HCG (human chorionic gonadotropin) is a heterodimeric glycoprotein hormone that binds and activates the luteinizing hormone/choriogonadotropin receptor (LHCGR), the same G protein-coupled receptor targeted by luteinizing hormone (LH). In laboratory research it is used primarily as a high-potency reference agonist for LHCGR in receptor-binding, cAMP-signaling, and in-vitro steroidogenesis assays. HCG is a two-chain glycoprotein of roughly 36-40 kDa (CAS 9002-61-3), and unlike short synthetic peptides it is quantified by bioactivity in International Units (IU) rather than by mass. This article summarizes its structure, receptor pharmacology, published in-vitro literature, and laboratory handling considerations. All information here is provided strictly for in-vitro and laboratory research use only.
What Is HCG?
HCG belongs to the glycoprotein hormone family, which also includes LH, follicle-stimulating hormone (FSH), and thyroid-stimulating hormone (TSH). Every member of this family is a non-covalently associated heterodimer built from a common alpha subunit and a hormone-specific beta subunit. The alpha subunit (92 amino acids in humans) is identical across all four hormones; receptor selectivity comes entirely from the beta subunit.
The hCG beta subunit is 145 amino acids and is the longest of the family. Its distinguishing feature is a C-terminal peptide extension (CTP) carrying multiple O-linked glycans, a segment absent from LH-beta. Because carbohydrate accounts for roughly a third of the molecule's mass, hCG is far larger and structurally more complex than the small synthetic peptides more commonly handled in research settings — a difference that has direct consequences for how it is standardized, stored, and reconstituted.
The Cystine Knot and the "Seat Belt"
Two independent structural studies published in 1994 established the architecture of hCG. Lapthorn and colleagues, writing in Nature, reported the crystal structure and showed that each subunit adopts a similar topology in which three disulfide bonds form a cystine knot — a fold shared with nerve growth factor, PDGF, and TGF-beta. The heterodimer is further stabilized by a segment of the beta subunit that wraps around the alpha subunit and is covalently fastened, seat-belt fashion, by the Cys26-Cys110 disulfide bond.
In parallel, Wu, Lustbader, Liu, Canfield, and Hendrickson solved the structure at 2.6 Å resolution in Structure, using multiwavelength anomalous diffraction (MAD) on recombinant selenomethionyl hCG expressed in mammalian cells and partially deglycosylated for crystallization. The two structures agree on the essential features and together explain why hCG is unusually rigid and thermally robust for a protein of its size: the cystine knot plus the seat-belt latch leaves relatively little conformational freedom.
Why HCG Is Sold in IU, Not Milligrams
Because hCG circulates and is manufactured as a heterogeneous mixture of glycoforms — differing in sialylation, in the extent of O-glycosylation on the CTP, and in whether the beta subunit has been proteolytically "nicked" — mass alone is a poor descriptor of activity. The field therefore uses bioassay-calibrated International Units traceable to WHO reference materials.
Bristow and colleagues documented this standardization framework in Clinical Chemistry (2005), describing the establishment and value assignment of WHO reference reagents for six distinct molecular forms of hCG: intact hCG, nicked hCG, the free alpha subunit, the free beta subunit, nicked free beta subunit, and the beta core fragment. Each preparation was purified, ampouled, and lyophilized, with value assignment and accelerated degradation testing performed under WHO protocols; the lyophilized standards proved highly stable. For a laboratory, the practical takeaway is that an hCG preparation labeled in IU is describing calibrated bioactivity, and that the specific molecular forms present are an analytical variable worth knowing about.
The LHCGR Receptor
Cloning and Architecture
The receptor was cloned in 1989 by McFarland and colleagues at Genentech, reporting in Science on the isolation of a cDNA for the rat luteal lutropin-choriogonadotropin receptor. The predicted protein comprises a 26-residue signal peptide, a 341-residue extracellular domain built from an internal leucine-rich repeat (LRR) structure, and a 333-residue region containing seven transmembrane segments with clear homology to the wider GPCR family. That combination — a large LRR ectodomain fused to a canonical 7TM core — is what made the receptor "an unusual member" of the family, in the authors' phrase, and it remains the defining feature of the glycoprotein hormone receptor subfamily.
Ascoli, Fanelli, and Segaloff's review in Endocrine Reviews (2002) consolidated the following decade of work on LHCGR structure, function, and regulation, and is still the standard entry point for the pre-structural-biology literature on this receptor.
