HCG

What HCG is

Human chorionic gonadotropin (hCG) is a glycoprotein hormone comprising two non-covalently joined subunits—alpha and beta—that together form the biologically active heterodimer ^[1]. While the alpha subunit is shared with other pituitary glycoprotein hormones, the beta subunit and its unique carboxyl-terminal peptide (CTP) extension are specific to hCG and confer a longer biological half-life compared to structurally similar luteinizing hormone (LH) ^[2]. hCG exists in multiple molecular forms, including classical hCG, hyperglycosylated hCG, and the free beta-subunit, each with distinct biological roles ^[1].

The hormone was discovered in 1927 when Selmar Ascheim and Bernhard Zondek demonstrated that the blood and urine of pregnant women contained a gonad-stimulating substance ^[3]. Injecting this substance into immature female mice produced follicular maturation, luteinization, and hemorrhage into the ovarian stroma—an observation that became the basis for the first clinical pregnancy test, known as the Ascheim-Zondek test ^[3]. Subsequent research in the 1940s confirmed that hCG is produced by the placental trophoblast rather than the pituitary gland ^[3]. Classical hCG is among the first molecules synthesized by the embryo; its RNA is transcribed as early as the eight-cell stage, and the blastocyst produces the protein before implantation ^[4]. Decades of scientific progress led to purified urinary preparations and, eventually, recombinant hCG (r-hCG) produced via recombinant DNA technology for use in clinical research and practice ^[5].

What it is being researched for

1. Ovulation induction and assisted reproduction

One of the most extensively studied applications of hCG is its use as a surrogate LH surge to trigger final oocyte maturation in controlled ovarian stimulation cycles. Because hCG binds the same LH/CGR receptor as endogenous LH, it can mimic the midcycle LH surge, promote oocyte meiosis completion, and initiate follicular luteinization ^[6]. Clinical trials and meta-analyses have confirmed that recombinant hCG (r-hCG) and urinary hCG (u-hCG) yield comparable ovulation rates, oocyte yields, and pregnancy outcomes in women undergoing IVF and intrauterine insemination ^[7]. A Phase III randomized trial in Japanese women with anovulation demonstrated that r-hCG was non-inferior to u-hCG for triggering ovulation, with an expected and acceptable safety profile ^[5]. Research has also explored how the timing interval between hCG administration and oocyte retrieval affects ART outcomes, with meta-analyses examining optimal retrieval windows to maximize oocyte maturity and fertilization competence ^[8].

2. Male hypogonadotropic hypogonadism and spermatogenesis

In men with hypogonadotropic hypogonadism (HH)—a condition in which impaired pituitary gonadotropin secretion leads to deficient testosterone and sperm production—hCG has been studied as a means of stimulating Leydig cells in the testes to restore intratesticular testosterone and support spermatogenesis ^[9]. A Phase III multi-centre open-label trial found that hCG pretreatment normalized testosterone in a majority of azoospermic HH men, and subsequent combined treatment with a long-acting FSH analog induced spermatogenesis in more than 75% of participants who had remained azoospermic on hCG alone ^[9]. A comparative retrospective study in adolescent males with HH further reported that hCG therapy produced significantly greater testicular volume than testosterone replacement, an effect believed to be relevant to future fertility preservation ^[10].

3. Fertility preservation during testosterone replacement therapy

Research has investigated whether co-administration of hCG alongside exogenous testosterone can prevent the suppression of intratesticular testosterone (ITT) and spermatogenesis that typically accompanies testosterone replacement therapy (TRT). A retrospective review of men on TRT who also received hCG found that no patient became azoospermic, sperm motility was maintained, and pregnancy was achieved in 9 out of 12 patients attempting to conceive during treatment ^[11]. The authors concluded that hCG appeared protective of continued spermatogenesis in hypogonadal men desiring pregnancy, while acknowledging that longer follow-up data were needed ^[11].

4. Luteal phase support in IVF

Following oocyte retrieval in IVF cycles, corpus luteum dysfunction can impair progesterone production and reduce implantation rates. hCG has been studied as a luteal support agent to stimulate corpus luteum progesterone secretion. A prospective randomized study in 310 IVF patients found that hCG and micronized vaginal progesterone were equally effective for luteal support, with no significant difference in pregnancy rates between groups ^[12]. Research also shows that the hCG trigger dose is positively associated with endogenous progesterone production during the mid-late luteal phase, though the clinical relevance for live birth rates requires further investigation ^[13]. The principal concern limiting broader use of hCG for luteal support is the risk of ovarian hyperstimulation syndrome (OHSS) ^[14].

