The Growth-Hormone Peptide Class: Secretagogues, the Hormone Itself, and Its Downstream Messengers

Walk down the "growth hormone" section of any research-peptide catalog and you will find a dozen compounds filed under one heading: sermorelin, tesamorelin, two CJC-1295 variants, a cluster of GHRPs, recombinant HGH, IGF-1 LR3, MGF. It is tempting to read that shelf as a dozen interchangeable ways of doing the same thing. It is not.

The single most common literacy error in this corner of the field is treating a growth-hormone secretagogue, growth hormone itself, and a downstream messenger such as IGF-1 as three flavors of one product. They act at three different points on the same biological axis, and those differences — not the shared "GH" label — are the whole story. This primer sorts the shelf by where each compound intervenes.

One axis, four places to intervene

Growth-hormone biology runs as a cascade. The hypothalamus releases growth-hormone-releasing hormone (GHRH), which tells somatotroph cells in the anterior pituitary to secrete growth hormone (GH). A second hypothalamic signal, ghrelin, amplifies that release through its own receptor, while somatostatin applies the brake. GH then travels to the liver and peripheral tissues, where much of its downstream effect is carried out by insulin-like growth factor 1 (IGF-1). IGF-1 in turn feeds back to suppress further GH release. We covered this loop in detail in the GH secretagogue axis explainer.

The catalog compounds map onto four distinct intervention points on that cascade:

• GHRH analogs — act at the GHRH receptor to prompt the pituitary.
• Ghrelin-mimetic GHRPs — act at a second pituitary receptor to amplify the same release.
• Growth hormone itself — supplies the finished hormone, bypassing the pituitary entirely.
• Downstream messengers — IGF-1 analogs and muscle splice variants that act below GH.

Read the shelf this way and the "which one is best?" question dissolves into "which point on the axis, and to what end?"

Branch one: GHRH analogs, the peptides that ask the pituitary

These are secretagogues — they do not contain any growth hormone. They present a GHRH-like signal to the GHRH receptor (a Gs/cAMP-coupled receptor) on somatotrophs, prompting the pituitary to synthesize and release its own GH. Because the pituitary and its feedback loops stay in the loop, release remains broadly pulsatile.

• Sermorelin is GRF(1–29), the minimal 29-amino-acid fragment that retains full GHRH activity. It is the archetype of the class and has a very short circulating half-life. Notably, it was once an approved drug (Geref) used in pediatric GH-deficiency diagnosis before being withdrawn from the U.S. market in 2008 for commercial rather than safety reasons.
• Tesamorelin is a stabilized GHRH(1–44) analog carrying an N-terminal modification that resists enzymatic cleavage. It is the most clinically validated member of the entire growth-hormone shelf: it is FDA-approved (marketed as Egrifta) and has been studied for reducing excess visceral abdominal fat in people with HIV-associated lipodystrophy. A reformulated weekly-reconstitution version, Egrifta WR, received FDA approval in March 2025.
• CJC-1295 No-DAC (also sold as Mod GRF 1-29) is a modified GRF(1–29) with amino-acid substitutions that resist degradation while keeping a short half-life and pulsatile profile.
• CJC-1295 DAC adds a Drug Affinity Complex that binds circulating albumin, extending the half-life to roughly a week and producing a sustained elevation rather than a sharp pulse. The trade-off between these two is significant enough that we gave it its own comparison.

The through-line: same receptor, same "ask the pituitary" strategy, different pharmacokinetic personalities.

Branch two: ghrelin-mimetic GHRPs, the second receptor

The growth-hormone-releasing peptides (GHRPs) are also secretagogues, but they work through a different receptor — GHS-R1a, the ghrelin receptor, which couples to a Gq/phospholipase-C/calcium pathway. Two different receptors, two different signaling cascades, both converging on GH release.

The members differ mainly in selectivity, and that is the axis that matters when reading them:

• Ipamorelin is a pentapeptide engineered to be a highly selective GHS-R1a agonist. It was designed to keep GHRP-6's GH-releasing potency while shedding the off-target cortisol, prolactin, and appetite effects — the "clean pulse" of the group.
• GHRP-2 is a potent hexapeptide with modest off-target cortisol and prolactin activity and a moderate appetite effect.
• GHRP-6 is the original hexapeptide, with strong ghrelin mimicry that produces pronounced hunger alongside more cortisol and prolactin stimulation.
• Hexarelin shows the highest GHS-R potency of the classic GHRPs and carries a distinct, non-GHS-R cardiac interaction via the CD36 scavenger receptor. It is also the most prone to receptor desensitization on repeated exposure.

