GnRH), delivered in pulses that differentially regulate FSH and LH secretion. Recombinant versions of both hormones are now cornerstones of fertility medicine. For a broader look at pituitary hormones, see the pillar article on prolactin and the pituitary hormone family.

Structure: shared alpha, unique beta

FSH and LH belong to the glycoprotein hormone family, a group that also includes thyroid-stimulating hormone (TSH) and human chorionic gonadotropin (hCG). All four hormones are heterodimers consisting of two non-covalently linked subunits: an alpha subunit and a beta subunit.

The alpha subunit is identical across all four glycoprotein hormones in humans. It is encoded by a single gene (CGA) on chromosome 6 and consists of 92 amino acids with two N-linked glycosylation sites. This shared alpha subunit is necessary for receptor binding but not sufficient; it provides the structural framework while the beta subunit determines specificity.

The beta subunit differs between hormones and is what makes FSH bind to FSH receptors and LH bind to LH receptors. FSH-beta is 111 amino acids with two N-linked glycosylation sites. LH-beta is 121 amino acids with one N-linked glycosylation site. The beta subunits share approximately 40% amino acid sequence homology with each other, reflecting their evolutionary origin from a common ancestral gene, but the differences in their three-dimensional structure and glycosylation patterns are sufficient to produce completely different receptor binding profiles.

Glycosylation is not merely a post-translational decoration. The sugar chains attached to FSH and LH are essential for protein folding, receptor binding affinity, signal transduction, and circulatory half-life. Different glycosylation patterns create isoforms with different biological potencies and pharmacokinetic profiles. Acidic (sialylated) isoforms have longer half-lives but lower receptor binding affinity, while less sialylated isoforms bind receptors more potently but are cleared faster from circulation. The body exploits this isoform heterogeneity to fine-tune the hormonal signal across the menstrual cycle.

How GnRH pulse frequency controls FSH vs. LH

A single hypothalamic peptide, GnRH, controls the release of both FSH and LH from the anterior pituitary. This seems paradoxical: how can one signal regulate two distinct hormones with different functions at different times in the reproductive cycle?

The answer is pulse frequency. GnRH is released in discrete pulses from hypothalamic neurons, and the frequency of these pulses determines which gonadotropin is preferentially secreted. Casati et al. (2023) reviewed the physiology and pharmacology of GnRH, establishing that low-frequency GnRH pulses (approximately one pulse every 2-4 hours) preferentially stimulate FSH transcription and release, while high-frequency pulses (approximately one pulse per hour) favor LH production.^[1]

This frequency-dependent coding is mediated by intracellular signaling pathways in pituitary gonadotrope cells. Low-frequency pulses activate different transcription factor cascades than high-frequency pulses, differentially regulating the FSH-beta and LH-beta gene promoters. The result is that a single releasing hormone can produce two different hormonal outputs by varying its temporal pattern, an elegant biological solution to a complex regulatory problem.

The kisspeptin connection

The neurons that generate GnRH pulses are themselves regulated by upstream peptide signals. Kisspeptin, produced by KNDy (kisspeptin/neurokinin B/dynorphin) neurons in the arcuate nucleus of the hypothalamus, is the primary activator of GnRH neuron firing.

Sliwowska et al. (2024) reviewed kisspeptin's dual role in regulating both reproduction and metabolism, noting that disruptions in metabolic conditions lead to simultaneous reproductive dysfunction through altered kisspeptin signaling. Only 15% of clinical studies address kisspeptin's metabolic role, despite growing evidence that metabolic status and reproductive function are tightly coupled through this peptide system.^[2]

Mills and Dhillo (2022) reviewed the translation of kisspeptin and neurokinin B biology into new therapies for reproductive health. Kisspeptin administration stimulates reproductive hormone secretion in both healthy individuals and those with reproductive disorders, opening a therapeutic avenue for conditions where GnRH agonists or antagonists are currently used.^[5]

What FSH and LH do in female reproduction

In females, FSH and LH orchestrate the menstrual cycle through a carefully timed sequence of hormonal signals.

Follicular phase (days 1-14). FSH is the dominant gonadotropin during the first half of the cycle. It stimulates the growth of ovarian follicles, with typically 5-15 follicles recruited each cycle. As follicles grow, they produce increasing amounts of estradiol and inhibin B, which feed back to the pituitary to suppress FSH secretion. This declining FSH creates a selection pressure: only the follicle with the most FSH receptors and the greatest sensitivity to declining FSH levels survives to become the dominant follicle. The rest undergo atresia (programmed death).

Ovulation (day 14). The dominant follicle's rising estradiol production eventually crosses a threshold that switches estradiol's effect on GnRH release from inhibitory to stimulatory (positive feedback). This triggers a massive surge of LH, with plasma LH concentrations increasing 6- to 10-fold over approximately 36-48 hours. The LH surge triggers ovulation: the dominant follicle ruptures and releases its oocyte.

