Gonadorelin vs HCG: Dosing and Syringe Units

Last updated: July 2026

Gonadorelin and HCG are both used to keep the testicles working while a man is on testosterone, but they act at opposite ends of the hormonal chain and are measured in different units: gonadorelin is dosed in micrograms (mcg) and works upstream at the pituitary, while HCG is dosed in international units (IU) and works downstream directly at the testis. Because the units are not interchangeable, "how many syringe units" only has an answer once you know the vial's concentration — and the answer differs for each drug.

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TL;DR — key takeaways

Different targets. Gonadorelin is synthetic GnRH and stimulates the pituitary to release LH and FSH; HCG mimics LH and stimulates the testis directly.
Different units. Gonadorelin is measured in mcg (mass); HCG in IU (biological activity). You cannot convert one number into the other.
Different evidence. HCG is well studied for maintaining intratesticular testosterone during TRT. Gonadorelin's evidence in men comes almost entirely from pulsatile pump delivery, not the twice-weekly injections marketed by TRT clinics.
Units follow concentration. Whether you draw 10 or 50 units depends on the mg/mL or IU/mL strength of your reconstituted vial — which is what a calculator works out for you.

Where each drug acts on the HPG axis

Testosterone therapy suppresses the brain's signal to the testes. The hypothalamus normally releases gonadotropin-releasing hormone (GnRH) in pulses, which tells the pituitary to make luteinizing hormone (LH) and follicle-stimulating hormone (FSH); those in turn drive the testis to make testosterone and sperm. Exogenous testosterone switches this chain off, so both testicular size and fertility can decline. Gonadorelin and HCG restart different links of that chain.

Gonadorelin is a synthetic copy of GnRH, so it acts one step higher — on the pituitary. HCG skips the brain entirely and behaves like LH at the Leydig cells of the testis. That single difference drives everything else: their units, their dosing rhythm, and how strong the evidence behind each one is.

Side-by-side: the two drugs at a glance

Property	Gonadorelin	HCG
What it is	Synthetic GnRH (decapeptide)	Placental hormone, LH-like glycoprotein
Acts on	Pituitary (upstream)	Testis / Leydig cells (downstream)
Measured in	Micrograms (mcg)	International units (IU)
Approx. half-life	~2–10 minutes	~24–36 hours
Physiologic signal	Must be pulsatile to work	Steady stimulation is fine
Form	Lyophilised powder, reconstitute	Lyophilised powder, reconstitute
Human approval status	No current FDA drug label; compounded	FDA-approved (e.g. Novarel)

Both arrive as a freeze-dried powder you mix with bacteriostatic water before drawing. The reconstitution maths is identical to any peptide: concentration = amount in the vial ÷ water added, and units = dose ÷ concentration × 100 on a U-100 syringe.

Typical doses (and why they are hard to compare)

In the TRT-support setting, HCG is commonly used off-label at roughly 250–500 IU two or three times a week. That range traces directly to the dose-finding work of Coviello and colleagues, who showed 125–500 IU every other day preserved intratesticular testosterone, and to Hsieh and colleagues, who used 500 IU every other day alongside TRT without azoospermia. Gonadorelin is typically compounded and dosed around 100–200 mcg two to three times a week or daily. Note that these numbers share no common unit — comparing "500" to "100" tells you nothing, because one is IU of activity and the other is mcg of mass.

Turning a dose into syringe units

The label dose (IU or mcg) is never what you read on the barrel — you read units, and a U-100 insulin syringe has 100 units per 1 mL. So find the concentration, divide your dose by it to get mL, then multiply by 100.

Worked example 1 — HCG, standard mix

A 5,000 IU vial reconstituted with 5 mL BAC water → 1,000 IU/mL. Dose = 500 IU.

500 ÷ 1,000 = 0.5 mL. 0.5 mL × 100 = 50 units on a U-100 syringe.

Worked example 2 — same HCG dose, more water

Same 5,000 IU vial but reconstituted with 10 mL → 500 IU/mL. Dose still 500 IU.

500 ÷ 500 = 1.0 mL × 100 = 100 units — a full 1 mL syringe for the identical dose. More water, more units.

Worked example 3 — gonadorelin

A 5 mg (5,000 mcg) gonadorelin vial + 5 mL BAC water → 1,000 mcg/mL. Dose = 100 mcg.

100 ÷ 1,000 = 0.1 mL × 100 = 10 units.

Worked example 4 — smaller gonadorelin vial

A 2 mg (2,000 mcg) vial + 2 mL water → 1,000 mcg/mL. Dose = 200 mcg.

200 ÷ 1,000 = 0.2 mL × 100 = 20 units. Same concentration as example 3, double the dose, double the units.

Worked example 5 — the unit trap

500 IU of HCG is not 500 mcg of gonadorelin. IU measures biological activity; mcg measures mass. There is no fixed conversion factor between them — treat each drug's number on its own scale.

Dose-to-units reference chart

Common doses shown as units on a U-100 syringe at a 1,000-per-mL concentration (1,000 IU/mL for HCG, 1,000 mcg/mL for gonadorelin). Always confirm against your own vial label.

Drug & dose	Volume	Units (U-100)
HCG 250 IU	0.25 mL	25 units
HCG 500 IU	0.5 mL	50 units
HCG 1,000 IU	1.0 mL	100 units
Gonadorelin 50 mcg	0.05 mL	5 units
Gonadorelin 100 mcg	0.1 mL	10 units
Gonadorelin 200 mcg	0.2 mL	20 units

Which one has the evidence?

This is where honesty matters. HCG's role in preserving testicular function during testosterone therapy is supported by controlled human studies and is reflected in clinical practice guidelines. Gonadorelin's clinical evidence in men, by contrast, comes overwhelmingly from pulsatile pump delivery — tiny doses infused every 90–120 minutes to imitate the body's natural GnRH rhythm. Because gonadorelin's half-life is only minutes, a bolus injected twice a week behaves nothing like a pump, and there is little direct trial evidence that this common TRT-clinic pattern works the way HCG does. It is also not sold under a current human FDA drug label; men's-clinic gonadorelin is compounded. None of this is a recommendation for or against either drug — it is context your prescriber weighs.

How this is calculated

Every unit figure here uses two facts only: a U-100 syringe holds 100 units per mL, and concentration is dose-per-volume. The arithmetic is identical whether the dose is written in IU or mcg — the unit label never changes the maths, only the number you start from. The calculators on this site automate exactly this and let you sanity-check against the chart above. This is an education and maths tool, not medical advice.

Frequently asked questions

Is gonadorelin just a cheaper HCG?

No. They work at different points in the hormonal chain and are measured in different units. Gonadorelin signals the pituitary; HCG acts like LH at the testis. They are not interchangeable milligram-for-IU or dose-for-dose.

Can I convert an HCG dose in IU to a gonadorelin dose in mcg?

No. IU measures biological activity and mcg measures mass, with no fixed conversion between the two drugs. Each has its own dosing scale set by your prescriber.

Why does the same dose sometimes need different syringe units?

Because units depend on concentration, not on the drug. More bacteriostatic water means a weaker concentration and more units for the same dose, as examples 1 and 2 show.

Does more water make the dose stronger?

No. Adding more water changes only the volume and the units you draw, never the amount of active drug in that dose. It is purely a measurement convenience.

Sources

This guide is for general educational purposes only and does not constitute medical advice. Always follow your prescriber’s specific instructions and consult a qualified clinician before changing any protocol.