So the GHRP sub-shelf is a selectivity spectrum running from ipamorelin (most selective) through GHRP-2 to GHRP-6 (least selective), with hexarelin as the potent, mechanistically distinct outlier.

Why the two secretagogue branches get blended

Because branch one and branch two hit separate receptors on the same cell, their signals add together — and a GHRP also helps blunt somatostatin's braking tone. In research settings, combining a GHRH analog with a GHRP has been studied for producing a larger, more synergistic GH release than either alone. That mechanistic rationale is exactly why the catalog carries a CJC-1295 + Ipamorelin blend rather than treating the two as competitors. The blend is not a marketing convenience; it is the two-receptor architecture expressed as a single vial.

Branch three: growth hormone itself

HGH 191AA is a different kind of object entirely. It is recombinant human growth hormone — the complete 191-amino-acid, ~22-kDa protein (somatropin), the same molecule the pituitary would otherwise secrete.

This is the crucial distinction: it is not a secretagogue. Nothing about it "asks" the pituitary to do anything. It supplies the finished hormone directly, bypassing the pituitary, its pulsatile release pattern, and the upstream signaling that every peptide in branches one and two depends on. It is still subject to the axis's downstream negative feedback — exogenous GH raises IGF-1, which suppresses endogenous GH — but the release step is skipped. Recombinant somatropin is an established, approved therapeutic class in clinical medicine; on a research shelf it is the reference standard for what "the actual hormone" looks like, against which the secretagogues are the indirect alternatives.

Branch four: the downstream messengers

The last group acts below GH, at or near the IGF-1 receptor — after the point where secretagogues and GH have done their work.

• IGF-1 LR3 (Long R3 IGF-1) is an 83-amino-acid IGF-1 analog with an arginine substitution at position 3 and a 13-residue N-terminal extension. Those changes sharply reduce its binding to IGF-binding proteins, giving it a much longer functional half-life and higher potency than native IGF-1 — properties that made it a long-standing cell-culture research reagent.
• MGF (Mechano Growth Factor) is a splice variant of IGF-1 (IGF-1Ec) produced locally in muscle in response to mechanical load or damage, distinguished by a unique C-terminal E-domain peptide and studied for its proposed role in activating muscle satellite cells.
• PEG-MGF is a PEGylated version of MGF intended to extend its otherwise brief stability.

One aside belongs here. HGH Fragment 176-191 carries the "growth hormone" name by sequence lineage — it is the C-terminal fragment of the GH molecule — but it does not act through the growth/IGF-1 axis at all. It isolates GH's fat-metabolism domain and has been studied for a proposed lipolytic mechanism. Its engineered cousin, AOD-9604, is the version with an actual completed human trial record — we covered that story separately. File these under "metabolic," not "growth."

Reading the shelf by evidence maturity

Sorting by mechanism also sorts by how much is actually known, and the spread is wide:

• Approved and clinically studied: tesamorelin (Egrifta, a GHRH analog) and recombinant somatropin (the drug class HGH 191AA represents) sit at the mature end.
• Formerly approved, withdrawn for commercial reasons: sermorelin.
• Research-use-only, never approved as drugs: both CJC-1295 variants, all four GHRPs, IGF-1 LR3, MGF, PEG-MGF, and the HGH fragment.

The label "growth hormone" flattens all of that into one word. Reading the shelf mechanism-first restores the distinctions that matter: which receptor, which point on the axis, and how much evidence stands behind each claim. For the compound-by-compound detail behind any name here, see the compound library, and for how purity and identity are established before a peptide is used in research, see our quality standards.

FAQ

Is using a secretagogue the same as supplying growth hormone? No. A secretagogue (branches one and two) prompts the pituitary to release its own GH and keeps the body's feedback and pulsatility in play. Recombinant GH (branch three) supplies the finished hormone and skips the release step entirely. They intervene at different points and behave differently as a result.

Why are there four GHRPs if they all hit the same receptor? They differ in selectivity. Ipamorelin is engineered to trigger a clean GH pulse with minimal cortisol, prolactin, or appetite effects; GHRP-2 and GHRP-6 carry progressively more of those off-target actions; hexarelin is the most potent at the receptor and adds a separate CD36-mediated cardiac interaction. Same target, different side profiles.

Does IGF-1 LR3 raise growth hormone? No — it acts downstream of GH, at the IGF-1 receptor. If anything, elevated IGF-1 signals back to suppress GH release. It is a messenger of GH's effects, not a trigger for its secretion.

This article is educational and for the laboratory research community. Trulogic Labs products are sold for laboratory and research use only and are not for human consumption.