What Cryo-EM Added
The mechanistic picture sharpened considerably with Duan and colleagues' 2021 Nature paper reporting cryo-EM structures of full-length glycoprotein hormone receptor signaling complexes, including the CG-LHCGR-Gs complex determined at 3.18-3.9 Å. The maps resolved both CG subunits, the complete receptor, the three Gs subunits, and the stabilizing nanobody Nb35.
Several features are directly useful for assay design. The LHCGR ectodomain contains eleven irregular LRRs forming a slightly curved tube, with CG binding the concave inner surface in a hand-clasp arrangement. The hinge region — comprising LRR10, an alpha-helix termed the hinge helix, and LRR11 — completes the ectodomain and mirrors the architecture seen in FSHR. Binding is driven substantially by electrostatic complementarity: discrete positively charged patches on the CG surface pair with negatively charged patches on the receptor ectodomain. Structures were also determined for inactive and constitutively active receptor states, and one included the small-molecule allosteric agonist Org 43553.
HCG and LH at the Same Receptor: Not Interchangeable
A recurring theme in the modern literature — and one of the more useful facts for anyone designing an LHCGR assay — is that hCG and LH are not equivalent agonists despite sharing a receptor.
Casarini and colleagues, publishing in PLoS ONE (2012), compared cAMP kinetics in COS-7 and hGL5 cells expressing LHCGR alongside cAMP, ERK1/2, AKT, and progesterone readouts in primary human granulosa lutein cells. In COS-7/LHCGR cells hCG was approximately 5-fold more potent than human LH, while human LH reached its maximal effect substantially faster (about 10 minutes for hLH versus roughly 1 hour for hCG). Under continuous exposure to equipotent doses for up to 36 hours, intracellular cAMP production in granulosa cells was oscillatory and significantly higher with hCG than with hLH.
Riccetti and colleagues extended this to the male gonadal model in Reproductive Biology and Endocrinology (2017), using primary Leydig cells from male mice — a standard gonadotropin bioassay system in which the murine LH receptor binds human LH and hCG. They reported hCG to be roughly 10-fold more potent than LH in cAMP recruitment and modestly but significantly more potent on cAMP-dependent Erk1/2 phosphorylation. Notably, those upstream differences did not propagate: Creb phosphorylation, Stard1 gene expression, and testosterone synthesis showed no significant differences between the two ligands.
The same group formalized this as biased agonism in Scientific Reports (2017), comparing recombinant hLH and hCG for cAMP, beta-arrestin 2 recruitment, and steroidogenesis using BRET and FRET readouts in living HEK293 and mLTC-1 cells. Recombinant hCG was more potent than recombinant hLH in both lines, and the human receptor discriminates between its two ligands in a manner that differs from the murine receptor at specific amino acid residues.
Practical Implication for Assay Design
Taken together, these results mean that potency, kinetics, and pathway bias must all be specified when reporting LHCGR work. An experiment that measures only endpoint steroid output may find hCG and LH indistinguishable, while the same cells assayed for early cAMP will show a roughly 5-10 fold potency separation. This is a common source of apparent contradiction between studies and is worth controlling for explicitly.
Genetic Models and Receptor Loss of Function
Receptor knockout models have supplied much of the causal evidence for LHCGR biology. The LuRKO (LH receptor knockout) mouse, produced roughly two decades ago, has been reviewed most recently by Huhtaniemi in Andrology (2026;14:1017-1028) — a useful current entry point to the model literature.
LuRKO animals of both sexes display pubertal delay, hypogonadism, and infertility, establishing the receptor's requirement for pubertal attainment and for maintenance of adult fertility. The review also underscores a less intuitive finding: LHCGR function proved redundant for prenatal sexual differentiation and maturation in both sexes, indicating that fetal gonadal development can proceed without receptor signaling. As of 2026 these models remain the reference standard against which pharmacological LHCGR manipulation in cell and tissue systems is interpreted.
Allosteric Modulators and Why HCG Remains the Reference Agonist
Interest in small-molecule LHCGR ligands has grown alongside the structural work. Lazzaretti, Paradiso, Simoni, and Casarini surveyed this area in Frontiers in Endocrinology (2023), covering synthetic compounds that bind allosteric sites on FSHR and LHCGR as well as modulation arising from membrane receptor interactions. Among LHCGR allosteric agonists, thieno[2,3-d]pyrimidine (TP) derivatives have attracted the most attention, including Org 43553 and the related compounds TP1 and TP3.
These molecules are valuable precisely because they engage the transmembrane domain rather than the orthosteric ectodomain site, allowing investigators to dissociate ectodomain-dependent from transmembrane-dependent signaling events. In practice this has reinforced rather than displaced hCG's role in the laboratory: allosteric agonists are typically benchmarked against hCG as the full orthosteric reference agonist, which is a large part of why the native glycoprotein continues to appear in method sections in 2026.
