The growth hormone axis, without mystery

Quick answer

The growth hormone axis starts in the hypothalamus, passes through the pituitary, reaches the liver, and acts on peripheral tissues. Four pharmacological classes act at different points along this chain: GHRH analogs (such as CJC-1295 and tesamorelin), GHRPs or peptide secretagogues (such as ipamorelin, GHRP-2, GHRP-6), non-peptide oral secretagogues (such as MK-677), and recombinant GH itself. Only tesamorelin and recombinant GH have approval from a regulatory agency, and in narrow indications — HIV-associated lipodystrophy for tesamorelin, GH deficiency for recombinant GH. The remaining compounds exist as compounded preparations under medical prescription and are banned in sport by WADA under section S2.

Why this guide exists

English-language search on the growth hormone axis runs into two opposite sources: commercial articles selling peptides as an aging solution, and dense academic texts with no bridge for the adult reader who simply wants to understand the landscape. What's missing is material that describes axis anatomy, shows where each pharmacological class enters, and situates everything in the Brazilian regulatory context. This guide is that bridge. It does not prescribe doses, does not recommend protocols, and does not advise intervention. The canonical Portuguese fact-sheets at /peptideos/cjc-1295, /peptideos/tesamorelina, /peptideos/ipamorelina, and /peptideos/mk-677 carry the molecule-level detail.

The GH/IGF-1 axis: minimal anatomy

The somatotropic axis is a chain of four stations with signals traveling in both directions. Understanding this architecture is the key to understanding why different pharmacological classes produce different effects — each class enters at a specific station, and the system's overall response depends on the entry point and the kinetics of the stimulus.

The first station is the hypothalamus, which produces two neuropeptides with opposite roles. GHRH (Growth Hormone-Releasing Hormone) stimulates GH release. Somatostatin inhibits it. The alternating interaction of the two defines the axis pulsatility. In parallel, the stomach produces ghrelin, which travels to the hypothalamus and pituitary and acts as an additional pro-GH signal.

The second station is the anterior pituitary, specifically the somatotrophs — cells that store GH and release it when stimulated. GHRH binds to the GHRH receptor (GHRH-R), ghrelin binds to the GH secretagogue receptor (GHSR-1a), and somatostatin binds to SSTR receptors to brake the system. The net result of this orchestration is pulsatile GH secretion into the bloodstream, with predominantly nocturnal pulses and near-zero levels between pulses. Hartman, Iranmanesh, Thorner, and Veldhuis described this pattern in a 1993 review (PMID 28548364) that remains a founding reference.

The third station is the liver, which receives circulating GH and responds by producing IGF-1 (Insulin-like Growth Factor 1). IGF-1 is the peripheral messenger that mediates most of the classical anabolic effects attributed to the axis — protein synthesis, cell division, tissue maintenance. IGF-1 also returns as a negative feedback signal, reducing pituitary GH secretion and increasing hypothalamic somatostatin.

The fourth station is the peripheral tissues — muscle, bone, adipose tissue, nervous system, immune system. IGF-1 receptors and GH receptors coexist with variable distribution; some effects are direct via GH, others are mediated by IGF-1.

Five numerical facts about this physiology:

1. Nocturnal pulsatility dominates: an estimated 70% of daily GH secretion occurs during sleep in a healthy young adult.
2. Pulses occur at average intervals of 3 to 4 hours, with amplitude and frequency regulated by age, sex, body composition, and nutrition.
3. The circulating half-life of GH is 10 to 20 minutes; that of IGF-1 is hours to days, because it circulates bound to carrier proteins (IGFBPs).
4. Total GH production drops approximately 50% between ages 20 and 60 — a physiological phenomenon called somatopause, not a disease.
5. Serum IGF-1 is the standard biomarker for axis evaluation.

The four pharmacological mechanisms available

Each class acting on the GH/IGF-1 axis enters at a different point in the chain. Identifying the point matters because it defines kinetics, reversibility, and potential risk profile.

GHRH analogs — act on the pituitary GHRH-R

This class mimics endogenous GHRH and activates the same receptor (GHRH-R) on anterior pituitary somatotrophs. The theoretical advantage is preserving physiological regulation of the axis: somatostatin still brakes the system, IGF-1 still provides negative feedback, and the resulting GH release follows a modulated pattern, though with an elevated floor. The main representatives with human clinical literature are:

• Tesamorelin, a synthetic GHRH 1-44 analog with N-terminal modification for resistance to DPP-IV cleavage. It is the only compound in the class with FDA approval, obtained in 2010 for reduction of visceral adipose tissue in people living with HIV and lipodystrophy. The regulatory basis is the phase 3 trials by Falutz and colleagues (NEJM 2007, PMID 18057338; JCEM 2010, PMID 20554713), with 816 combined participants.
• CJC-1295, a GHRH 1-29 analog with 4 amino acid substitutions for degradation resistance. It exists in two variants — with DAC (extended half-life via albumin binding, ~8 days) and without DAC (called Mod-GRF 1-29, ~30 minutes half-life). Human literature is restricted to phase 1 studies in healthy adults, with Teichman 2006 (PMID 16352683) being the seminal one.
• Sermorelin, a GHRH 1-29 analog without resistance modifications. It was registered as Geref in the United States for short stature in children, but the product was commercially discontinued. It now circulates only in compounded form.

