GH vs. GHRH vs. GHRP: Untangling the Growth-Hormone Axis

The growth-hormone axis is often discussed as if it were one thing, and the compounds that act on it are often lumped together under vague labels. In fact the axis has a clear architecture with at least three points at which a molecule can intervene, and the compounds studied in this field fall into three classes that correspond to those points. Growth hormone acts directly on peripheral tissues. Growth hormone-releasing hormone and its analogues act on the pituitary through one receptor. Growth hormone-releasing peptides act on the pituitary and hypothalamus through a different receptor, one that turned out to belong to a stomach hormone nobody had yet found. Understanding which class a compound belongs to explains most of what the literature reports about it: how long it acts, what limits its effect, what feedback it triggers and what it can and cannot do. This note lays out the axis, places each class on it, and describes the experimental evidence that established the distinctions.

The axis in outline

Growth hormone (GH) is a 191-amino-acid protein secreted by somatotroph cells in the anterior pituitary. It is released in pulses, with the largest pulses occurring in the first hours of slow-wave sleep, and the pulsatile pattern is not incidental: tissue responses to GH depend on the pulse frequency and amplitude, not just the average level. GH acts on the liver and other tissues to stimulate production of insulin-like growth factor 1 (IGF-1), which mediates many of its growth-promoting effects, and it also acts directly on adipose tissue, muscle and bone. Møller and Jørgensen’s review of GH’s metabolic actions in humans describes the direct effects: GH is lipolytic, promotes protein synthesis and is counter-regulatory to insulin, raising glucose production and reducing peripheral glucose uptake.¹

The pituitary does not decide on its own when to release GH. It is governed by two hypothalamic hormones with opposing actions. GHRH stimulates synthesis and release; somatostatin inhibits release. The pulse pattern emerges from their alternation: a GH pulse occurs when GHRH secretion rises and somatostatin secretion falls, and the pulse ends when the balance reverses. GH and IGF-1 feed back on both the hypothalamus and the pituitary to restrain further secretion, closing the loop. Giustina and Veldhuis’s comprehensive review remains the standard reference on how these components interact in animals and humans, and on how the system changes with age, sex, nutrition and disease.²

A third input was identified more recently. In 1999, Kojima and colleagues isolated a 28-residue peptide from rat stomach that potently stimulated GH release through a receptor already known in the pituitary, and named it ghrelin.³ Ghrelin is secreted mainly by the stomach, rises before meals, stimulates appetite and, through its receptor on somatotrophs and hypothalamic neurons, amplifies GH release. Its discovery completed a picture in which GH secretion is controlled by two stimulatory signals acting through distinct receptors and one inhibitory signal.

Class one: growth hormone itself

Recombinant human GH, produced in bacteria since the 1980s, is the pituitary hormone supplied from outside. Its pharmacology is simple to state: it acts on GH receptors in peripheral tissues directly, bypassing the hypothalamus and pituitary entirely. This has two consequences that distinguish it from everything else discussed here. First, it works regardless of pituitary function, which is why it is the therapy for pituitary GH deficiency. Second, it does not produce pulses. A subcutaneous injection produces a sustained elevation of GH lasting many hours, and the pituitary’s own secretion is suppressed by feedback while exogenous GH is present. Long-acting GH formulations extend this to weekly profiles. Whatever the merits of continuous exposure in specific clinical settings, it is physiologically unlike endogenous secretion, and this difference is the basis for much of the interest in the upstream classes.

Class two: GHRH and its analogues

GHRH was isolated in 1982 from pancreatic tumors that were causing acromegaly by secreting it ectopically; Guillemin’s group characterized the 44-residue form that proved to be the principal hypothalamic species.⁴ Its receptor, cloned a decade later, is a G-protein-coupled receptor on somatotrophs that signals through cyclic AMP. Activation produces immediate release of stored GH, stimulates GH gene transcription, and over time promotes somatotroph proliferation. Loss-of-function mutations in the receptor, in the little mouse and in humans, cause severe GH deficiency and pituitary hypoplasia, confirming that GHRH signaling is required for normal pituitary development as well as for secretion.⁵

The analogues studied in research differ mainly in stability. Native GHRH and sermorelin, the 1–29 fragment, are inactivated within minutes by dipeptidyl peptidase IV. Tesamorelin adds an N-terminal hexenoyl group that resists this enzyme. CJC-1295 substitutes four residues to resist several proteases and, in its DAC form, adds a linker that binds serum albumin to extend circulation for days.⁶ All of them act through the GHRH receptor and all share its defining constraint: they can only release GH the pituitary is capable of making, and their effect is limited by somatostatin. When somatostatin tone is high, as during much of the day, a GHRH pulse produces a small GH response; when somatostatin tone is low, the same pulse produces a large one. This is why GHRH analogues preserve, rather than override, the physiological pattern. Our notes on sermorelin and tesamorelin review the individual compounds.

GHRH turns the tap on. Somatostatin sets how far it opens. A GHRP does something neither of them does: it loosens the brake.