5. Early embryo implantation and placentation

hCG and its receptor (LH/CGR) are expressed in numerous gonadal and extra-gonadal reproductive tissues, and preclinical research indicates roles in oocyte maturation, fertilization, implantation, and early embryo development ^[2]. A variant form—hyperglycosylated hCG—is produced by extravillous invasive cytotrophoblast cells and acts as an autocrine factor that promotes cell invasion essential for hemochorial placentation ^[1]. Researchers have described hCG as a multi-effect hormone with significant cytokine interactions that regulate the maternal immune response during conception, implantation, and the early maintenance of pregnancy ^[4].

6. Oncology and tumor biology

Beyond its reproductive functions, hCG has been studied in the context of cancer biology. Certain non-trophoblastic malignancies produce hyperglycosylated hCG free beta-subunit, which has been linked to promotion of cell invasion and inhibition of apoptosis ^[1]. Research has examined the roles of hCG and its receptor in endometrial adenocarcinoma and breast cancer, noting that the hormone's impact is not limited to pregnancy physiology and may influence tumor formation and metastatic outgrowth ^[15]. Immunological strategies targeting hCG—including vaccine approaches—have also been explored in both fertility control research and as potential tools in the management of hCG-expressing cancers ^[16].

7. Oligopeptide-derived drug discovery

Small degradation products of the hCG molecule have attracted research interest as potential therapeutic leads. Studies have examined oligopeptides derived from the beta-subunit of hCG and found that certain sequences can inhibit severe inflammation, the onset of experimental type I diabetes, renal failure, and tumorigenesis in model systems ^[17]. One such oligopeptide was reported to accelerate recovery after lethal radiation in mice and has been evaluated in early-phase human clinical trials, illustrating hCG's utility as a model molecule for oligopeptide-based drug discovery beyond its hormonal functions ^[17].

How it is thought to work

hCG exerts its principal biological effects by binding to the luteinizing hormone/choriogonadotropin receptor (LHCGR), a G protein-coupled receptor (GPCR) belonging to the rhodopsin-like class A family, characterized by a large extracellular leucine-rich repeat domain ^[6]. Although hCG and LH share this receptor, research has demonstrated that they are not biologically equivalent: hCG tends to generate more potent and sustained activation of the cAMP/protein kinase A (PKA) steroidogenic pathway, while LH binding results in faster and more potent activation of the proliferative ERK1/2 and AKT pathways ^[2]. Upon receptor activation, LHCGR primarily couples to Gαs proteins, stimulating adenylyl cyclase and triggering cAMP accumulation, PKA activation, and downstream regulation of steroidogenic enzymes ^[6]. At higher hormone and receptor concentrations, LHCGR can also couple to Gαq proteins to activate phospholipase C and increase intracellular calcium ^[6].

The pharmacological distinction between hCG and LH—now described as biased agonism—means the two hormones elicit quantitatively and qualitatively different intracellular signalling outcomes even when binding the same receptor ^[2]. hCG's longer biological half-life, attributable to its carboxyl-terminal peptide extension and higher degree of glycosylation, also means it sustains receptor occupancy for longer than the pulsatile LH signal, resulting in more prolonged cAMP production in gonadal cells ^[2]. Using multiple validated in vitro cell models, researchers have confirmed that recombinant hCG was more potent than LH for cAMP responses and progesterone production, while LH showed relatively greater effects on β-arrestin recruitment ^[2]. This receptor-level complexity has implications for understanding how hCG supports corpus luteum steroidogenesis during early pregnancy and why its clinical applications in reproductive medicine differ meaningfully from those of recombinant LH.

Where the evidence stands

The clinical evidence base for hCG is most robust in the area of assisted reproduction. Multiple prospective, randomized controlled trials and systematic reviews have demonstrated that r-hCG and u-hCG produce comparable outcomes—including similar numbers of oocytes retrieved, fertilization rates, and clinical pregnancy rates—when used as ovulation triggers in IVF and ovulation induction cycles ^[7]. A meta-analysis examining follicular phase hCG supplementation found that clinical pregnancy rates were significantly higher in the hCG group compared to standard FSH-alone protocols at the late follicular phase, though live birth rates were not significantly different across compared regimens ^[18]. In male hypogonadotropic hypogonadism, a Phase III multi-centre trial provided strong evidence that hCG-based gonadotropin therapy can normalize testosterone and induce spermatogenesis, with spermatogenesis achieved in the majority of men who were refractory to hCG monotherapy when FSH was added ^[9]. A retrospective comparative study in adolescents with HH confirmed significantly greater testicular volume with hCG versus testosterone, though its small sample size (n=4 in the hCG group) limits generalizability ^[10].