Mechanically, GHRH analogs preserve physiological regulation via somatostatin — somatostatin peaks still brake GH release even under GHRH-R stimulation. This detail differentiates the class from exogenous recombinant GH, which suppresses the axis through feedback.

GHRPs and peptide secretagogues — act on GHSR-1a

This class mimics ghrelin and activates the GH secretagogue receptor (GHSR-1a). Main representatives include:

• Ipamorelin, a synthetic pentapeptide developed by Novo Nordisk in the 1990s. Distinguishing feature: selectivity — it releases GH without relevant elevation of cortisol or prolactin, unlike GHRP-2 and GHRP-6, which tend to stimulate those other axes.
• GHRP-2 (pralmorelin), a hexapeptide with greater potency in GH stimulation but with effect also on cortisol and prolactin. Approved in Japan for diagnostic GH deficiency testing, not for chronic therapeutic use.
• GHRP-6, an analog hexapeptide. It elevates appetite via a ghrelin-like mechanism, a feature frequently described as a relevant side effect.
• Hexarelin, a high-potency analog. Studies suggest receptor desensitization with prolonged use.

The Sigalos and Pastuszak review in Sexual Medicine Reviews (2018, PMID 28400207) synthesizes the available literature: the class suggests acceptable safety in short trials, but sample size and duration limit conclusions.

Non-peptide oral secretagogues — same receptor, oral route

This class activates the same GHSR-1a as GHRPs but with a non-peptide molecule and oral bioavailability — a relevant pharmaceutical differential, since peptides are degraded in the gastrointestinal tract and require injection. The clinical representative is MK-677, also known as ibutamoren mesylate. It was developed by Merck in the 1990s with the stated goal of treating frailty in older adults and short stature in children. The phase 3 program was discontinued.

The Adunsky 2011 phase IIb trial in older adults post-hip fracture (n=123, PMID 21067829) is representative: IGF-1 elevated 84% in the active arm, but the incidence of congestive heart failure was higher (6.5% vs 1.7% placebo) — a safety signal that contributed to the closure of the clinical program.

Recombinant GH — non-peptide, outside the main scope

Recombinant GH (somatropin) is the human hormone itself produced by recombinant DNA, identical in sequence to endogenous GH. It has multiple regulatory registrations in Brazil and abroad for specific indications: GH deficiency in children and adults, Turner syndrome, short stature due to chronic kidney disease, Prader-Willi syndrome, among others.

The mechanistic difference is fundamental: recombinant GH replaces the hormone directly, delivering the axis end-product to circulation and suppressing endogenous secretion via negative feedback. The other three classes stimulate the endogenous system to produce more GH, preserving some degree of physiological regulation.

Pulsatile vs sustained: why it matters

Endogenous GH in a healthy person is pulsatile — nocturnal pulses dominate, with near-zero levels between them. This temporal pattern appears to carry biologically relevant information for target tissues, not just cumulative exposure.

Pharmacological classes relate to this pattern in different ways. Short half-life GHRPs like ipamorelin (minutes) produce an acute pulse closer to the endogenous profile. Mod-GRF 1-29 (CJC-1295 without DAC) also preserves pulsatility. CJC-1295 with DAC maintains continuous GHRH-R stimulus for days — Ionescu and Frohman (2006) showed that endogenous pulses persist even in this scenario, but basal GH between pulses rises substantially and hepatic IGF-1 stays elevated for 9 to 11 days. Oral MK-677 daily produces tonic elevation over months. Daily recombinant GH produces fully non-pulsatile exposure.

Why this matters clinically. In acromegaly (pathological GH excess), sustained exposure is associated with increased cardiovascular, metabolic, and neoplastic risk over decades. In intermittent use of long-acting analogs outside that pathological context, how much of that risk materializes in humans is unknown — there is no trial with follow-up longer than 12 months for most of these compounds. Absence of data is not evidence of safety; it is documented ignorance.

Evidence hierarchy by compound

Indexed human clinical evidence varies dramatically across the four mechanisms.

[Tesamorelin](/peptideos/tesamorelina): the only peptide compound with a robust phase 3 program. Falutz 2007 (NEJM, n=412) and Falutz 2010 (JCEM, pooled analysis of 816 participants) support FDA approval for HIV lipodystrophy, with safety extension to 52 weeks. Outside that indication, evidence is not reproduced.

[CJC-1295](/peptideos/cjc-1295): literature restricted to 3 small studies in healthy adults (2006-2009). Teichman 2006 (n=43) and Ionescu 2006 (n=20) are the main ones, in phase 1 design. No RCT in clinical populations, no long-term data.