Class three: GHRPs and the ghrelin receptor

The growth hormone-releasing peptides have an unusual history, because the compounds were made before anyone knew what they bound. In the late 1970s, Cyril Bowers and colleagues at Tulane were synthesizing analogues of the opioid peptide met-enkephalin and noticed that some of them released GH from pituitary cells in vitro. Systematic modification produced, in 1984, a hexapeptide with no opioid activity that released GH potently and specifically both in vitro and in vivo.⁷ This compound, GHRP-6, was the first GH secretagogue, and it clearly did not act through the GHRH receptor: it released GH in the presence of GHRH antibodies, its effect was additive to or synergistic with GHRH, and it worked through a different second-messenger pathway, raising intracellular calcium through phospholipase C rather than raising cyclic AMP.

The receptor was identified in 1996, when Howard and colleagues at Merck used a nonpeptide secretagogue developed from the GHRP template to clone a G-protein-coupled receptor expressed in pituitary and hypothalamus, which they named the growth hormone secretagogue receptor.⁸ It was an orphan receptor; its natural ligand was unknown. Three years later, Kojima and colleagues found that ligand in the stomach and named it ghrelin.³ The receptor is now called GHS-R1a or the ghrelin receptor, and the GHRPs are understood as synthetic ghrelin mimetics discovered fifteen years before ghrelin itself.

GHRPs act at two sites. On the pituitary somatotroph, they stimulate GH release directly through GHS-R1a. In the hypothalamus, they stimulate GHRH-secreting neurons and, crucially, appear to reduce somatostatin’s inhibitory influence, either by reducing somatostatin release or by functionally antagonizing its action on the somatotroph. This second action is what makes GHRPs different in kind from GHRH analogues: they do not merely add stimulation, they partially remove inhibition. The result, demonstrated repeatedly in animal and human studies, is that GHRP and GHRH given together release more GH than the sum of either alone.^2,7 That synergy is the experimental rationale for studying the two classes in combination, a subject covered in our note on CJC-1295 and ipamorelin.

Selectivity within the GHRP class

Early GHRPs were not selective for GH. GHRP-6 and its successor GHRP-2 also released prolactin, ACTH and cortisol, and because the ghrelin receptor mediates appetite, they stimulated hunger. Raun and colleagues at Novo Nordisk reported in 1998 a pentapeptide, ipamorelin, that released GH with potency similar to GHRP-6 but without the ACTH, cortisol or prolactin release seen with earlier compounds, even at high doses in animal models.⁹ Ipamorelin’s selectivity made it the preferred research GHRP where the confounding effects of cortisol and prolactin would complicate interpretation. It is described in more detail in our note on ipamorelin.

Property	Growth hormone (rhGH)	GHRH analogues	GHRPs
Examples	Somatropin	Sermorelin, tesamorelin, CJC-1295	GHRP-6, GHRP-2, ipamorelin, hexarelin
Site of action	Peripheral GH receptors	Pituitary somatotroph	Pituitary somatotroph and hypothalamus
Receptor	GH receptor	GHRH receptor	GHS-R1a (ghrelin receptor)
Second messenger	JAK–STAT	Cyclic AMP	Phospholipase C, calcium
Requires functional pituitary	No	Yes	Yes
Preserves pulsatility	No	Yes	Yes
Interaction with somatostatin	Suppresses endogenous GH via feedback	Effect limited by somatostatin tone	Partially counteracts somatostatin
Other hormones released	None directly	None	Varies: prolactin, ACTH, cortisol with early GHRPs; minimal with ipamorelin
Natural ligand	GH	GHRH	Ghrelin

Why the distinctions matter in research

The classification is not taxonomic tidiness; it predicts experimental behavior. Three examples illustrate this.

Duration and pattern. A GHRH analogue produces a GH pulse whose size depends on the moment’s somatostatin tone and whose duration depends on the analogue’s half-life. Teichman and colleagues showed in healthy adults that CJC-1295, with its albumin-binding extension, raised mean GH and IGF-1 for days after a single administration while the pulsatile pattern of GH secretion was preserved; the amplitude of pulses increased but their frequency did not change.⁶ Exogenous GH, by contrast, replaces pulses with a plateau. A researcher interested in pulse physiology needs the former; one interested in sustained exposure needs the latter.

Dependence on pituitary state. Both upstream classes fail in the absence of a functional pituitary, and both are attenuated when the pituitary is already maximally stimulated. In aging, where GH secretion declines because of reduced GHRH drive and increased somatostatin tone rather than loss of somatotroph capacity, GHRH analogues restore secretion because the capacity remains. The same logic predicts that GHRPs, which reduce somatostatin’s influence, would be particularly effective when high somatostatin tone is the limiting factor, and experimental data support this.²

Off-target signaling. The GHRH receptor is expressed almost exclusively in the pituitary, with lower levels in a few other tissues, so GHRH analogues have a narrow profile. The ghrelin receptor is expressed in the hypothalamus, hippocampus, heart, pancreas and elsewhere, so GHRPs have effects on appetite, gastric motility and other systems that are independent of GH release. In an experiment designed to isolate GH effects, this is a confound; in an experiment on ghrelin signaling itself, it is the point.