Luteal phase (days 15-28). After ovulation, LH sustains the corpus luteum (the remnant of the ruptured follicle), which produces progesterone to prepare the uterine lining for potential implantation. If pregnancy occurs, human chorionic gonadotropin (hCG) from the developing placenta takes over the LH function, maintaining the corpus luteum until the placenta itself can produce sufficient progesterone.

What FSH and LH do in male reproduction

In males, FSH and LH work in concert to support spermatogenesis and testosterone production, though the hormonal dynamics are continuous rather than cyclical.

LH stimulates Leydig cells in the testes to produce testosterone. The testosterone produced acts locally within the testes to support spermatogenesis and systemically to maintain secondary sexual characteristics, muscle mass, bone density, and libido. Testosterone feeds back to the hypothalamus and pituitary to suppress GnRH and LH secretion, maintaining homeostatic control.

Amodei et al. (2020) demonstrated that kisspeptin and neurokinin B regulate LH and testosterone secretion in the developing fetus, establishing that this regulatory axis is active well before puberty and plays a role in prenatal sexual development.^[6]

FSH stimulates Sertoli cells, the "nurse cells" of the seminiferous tubules that provide structural and nutritional support to developing sperm cells. FSH increases Sertoli cell production of androgen-binding protein, which concentrates testosterone within the tubules at the levels required for spermatogenesis (approximately 100-fold higher than circulating levels). FSH also promotes the production of inhibin B by Sertoli cells, which feeds back to selectively suppress FSH secretion without affecting LH.

Clinical applications: recombinant gonadotropins in fertility

Recombinant human FSH (follitropin alfa, follitropin beta) and recombinant human LH (lutropin alfa) are manufactured by expressing the human alpha and beta subunit genes in Chinese hamster ovary (CHO) cells. These recombinant proteins have replaced older urinary-derived gonadotropin preparations (human menopausal gonadotropin, hMG) in many clinical settings because they offer higher purity, defined potency, and batch-to-batch consistency.

Controlled ovarian stimulation for IVF

The primary clinical application of recombinant FSH is controlled ovarian hyperstimulation during in vitro fertilization (IVF). Exogenous FSH is administered to override the natural single-follicle selection process and stimulate the development of multiple follicles simultaneously, increasing the number of oocytes available for fertilization. Typical protocols use 150-300 IU of recombinant FSH daily for 8-12 days, with monitoring by ultrasound and estradiol measurements.

LH supplementation is added in specific clinical scenarios. Women with severe LH deficiency (WHO Group I hypogonadotropic hypogonadism) require exogenous LH in addition to FSH, because their pituitary does not produce enough endogenous LH to support follicular steroidogenesis. In older women undergoing IVF, LH supplementation may improve oocyte quality, though this remains an area of ongoing investigation.

Ovulation induction

For women with anovulatory infertility (WHO Group II, including some cases of PCOS), low-dose FSH protocols aim to develop a single dominant follicle rather than multiple follicles. The goal is to restore the natural cycle rather than produce supraphysiological stimulation. Starting doses of 50-75 IU of FSH with gradual increases allow careful titration toward mono-follicular development.

Male infertility

Recombinant FSH combined with hCG (which mimics LH's action on Leydig cells) is used to treat male infertility caused by hypogonadotropic hypogonadism. Treatment durations of 3-6 months are typically needed before spermatogenesis improves, reflecting the approximately 74-day duration of the human spermatogenic cycle.

GnRH manipulation: controlling FSH and LH pharmacologically

Because FSH and LH are controlled by pulsatile GnRH, manipulating GnRH provides indirect pharmacological control over gonadotropin secretion.

GnRH agonists (leuprolide, goserelin, nafarelin) initially stimulate FSH and LH release (the "flare effect") but then desensitize pituitary GnRH receptors through continuous rather than pulsatile stimulation, leading to suppression of both gonadotropins within 1-2 weeks. This is exploited in endometriosis treatment, breast cancer therapy, and IVF protocols where premature LH surges must be prevented.

GnRH antagonists (cetrorelix, ganirelix, elagolix, relugolix) directly block GnRH receptors, causing immediate suppression of FSH and LH without the initial flare. Patel et al. (2024) compared the chemistry and formulation of GnRH peptide antagonists, analyzing their implications for clinical safety and efficacy across different formulations and delivery systems.^[7]

Moradi et al. (2015) demonstrated that glycosylation of LHRH (GnRH) analogs significantly increased their enzymatic stability, with half-lives extending from 3 minutes to up to 103 minutes when exposed to kidney membrane enzymes. These glycopeptides also showed significant antiproliferative activity against cancer cell lines, illustrating the dual therapeutic and oncological potential of modified GnRH peptides.^[8]

Gonadotropin pathology: when FSH and LH go wrong

Hestiantoro et al. (2024) examined altered expression of kisspeptin, dynorphin, and related neuropeptides in polycystic ovary syndrome (PCOS). They found that disrupted neuropeptide signaling upstream of GnRH contributes to the tonically elevated LH and relatively suppressed FSH that characterize PCOS, driving the excess androgen production and anovulation that define the condition.^[3]

