Three compounds, three levels of the same system: kisspeptin, HCG and PT-141 are often grouped together because they all touch reproductive and sexual-function research. But they act at completely different points. Kisspeptin works at the top, in the hypothalamus. HCG works at the bottom, directly on the gonads. PT-141 works in a separate brain pathway altogether. This guide maps where each one acts and what the research shows.
The HPG axis in one picture
The hypothalamic-pituitary-gonadal (HPG) axis is a three-step relay. The hypothalamus releases GnRH; the pituitary responds by releasing LH and FSH; the gonads respond to LH and FSH by producing sex hormones. Kisspeptin sits above GnRH and controls the whole relay. HCG skips the first two steps and acts directly on the gonads. PT-141 is not part of the relay at all.
Side by side
| Kisspeptin-10 | HCG | PT-141 | |
|---|---|---|---|
| Type | 10-amino-acid peptide | Glycoprotein hormone | Cyclic heptapeptide |
| Receptor | KISS1R (GPR54) | LH/CG receptor | MC4R (and MC3R) |
| Level of action | Hypothalamus (top of HPG axis) | Gonads (bottom of HPG axis) | Central melanocortin pathway |
| Downstream effect studied | GnRH, then LH/FSH release | Direct LH-like signalling | Brain signalling, not hormonal |
| Clinical status | Research tool | Decades of fertility-medicine use | Approved drug in the US (2019) |
Kisspeptin: the master switch
In 2003, two research groups independently found that mutations in the kisspeptin receptor prevent puberty. That made kisspeptin the master upstream regulator of GnRH. Dhillo and colleagues (2005, JCEM) then showed that kisspeptin potently stimulated LH and testosterone release in healthy men. Because it works at the top of the axis, it preserves the body’s own feedback loops, which makes it a favoured tool in reproductive endocrinology. See Kisspeptin-10.
HCG: the LH stand-in
Human chorionic gonadotropin is made by the placenta in pregnancy. It shares a subunit with LH and activates the same receptor, but lasts much longer. In research it is used to stimulate gonadal steroid production directly, bypassing the hypothalamus and pituitary. It is a glycoprotein, not a short peptide, and its activity is measured in International Units. See HCG.
PT-141: a different road entirely
PT-141 (bremelanotide) came out of Melanotan II research. It activates melanocortin receptors in the brain rather than any reproductive hormone. That distinction is the whole point: it is studied for central effects on sexual function that are independent of sex-hormone levels, and it became an approved prescription medicine in the United States in 2019 for a specific indication. See PT-141.
Frequently asked questions
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What is the difference between kisspeptin and HCG?
Kisspeptin acts at the top of the reproductive hormone axis, triggering GnRH and then LH and FSH. HCG acts at the bottom, mimicking LH directly on the gonads.
Is PT-141 a hormone?
No. PT-141 activates melanocortin receptors in the brain and does not act through reproductive hormones.
What is the HPG axis?
The hypothalamic-pituitary-gonadal axis: the hypothalamus releases GnRH, the pituitary releases LH and FSH, and the gonads produce sex hormones.
Related reading
- What is PT-141?
- PT-141 side effects
- HCG side effects
- Kisspeptin side effects
- Melanotan II side effects: moles, melanoma and the evidence
- Peptides 101: beginner’s guide
- CJC-1295 vs Tesamorelin vs Ipamorelin
Selected references
- Dhillo WS, et al. Kisspeptin-54 stimulates the hypothalamic-pituitary gonadal axis in human males. J Clin Endocrinol Metab. 2005;90(12):6609–6615. PubMed
- Seminara SB, et al. The GPR54 gene as a regulator of puberty. N Engl J Med. 2003;349(17):1614–1627. PubMed
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