HCG — human chorionic gonadotropin — is a natural hormone most people know from one place: it's the hormone a pregnancy test looks for. It's produced in large amounts during pregnancy. It's also a long-established approved medicine, and it shows up in TRT-adjacent research discussion for a specific reason tied to how it signals.
In plain terms: it's a real hormone that happens to press the same button that tells the testes to make testosterone.
How it works — the LH mimic
Your body has a hormone called luteinizing hormone (LH). LH travels from the pituitary to the testes and tells them to produce testosterone. HCG's trick is that it binds the very same receptor — the LH/CG receptor, or LHCGR2. To the testes, a dose of HCG reads a lot like a surge of LH.
That's the whole reason it appears in a testosterone-therapy context. When someone takes external testosterone, the body senses it has "enough" and dials down its own LH — which can leave the testes idle. Because HCG mimics LH, it's discussed as a way to keep that testicular signal switched on1. (This page explains the mechanism; it is not a protocol or a recommendation.)
Why it's measured in IU, not mg
This is HCG's most distinctive practical quirk. Nearly every compound on this site is measured by weight (milligrams or micrograms). HCG is measured in International Units (IU) — a unit based on biological activity, not mass1.
The reason is historical and sensible: HCG was originally purified from a natural source and varied from batch to batch, so defining its strength by *what it does* (activity) rather than *what it weighs* kept doses consistent. The catch: there's no universal IU-to-mg conversion — the factor is specific to the substance (units explainer). This is the single biggest way HCG math differs from peptide math.
HCG at a glance
| Property | HCG |
|---|---|
| What it is | Human chorionic gonadotropin — a glycoprotein hormone |
| Receptor | LHCGR — the same receptor luteinizing hormone (LH) uses |
| Effect on the testes | Mimics LH, signalling testosterone production |
| Measured in | International Units (IU), by biological activity |
| Molecule type | Large glycoprotein (not a short peptide) |
| Regulatory status | Approved medicine (e.g. Pregnyl) |
It's a glycoprotein, not a small peptide
Worth stating plainly: HCG is a large glycoprotein — a sizable protein decorated with sugar chains — not one of the short amino-acid chains this site usually calls "peptides"2. Those sugar chains are part of why it lingers in the body longer than LH does. It's often supplied lyophilized (freeze-dried) and reconstituted like a peptide, but always on the IU basis printed on the specific product, never a mg guess.
Latest research
- The modern understanding of HCG and LH is that they are not perfectly interchangeable: although they share the LHCGR receptor, research shows they can signal through it in subtly different ("biased") ways2. That's an active area of endocrine research.
- Its established medical uses (fertility, and the specific approvals on the label) remain the settled evidence base1.
- We update this section as new research reports.
The short version
HCG is a natural, approved hormone that mimics luteinizing hormone at the LHCGR receptor — which is why it comes up in testosterone-therapy discussion as a way to keep the testes signalled. Its defining practical feature is that it's dosed in International Units, not milligrams, because its strength is set by biological activity. This is an educational overview of what it is and how it signals — not medical advice or usage guidance.