Important gaps and limitations persist across the evidence base. Most luteal phase support studies with hCG are limited by small sample sizes and heterogeneous protocols, and the consistent concern about hCG-associated ovarian hyperstimulation syndrome (OHSS) has made progesterone supplementation the preferred first-line luteal support option in high-risk patients ^[14]. Evidence for hCG's roles in implantation biology, immune modulation, and oncology remains largely preclinical—derived from in vitro cell studies and animal models—and has not been comprehensively translated into human interventional trials ^[15][17]. The oligopeptide-derived hCG research, while promising in animal models, is still in early-phase human evaluation ^[17]. Much of the foundational mechanistic understanding of the LHCGR signalling differences between hCG and LH also comes from cell line models and requires further validation in primary human tissue systems ^[2][6].

Frequently asked questions

What is HCG and why is it called the pregnancy hormone?

HCG (human chorionic gonadotropin) is a glycoprotein hormone produced by the developing embryo and then the placenta beginning very early in pregnancy. Its primary physiological role is to signal the corpus luteum to continue producing progesterone, thereby maintaining the uterine lining and supporting early embryo development. Because its levels rise sharply after implantation, hCG detection in blood or urine is the basis for most pregnancy tests. It is often called 'the pregnancy hormone' because it is one of the first signals the embryo sends to the maternal body.

How does HCG differ from LH?

hCG and LH are both glycoprotein hormones that bind to the same receptor (LHCGR), but they are structurally and functionally distinct. hCG has a unique carboxyl-terminal peptide extension on its beta subunit that greatly extends its half-life compared to LH. Research shows that while LH produces faster but shorter signalling bursts, hCG elicits more sustained activation of the cAMP steroidogenic pathway. These pharmacological differences mean the two hormones act as natural 'biased agonists' at their shared receptor.

What is HCG studied for in men?

In men, hCG is primarily researched for its ability to stimulate testicular Leydig cells to produce testosterone and to support spermatogenesis. Clinical studies have focused on men with hypogonadotropic hypogonadism, where the pituitary does not produce adequate LH or FSH. Research also examines whether hCG can preserve fertility in men who require testosterone replacement therapy, where exogenous testosterone alone would otherwise suppress sperm production.

Is HCG used in fertility treatments?

Yes, hCG is extensively researched and used in fertility medicine. In women undergoing assisted reproduction, it is administered to trigger final oocyte maturation and ovulation after follicular stimulation with FSH. It has also been studied for luteal phase support in IVF to help maintain corpus luteum function and progesterone production after oocyte retrieval. Both urinary-derived and recombinant forms of hCG have been evaluated in clinical trials.

What are the different molecular forms of HCG?

hCG exists in several molecular variants, including classical (regular) hCG, hyperglycosylated hCG, and the free beta-subunit. Classical hCG is the predominant form made by the fused syncytiotrophoblast cells of the placenta and drives the main steroidogenic effects via the LHCGR. Hyperglycosylated hCG is produced by invasive cytotrophoblast cells and acts primarily as an autocrine promoter of cell invasion. The free beta-subunit has been studied in the context of certain malignancies.

When was HCG discovered?

hCG was discovered in 1927 by Selmar Ascheim and Bernhard Zondek, who demonstrated that the blood and urine of pregnant women contained a gonad-stimulating substance. When injected into immature female mice, this substance caused follicular maturation and luteinization. Their observation led directly to the development of the first clinical pregnancy test, known as the Ascheim-Zondek pregnancy test.

What is the LHCGR receptor and why does it matter for HCG research?

The LHCGR (luteinizing hormone/choriogonadotropin receptor) is a G protein-coupled receptor found mainly in the ovaries and testes that serves as the shared receptor for both hCG and LH. Understanding this receptor is central to hCG research because it mediates all of the hormone's known endocrine effects, including steroidogenesis, ovulation, and luteal support. Research into biased agonism at LHCGR—where hCG and LH trigger different intracellular signalling patterns despite binding the same receptor—may help refine and personalize assisted reproduction protocols.

What are the risks or limitations associated with HCG use in research and clinical contexts?

The most significant clinical concern associated with hCG is ovarian hyperstimulation syndrome (OHSS), a potentially serious complication in women undergoing ovarian stimulation, which is more common with hCG than with GnRH agonist triggers in high-risk patients. Researchers have noted this risk as a key limitation for using hCG as luteal phase support in IVF. In male populations, hCG therapy can lead to elevated estrogen levels due to increased aromatization, which is monitored in clinical studies. Evidence for many of hCG's non-reproductive research applications (e.g., immune modulation, oncology) remains largely preclinical.