Regulatory status across the classes

The three classes also differ in their regulatory histories, which affects what human data exist. Recombinant GH has been approved for decades and has the largest clinical literature. Among GHRH analogues, tesamorelin is approved for one indication and sermorelin was formerly approved and discontinued for commercial reasons. Among GHRPs, no peptide has reached approval; the nonpeptide ghrelin agonist macimorelin is approved as a diagnostic agent for GH deficiency, and ipamorelin’s clinical development for postoperative ileus was discontinued after phase 2. Research-grade versions of any of these compounds are not the approved products and carry no approved indication; their composition is established by analytical testing, as described in our note on third-party lab testing, and Wednesday’s per-lot certificates are in the COA library.

The axis is a three-level system: a peripheral hormone, a pituitary stimulus and a hypothalamic modulator, with somatostatin as the brake on the middle level. Each compound class engages one level. Keeping that architecture in view resolves most of the apparent contradictions in the secretagogue literature and clarifies what any particular study can be expected to show.

Frequently asked questions

What is the difference between GHRH and GHRP?

GHRH and its analogues bind the GHRH receptor on pituitary cells and stimulate GH release through cyclic AMP. GHRPs bind the ghrelin receptor (GHS-R1a) on pituitary and hypothalamic cells, signal through calcium, and also reduce somatostatin’s inhibitory influence. They act through separate mechanisms and are synergistic in experimental models.

Is sermorelin a GHRP?

No. Sermorelin is a GHRH analogue: the first 29 residues of growth hormone-releasing hormone, acting on the GHRH receptor. GHRPs such as ipamorelin and GHRP-6 act on the ghrelin receptor. The two classes are frequently confused because both release GH.

What receptor does ipamorelin bind?

Ipamorelin binds GHS-R1a, the growth hormone secretagogue receptor, whose natural ligand is ghrelin. It was reported in 1998 as the first GHRP that released GH without significant release of ACTH, cortisol or prolactin in animal models.

Why do GHRH and GHRP work synergistically?

Because they act through different receptors and second messengers on the same pituitary cells, and because GHRPs also act in the hypothalamus to increase GHRH release and reduce somatostatin’s braking effect. In animal and human studies, the two together release more GH than the sum of each alone.

How does growth hormone differ from GH secretagogues?

Growth hormone is the pituitary hormone itself, acting directly on peripheral tissues regardless of pituitary function and producing sustained rather than pulsatile levels. Secretagogues, whether GHRH analogues or GHRPs, act upstream to make the pituitary release its own GH in pulses, and they require a functional pituitary.

References & further reading

1. Møller N, Jørgensen JO. Effects of growth hormone on glucose, lipid, and protein metabolism in human subjects. Endocr Rev. 2009;30(2):152-177. PMID 19240267
2. Giustina A, Veldhuis JD. Pathophysiology of the neuroregulation of growth hormone secretion in experimental animals and the human. Endocr Rev. 1998;19(6):717-797. PMID 9861545
3. Kojima M, Hosoda H, Date Y, Nakazato M, Matsuo H, Kangawa K. Ghrelin is a growth-hormone-releasing acylated peptide from stomach. Nature. 1999;402(6762):656-660. doi:10.1038/45230 / PMID 10604470
4. Guillemin R, Brazeau P, Böhlen P, Esch F, Ling N, Wehrenberg WB. Growth hormone-releasing factor from a human pancreatic tumor that caused acromegaly. Science. 1982;218(4572):585-587. doi:10.1126/science.6812220 / PMID 6812220
5. Granata R, Leone S, Zhang X, et al. Growth hormone-releasing hormone and its analogues in health and disease. Nat Rev Endocrinol. 2025;21(3):180-195. doi:10.1038/s41574-024-01052-1 / PMID 39537825
6. Teichman SL, Neale A, Lawrence B, Gagnon C, Castaigne JP, Frohman LA. Prolonged stimulation of growth hormone (GH) and insulin-like growth factor I secretion by CJC-1295, a long-acting analog of GH-releasing hormone, in healthy adults. J Clin Endocrinol Metab. 2006;91(3):799-805. doi:10.1210/jc.2005-1536 / PMID 16352683
7. Bowers CY, Momany FA, Reynolds GA, Hong A. On the in vitro and in vivo activity of a new synthetic hexapeptide that acts on the pituitary to specifically release growth hormone. Endocrinology. 1984;114(5):1537-1545. PMID 6714155
8. Howard AD, Feighner SD, Cully DF, et al. A receptor in pituitary and hypothalamus that functions in growth hormone release. Science. 1996;273(5277):974-977. doi:10.1126/science.273.5277.974 / PMID 8688086
9. Raun K, Hansen BS, Johansen NL, et al. Ipamorelin, the first selective growth hormone secretagogue. Eur J Endocrinol. 1998;139(5):552-561. PMID 9849822