Laboratory Research Applications
Within in-vitro and preclinical model systems, hCG appears in the published literature in several recurring roles:
- LHCGR reference agonist. Concentration-response calibration in receptor-transfected lines such as COS-7/LHCGR, HEK293, and mLTC-1.
- Steroidogenesis assays. Stimulation of primary or immortalized Leydig cell preparations to read out Stard1 expression, StAR protein, and steroid output.
- Granulosa cell models. Human granulosa lutein cell cultures for cAMP oscillation, ERK1/2 and AKT phosphorylation, and progesterone endpoints.
- Receptor binding and structural work. Ligand for co-structure determination and for mapping ectodomain and hinge-region contacts.
- Assay standardization. Bioactivity calibration against WHO reference materials in gonadotropin bioassays.
- Glycoform analysis. Comparison of intact, nicked, free-subunit, and beta core fragment forms in analytical method development.
Handling, Reconstitution, and Storage
HCG is supplied as a lyophilized powder. Its handling requirements differ from those of small synthetic peptides in a few respects worth noting.
Store lyophilized material at -20°C, protected from light and moisture. The cystine-knot core is robust, but glycoprotein preparations are sensitive to moisture ingress; allow vials to equilibrate to room temperature before opening to avoid condensation on cold powder.
Reconstitute gently. Direct a slow stream of diluent down the vial wall rather than onto the cake, and swirl rather than shake. Vigorous agitation of a glycoprotein solution promotes interfacial denaturation and aggregation, which will reduce measured bioactivity without necessarily being visible.
Minimize freeze-thaw cycles. Aliquot reconstituted material into single-use volumes. Repeated freezing and thawing is a well-recognized contributor to potency loss in gonadotropin preparations.
Be aware of nicking. Proteolytic cleavage within the beta subunit loop generates nicked hCG, a distinct molecular form with altered receptor activity. Elevated temperature and residual protease activity both favor it, which is the mechanistic reason cold-chain discipline matters more here than for a chemically synthesized peptide. The WHO panel described by Bristow et al. explicitly includes nicked hCG as a separate reference reagent for this reason.
Verify with documentation. Because IU labeling encodes bioactivity rather than mass, a certificate of analysis and independent purity testing are the relevant confirmation for a research-grade preparation.
Common Questions
What receptor does HCG bind?
The luteinizing hormone/choriogonadotropin receptor (LHCGR), a class A GPCR with a large leucine-rich-repeat extracellular domain and a seven-transmembrane core. The receptor was cloned from rat luteal tissue in 1989 and its full-length complex with CG and Gs was resolved by cryo-EM in 2021.
Is HCG a peptide?
Not in the usual sense. It is a heterodimeric glycoprotein of roughly 36-40 kDa built from two separate polypeptide chains plus extensive N- and O-linked carbohydrate, whereas most research peptides are single synthetic chains under 5 kDa. This distinction drives the differences in standardization and handling described above.
How does HCG differ from LH if they share a receptor?
They differ in the beta subunit — hCG-beta carries a glycosylated C-terminal extension absent from LH-beta — and they behave as biased agonists at the shared receptor. Published in-vitro work reports hCG as roughly 5-fold more potent than LH in COS-7/LHCGR cells and roughly 10-fold more potent in mouse primary Leydig cells for cAMP, while LH reaches maximal effect faster and several downstream endpoints converge.
Is HCG studied for weight loss?
No credible peer-reviewed evidence supports that use, and it falls entirely outside the scope of this material. HCG supplied as a research chemical is intended for in-vitro and laboratory investigation of gonadotropin receptor signaling only. It is not for human consumption and is not intended to diagnose, treat, cure, or prevent any disease.
What is HMG and how does it relate?
Human menopausal gonadotropin (HMG) is a distinct preparation containing both FSH and LH activity. Because it engages FSHR in addition to LHCGR, it is used in different experimental contexts than hCG and is not a substitute for it in LHCGR-specific assays.
Research Use Only
All products and information referenced here are supplied strictly for laboratory research use only. HCG is not a dietary supplement, is not approved for human or veterinary use, and is not intended to diagnose, treat, cure, or prevent any disease. Handling should be performed only by qualified personnel in an appropriate laboratory setting and in accordance with applicable institutional and regulatory requirements. Nothing in this article constitutes medical advice or a recommendation for use in humans or animals.
This article was researched and last updated August 2026. Citations below link to indexed primary literature and peer-reviewed reviews.