Is there a recombinant version of HCG and how does it compare to urinary HCG?

Yes, recombinant hCG (r-hCG), produced via recombinant DNA technology, is a pure form of the hormone free of the urinary proteins present in traditional urinary-derived hCG (u-hCG). Multiple prospective randomized trials have demonstrated that r-hCG and u-hCG are clinically equivalent for ovulation triggering in terms of oocyte yield and pregnancy rates. r-hCG has been reported to cause fewer local injection site reactions than u-hCG due to its higher purity.

Can HCG be detected in non-pregnant individuals?

Yes, very low levels of hCG can be detected in some non-pregnant individuals, including a pituitary-derived form of hCG found in both men and women. Certain non-trophoblastic cancers can also produce hCG or its subunits, which is why hCG measurement can serve as a tumor marker in some oncological contexts. Rare cases of 'inherited hCG'—persistent low levels of circulating hCG in otherwise healthy non-pregnant individuals—have also been documented in the scientific literature.

Glossary

Glycoprotein hormone

A class of hormones characterized by a protein backbone attached to carbohydrate (sugar) chains, which influence the hormone's biological activity, half-life, and receptor binding.

LHCGR (LH/CGR)

The luteinizing hormone/choriogonadotropin receptor, a G protein-coupled receptor found primarily on ovarian and testicular cells that mediates the biological effects of both LH and hCG.

Hypogonadotropic hypogonadism (HH)

A clinical condition in which impaired pituitary secretion of LH and FSH results in insufficient gonadal function, low sex hormone levels, and often reduced fertility.

Biased agonism

A pharmacological phenomenon in which two different ligands binding the same receptor preferentially activate different downstream signalling pathways, leading to distinct biological outcomes.

Hyperglycosylated hCG

A molecular variant of hCG produced by invasive cytotrophoblast cells that carries additional carbohydrate chains and acts as an autocrine promoter of cell invasion during early placentation.

Luteal phase support

Supplementation provided after ovulation or oocyte retrieval in ART cycles to maintain corpus luteum function and adequate progesterone levels, supporting endometrial receptivity and early embryo implantation.

Ovarian hyperstimulation syndrome (OHSS)

A potentially serious complication of ovarian stimulation in which the ovaries become swollen and fluid shifts from blood vessels to body cavities, associated with hCG exposure in susceptible individuals.

Spermatogenesis

The biological process by which sperm cells are produced in the testes, dependent on adequate intratesticular testosterone levels and FSH signalling.

References

1. New discoveries on the biology and detection of human chorionic gonadotropin — Reproductive Biology and Endocrinology / PubMed
2. LH and hCG Action on the Same Receptor Results in Quantitatively and Qualitatively Different Intracellular Signalling — PLOS ONE / PMC
3. Human Chorionic Gonadotropin and Early Embryogenesis: Review — PMC / Frontiers in Endocrinology
4. Human Chorionic Gonadotropin: The Pregnancy Hormone and More — International Journal of Molecular Sciences
5. Phase III trial comparing the efficacy and safety of recombinant- or urine-derived human chorionic gonadotropin for ovulation triggering — Reproductive Medicine and Biology
6. Human Luteinizing Hormone and Chorionic Gonadotropin Display Biased Agonism at the LH and LH/CG Receptors — Scientific Reports / PMC
7. Use of recombinant human chorionic gonadotropin in ovulation induction — Fertility and Sterility / PubMed
8. Time interval between hCG administration and oocyte retrieval and ART outcomes: an updated systematic review and meta-analysis — Reproductive Biology and Endocrinology / PMC
9. An open-label clinical trial to investigate the efficacy and safety of corifollitropin alfa combined with hCG in adult men with hypogonadotropic hypogonadism — Reproductive Biology and Endocrinology / PMC
10. Testosterone versus hCG in Hypogonadotropic Hypogonadism - Comparing Clinical Effects and Evaluating Current Practice — PubMed
11. Low dose human chorionic gonadotropin prevents azoospermia and maintains fertility in hypogonadal men on testosterone replacement therapy — Fertility and Sterility
12. Human chorionic gonadotropin and intravaginal natural progesterone are equally effective for luteal phase support in IVF — Gynecological Endocrinology / PubMed
13. Does the HCG trigger dose used for IVF impact luteal progesterone concentrations? A randomized controlled trial — Human Reproduction / PubMed
14. Luteal phase support for assisted reproduction cycles (Cochrane Review) — Cochrane Database of Systematic Reviews / PubMed
15. The importance of hCG in human endometrial adenocarcinoma and breast cancer — International Journal of Biological Markers / PubMed