Emerging research: FSH receptor targeting beyond fertility

Rahman et al. (2025) demonstrated a novel application of FSH biology: using the FSH receptor as a targeting mechanism for cancer therapy. They created a peptide conjugate (Hecate-FSH-beta-33-53-C/S) consisting of a lytic antimicrobial peptide linked to a fragment of the FSH beta subunit. This conjugate selectively killed FSHR-positive cancer cells (KGN granulosa tumor cells and FSHR-transfected HEK293 cells) while showing minimal toxicity to FSHR-negative controls.^[4] This approach exploits the fact that FSH receptors are overexpressed on certain ovarian and prostate cancers, potentially opening a peptide-based targeted therapy pathway.

What remains uncertain

Despite decades of clinical use, several aspects of FSH and LH biology remain incompletely understood.

The optimal ratio of FSH to LH during IVF stimulation is debated. Some evidence suggests that LH supplementation improves outcomes in certain patient subgroups (older women, poor responders), but large randomized trials have produced conflicting results. Whether recombinant FSH plus recombinant LH is clinically superior to urinary hMG (which contains both FSH and LH activity) remains an active area of investigation.

The role of FSH isoform heterogeneity in natural reproduction and clinical outcomes is poorly characterized. Different glycosylation patterns create FSH isoforms with different potencies and half-lives, but current clinical assays do not distinguish between isoforms, and recombinant FSH preparations produce a narrower isoform range than natural pituitary secretion.

The recently proposed existence of a specific FSH-releasing hormone (FSH-RH), distinct from GnRH, would fundamentally alter the understanding of differential gonadotropin regulation if confirmed.

The Bottom Line

FSH and LH are glycoprotein hormones that share an alpha subunit but differ in their beta subunits, which determine receptor specificity. They are regulated by pulsatile GnRH release from the hypothalamus, with pulse frequency determining which hormone predominates. Together they drive follicular development, ovulation, spermatogenesis, and steroidogenesis. Recombinant forms are foundational tools in fertility medicine, while upstream manipulation of GnRH provides pharmacological control for conditions ranging from endometriosis to hormone-dependent cancers.

Frequently Asked Questions

What is the difference between FSH and LH?

FSH (follicle-stimulating hormone) and LH (luteinizing hormone) are both glycoprotein hormones from the anterior pituitary that share an identical 92-amino acid alpha subunit. They differ in their beta subunits: FSH-beta is 111 amino acids, LH-beta is 121 amino acids. Functionally, FSH stimulates follicle growth and sperm production, while LH triggers ovulation and testosterone synthesis. GnRH pulse frequency determines which hormone is preferentially released.

What happens when FSH is high?

Elevated FSH typically indicates reduced ovarian reserve in women or testicular failure in men. The pituitary increases FSH production when negative feedback from the gonads (estradiol and inhibin in women, testosterone and inhibin in men) decreases. In women, consistently elevated FSH levels above 10-15 mIU/mL in the early follicular phase suggest diminishing egg supply and are used as one marker of ovarian reserve alongside AMH testing.

How are FSH and LH used in IVF?

Recombinant FSH (follitropin alfa) is administered at 150-300 IU daily for 8-12 days to stimulate multiple follicular development during controlled ovarian hyperstimulation. LH supplementation is added for women with severe LH deficiency. GnRH agonists or antagonists are used concurrently to prevent premature LH surges that would trigger ovulation before egg retrieval. The timing of a final hCG or GnRH agonist trigger mimics the natural LH surge to induce final oocyte maturation.

What controls the release of FSH and LH?

GnRH from the hypothalamus is the primary controller. It is released in pulses, with low-frequency pulses (every 2-4 hours) favoring FSH and high-frequency pulses (approximately hourly) favoring LH (Casati et al., 2023). Upstream, kisspeptin neurons activate GnRH release. Downstream, estradiol, progesterone, testosterone, and inhibin feed back to modulate GnRH pulse frequency and pituitary sensitivity, creating a self-regulating loop.

What is the LH surge and why does it matter?

The LH surge is a 6- to 10-fold increase in plasma LH concentration occurring over approximately 36-48 hours at midcycle. It is triggered when rising estradiol from the dominant follicle switches from inhibiting to stimulating GnRH release (positive feedback). The LH surge causes ovulation: the dominant follicle ruptures and releases its egg. In IVF, preventing premature LH surges with GnRH antagonists is essential to control the timing of egg retrieval.

Can FSH and LH levels indicate PCOS?

In polycystic ovary syndrome, the typical pattern is elevated LH relative to FSH, often with an LH:FSH ratio greater than 2:1, though this ratio is not present in all cases. Hestiantoro et al. (2024) found that altered kisspeptin and dynorphin expression upstream of GnRH contributes to the tonically elevated LH pulsatility that drives excess androgen production in PCOS. FSH is relatively suppressed, contributing to impaired follicular development and anovulation.