HCG (Human Chorionic Gonadotropin): Mechanism and Evidence

Human chorionic gonadotropin, better known under the brand names Pregnyl, Ovitrelle and Novarel, is one of the oldest hormone therapies still in routine clinical use. It is also one of the most misunderstood, thanks partly to a decades-old weight-loss fad that has nothing to do with its real, approved medical function. This article explains what hCG actually is, how it works at the receptor level, what the controlled trial data show for fertility preservation, hypogonadism and muscle, and why it sits entirely outside the growth-hormone-axis research peptides this site covers.

What is hCG?

Human chorionic gonadotropin is a glycoprotein hormone naturally produced by the placenta during pregnancy, where it signals the corpus luteum to keep producing progesterone. Structurally it belongs to the same hormone family as luteinizing hormone (LH), sharing an alpha subunit with LH, follicle-stimulating hormone (FSH) and thyroid-stimulating hormone, and a hormone-specific beta subunit that gives it its distinct receptor binding and its much longer circulating half-life than LH.

Pharmaceutically, hCG is manufactured either by extraction and purification from the urine of pregnant women or, in newer products, recombinantly. It has been an approved prescription medicine for decades and is marketed under names including Pregnyl, Ovitrelle and Novarel. Clinically it is used almost exclusively in two settings: female fertility treatment, where it triggers ovulation and supports IVF protocols, and male hypogonadism and infertility, where it substitutes for the body's own LH signal. This article focuses on the male-hypogonadism use case, since that is the context in which hCG regularly comes up in conversations about muscle, body composition and the GH axis. Female fertility applications are outside its scope.

Mechanism: hCG as a long-acting LH mimetic

The reason hCG works in men at all is receptor overlap. Both LH and hCG bind the same receptor, LHCGR, expressed on Leydig cells in the testes. A 2017 mechanistic study comparing the two hormones side by side in Leydig cells found that hCG is roughly ten times more potent than LH at recruiting the cAMP second messenger, and that both hormones engage the same cAMP/PKA/ERK1/2 signalling cascade, ultimately producing equal final testosterone output (PMID 28056997). In practical terms, hCG is not a distinct pathway or a novel hormone action. It is a longer-acting, more potent stand-in for the body's own LH signal, stimulating the same Leydig cells to synthesize testosterone through the same downstream machinery.

This matters clinically because the testes need a continuous LH-type signal to keep producing testosterone locally (intratesticular testosterone) and to keep sperm-producing tissue active. When a man starts exogenous testosterone replacement therapy (TRT), the pituitary senses adequate circulating testosterone and shuts down its own LH and FSH output through negative feedback. Without any LH-type signal reaching the testes, intratesticular testosterone collapses, spermatogenesis stalls and the testes can shrink. hCG is used specifically to keep that local signal alive while external testosterone is being administered.

The clinical evidence: fertility preservation and hypogonadism monotherapy

The best-established use of hCG is as a co-therapy alongside TRT to preserve fertility. In a study following 26 hypogonadal men on TRT combined with low-dose intramuscular hCG for a mean of about 6.2 months, no patient became azoospermic (zero sperm count), and partners in 9 of the couples achieved pregnancy during the study period (PMID 23260550). A 2021 review of the broader literature reached the same conclusion at a mechanistic level: hCG's LH-like action drives testosterone and sperm production without the fertility-suppressing effect seen with testosterone replacement alone, because it keeps the local testicular signal intact even while circulating testosterone from TRT suppresses the pituitary (PMID 33345656).

hCG has also been studied as a standalone therapy, not just an add-on to TRT. In a retrospective study of 20 men with hypogonadal symptoms but total testosterone still above 300 ng/dL, hCG monotherapy raised mean testosterone by 49.9 percent, from 362 to 519.8 ng/dL, over a median follow-up of 8 months. Roughly half the patients reported symptom improvement, and no adverse events were recorded (PMID 31408289). This positions hCG monotherapy as a plausible option specifically for men who want to raise endogenous testosterone without shutting down natural production and fertility, an outcome that exogenous testosterone cannot offer.

The "hCG diet" myth: what the trials actually found

hCG's popular reputation as a weight-loss aid traces back to a protocol developed in the 1950s combining hCG injections with an extremely low-calorie diet (around 500 kcal per day), often called the Simeons protocol. The claim was that hCG itself mobilized stored fat and suppressed hunger. This claim has been tested directly and repeatedly, and it does not hold up.

