The growth hormone axis: GHRH, ghrelin, GH and IGF-1
Growth hormone is not released just like that. There are two buttons that switch it on and a third that holds it back. Almost every peptide in this group sits on one of those buttons.
The chain from top to bottom
- The hypothalamus releases GHRH, the hormone that says: make growth hormone.
- The pituitary responds and lets growth hormone into the blood.
- The liver responds to growth hormone by making IGF-1.
- IGF-1 does most of the work in the tissues that growth hormone gets the credit for.
There is also a brake: somatostatin, likewise from the hypothalamus, which keeps the pituitary quiet. Growth hormone is therefore released in pulses, not as a steady stream. Those pulses are the natural pattern a lot of research tries to imitate.
Two buttons, not one
GHRH is the first button. The second is the ghrelin receptor, discovered because substances existed that released growth hormone without resembling GHRH. Only later did it turn out the body has its own hormone for it: ghrelin, released by an empty stomach.
That explains why peptides in this group fall into two families.
| Family | Works through | Examples |
|---|---|---|
| GHRH analogues | the GHRH receptor | Sermorelin, CJC-1295, tesamorelin |
| Ghrelin mimetics | the ghrelin receptor | Ipamorelin, GHRP-2, GHRP-6, hexarelin |
Two different buttons for the same result means they can reinforce each other. A GHRH peptide and a ghrelin peptide together give a larger effect in research than either alone. That is why the combination of CJC-1295 with ipamorelin is the most studied blend in this group.
Selectivity is the difference within the second family
The ghrelin mimetics resemble each other but differ in how tidy they are. GHRP-6 gives a strong hunger signal. GHRP-2 also touches cortisol and prolactin. Ipamorelin is the most selective: it switches on the pituitary without dragging those side routes along. Hexarelin is the most potent and additionally reaches a receptor in the heart.
That makes the choice within this family not a matter of better or worse, but of what you want to rule out in your study design.
And the hormone itself?
Growth hormone is also available as research material: a protein of 191 amino acids. The difference from all the peptides above is fundamental. The peptides ask the body to make hormone, within the natural pulses and with the natural brake in place. The protein itself bypasses that whole regulation.
In practical terms it is also a very different product: 191 amino acids is a long, folded chain far more sensitive to heat and shaking than a peptide of ten or twenty building blocks.
Where IGF-1 fits
IGF-1 is the last link. Research often uses the LR3 variant: modified so the binding proteins in the blood cannot capture it, which keeps it active far longer. That skips the whole axis and looks directly at what the end substance does in tissue.
This site is made by the team behind Apex Bio Research, a Dutch supplier of research peptides with a certificate of analysis per batch.
Visit the shopThis site explains what the scientific literature says about peptides. It is not medical advice and not a set of instructions. The substances discussed here are research compounds: they are not approved for use in humans or animals. If you have a health question, see a doctor.
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