The landmark study is a 1976 double-blind, placebo-controlled crossover trial of 202 patients, in which each participant received both hCG and placebo injections at different points in the trial while on the same restricted diet. The result: no statistically significant difference between hCG and placebo on weight loss, skin-fold thickness or dropout rate at any phase of the study (PMID 792477). Subsequent placebo-controlled trials reached the same conclusion. Whatever weight loss occurs on the "hCG diet" is attributable entirely to the severe calorie restriction, not to the hormone. US regulators eventually caught up with the science: the FDA and FTC issued joint warning letters in December 2011 declaring hCG weight-loss products "unproven and illegal," a designation that still applies to over-the-counter and homeopathic hCG products marketed for weight loss today.

Muscle and the GH axis: what the 2002 trial actually shows

Because hCG raises testosterone, and testosterone is loosely associated with muscle in the popular imagination, hCG sometimes gets discussed as a muscle-building agent. The controlled evidence tells a more precise, and more limited, story.

The only randomized, placebo-controlled trial examining hCG's effect on muscle and physical function was conducted in 40 men aged 60 to 85 with partial age-related androgen deficiency. Over 3 months, recombinant hCG raised testosterone and estradiol by roughly 150 percent, increased lean body mass by approximately 2 kg and reduced fat mass by roughly 1 kg. But the same trial found no improvement in muscle strength, physical function, gait speed or balance, and testis volume actually decreased by about 5 mL over the study period (PMID 12107212). In other words, hCG produced a measurable hormonal and body-composition shift without any corresponding functional muscle benefit.

This distinction matters for anyone specifically interested in the growth-hormone/IGF-1 axis and tissue repair, because hCG has zero mechanistic overlap with that system. It works exclusively through the LH receptor on gonadal tissue, not through the GHRH receptor or the ghrelin receptor that drive endogenous growth hormone release. It does not raise GH, does not raise IGF-1, and does not engage the signalling pathways associated with muscle-protein synthesis and connective-tissue repair the way GH-secretagogue peptides do. The 2002 trial's finding, lean mass up but strength and function flat, is a useful illustration of that gap: raising sex hormones changes body composition numbers, but it is not the same lever as stimulating the GH/IGF-1 axis directly.

What the community reports

Why we do not sell hCG

hCG also falls outside our catalog for a structural reason beyond its legal status: it works through the LH receptor on gonadal tissue, an entirely different system from the GHRH-receptor and ghrelin-receptor pathways that our GH-secretagogue research peptides target. Someone researching GH-axis-driven tissue effects, IGF-1 signalling or lean-mass mechanisms is, mechanistically speaking, looking at a different hormonal system than the one hCG operates on.

The research peptides we carry instead

For research into the GHRH-receptor and ghrelin-receptor pathways that drive endogenous growth hormone and IGF-1 signalling, the following peptides are available for laboratory research use:

Sermorelingrowth

GHRH(1-29) analog for physiological growth hormone stimulation research

CJC-1295 (No DAC)growth

CJC-1295 without DAC (Mod GRF 1-29) is a short-acting GHRH(1-29) analog for GH/IGF-1 research. Research-grade lyophilized powder, specified purity >=99% (HPLC). Laboratory use only.

Ipamorelingrowth

Highly selective growth hormone releaser that triggers natural GH pulses without raising cortisol or prolactin. Clean GH stimulation with minimal side effects - the most targeted growth hormone peptide available.

Growth & Performancegrowth

Growth hormone secretagogues and gonadotropins

Sermorelin is GHRH(1-29), the shortest active fragment of natural growth-hormone-releasing hormone, and activates the GHRH receptor to produce a pulsatile, physiological GH release. CJC-1295 (the No-DAC form carried here) is a DPP-IV-resistant GHRH(1-29) analogue with the same receptor target but a metabolically hardened structure, often paired with ipamorelin for synergistic GHRH/ghrelin-receptor signalling. Ipamorelin itself is a selective ghrelin-receptor agonist (a GHRP) that stimulates GH release with minimal effect on cortisol or prolactin. None of these three engages the LH receptor or the gonadal axis at all, they are mechanistically unrelated to hCG, and none is positioned here as a substitute for it. They are relevant only because they are the actual GH-axis tools available for laboratory research into the pathways hCG does not touch. Readers following the GLP-1-and-muscle-loss research thread on this site will recognize these same GHRH/ghrelin pathways as the mechanistic bridge between GH-axis signalling and lean-tissue research.

This article is for informational and educational purposes only. All mentioned peptides are intended exclusively for laboratory research and not for human consumption. We do not sell the drug this article is about. For Research Purposes Only.