[Ipamorelin](/peptideos/ipamorelina): limited phase 1 and phase 2 studies, mainly in postoperative ileus contexts and pharmacological research. Sigalos and Pastuszak (2018) concluded that the literature suggests acceptable tolerability in short trials, with no data supporting approved chronic use.

[MK-677](/peptideos/mk-677): a relatively extensive phase 2 and phase IIb program. Adunsky 2011 (n=123, phase IIb) is the most robust trial, with a safety signal (congestive heart failure) that contributed to discontinuation. Phase 3 not completed.

GHRP-2, GHRP-6, sermorelin: scattered studies, no active phase 3 program. Sermorelin had an old FDA registration for short stature in children (Geref product), commercially discontinued.

What we know

1. The axis anatomy is well characterized: hypothalamus releases GHRH and somatostatin; pituitary releases GH in pulses; liver produces IGF-1; tissues respond.
2. Nocturnal pulsatility is a documented physiological feature in humans, with differences by age, sex, and body composition.
3. Tesamorelin reduces visceral adipose tissue in people living with HIV and lipodystrophy, based on phase 3 RCTs totaling 816 participants and safety extension to 52 weeks.
4. Recombinant GH has established efficacy in GH deficiency confirmed by specific tests.
5. WADA bans all axis classes (S2.2.3 and S2.2.4) in and out of competition.
6. Sustained IGF-1 signaling in pathological condition (acromegaly) is associated with increased cardiovascular and neoplastic risk over decades.

What we don't yet know

• Long-term safety of pharmacologically sustained IGF-1 in people without an approved clinical indication. Human data with follow-up beyond 12 months are scarce for most compounds.
• Clinical equivalence between endogenous pulsatile and pharmacological sustained patterns in prolonged use. The difference is pharmacodynamically clear, but translation into relevant clinical outcomes is not established.
• Efficacy of CJC-1295, ipamorelin, and MK-677 in specific populations such as people with obesity, sarcopenia in older adults, partial GH deficiency, or surgical recovery. There are no RCTs in clinical populations for these compounds.
• Neoplastic risk in long-term intermittent use. Biological plausibility exists; clinical data does not.
• Combinations like CJC-1295 + ipamorelin or other associations: in vitro and short-term in vivo data suggest synergy; human RCTs with clinical outcomes do not exist.
• Candidate selection criteria. There is no prescriptive literature on who benefits, at what dose, for how long, outside approved indications.

Regulatory status in Brazil

ANVISA (Brazil's federal drug regulatory agency) holds no marketing authorization for medicines containing CJC-1295, ipamorelin, GHRP-2, GHRP-6, hexarelin, sermorelin, MK-677, or tesamorelin for commercial use in humans in Brazil. Recombinant GH (somatropin) has registrations for specific indications.

For peptide compounds without registration, magistral compounding is the legal route, exclusively in pharmacies licensed for hormones and under medical prescription. Technical Note No. 200/2025/SEI/GIMED/GGFIS/DIRE4/ANVISA, although focused on GLP-1 analogs, consolidated quality criteria for imported peptide APIs that apply by extension to GH-axis peptides: HPLC/UV identification, peptide mapping, assay, pH, impurities, sterility, and endotoxins. Importation by a compounding pharmacy is restricted to authorized distributors.

ANVISA has publicly addressed, throughout 2025 and 2026, the growth of the compounded peptide market without valid prescription — characterized by the agency as a sanitary infraction.

WADA and sport

The World Anti-Doping Agency 2026 Prohibited List includes all axis classes in section S2 (Peptide Hormones, Growth Factors, and Related Substances):

• S2.2.3 — GH, its analogs, and fragments (somatropin, AOD-9604).
• S2.2.4 — GH-releasing substances: GHRH analogs (CJC-1295, tesamorelin, sermorelin, CJC-1293), GHRPs (ipamorelin, GHRP-2/pralmorelin, GHRP-6, alexamorelin, hexarelin), and secretagogues such as MK-677/ibutamoren.

The ban applies in and out of competition. Athletes in federations signatory to the World Anti-Doping Code with a positive test face automatic suspension. Detection combines IGF-1 biomarkers, mass spectrometry for synthetic peptides, and differential isoforms for recombinant vs endogenous GH.

When it makes sense to ask the endocrinologist

pephealth does not direct intervention. Asking the endocrinologist about the GH axis makes sense in scenarios where clinical literature recognizes evaluation:

• Suspected adult GH deficiency, based on symptoms (persistent unexplained fatigue, loss of muscle mass with increased central adiposity, reduced quality of life) and ideally with a predisposing factor — pituitary tumor, cranial irradiation, severe head trauma, specific syndromes. Confirmation involves stimulation tests (clonidine, GHRH-arginine, insulin-induced hypoglycemia, glucagon); a low IGF-1 alone is not enough.
• Lipodystrophy in a person living with HIV on antiretroviral therapy, with recognized indication for tesamorelin.
• Specific syndromes with indication for recombinant GH (Turner, Prader-Willi, confirmed deficiency in children).
• Clinical research context in a specialized center, within an ethics-committee-approved protocol.

Outside these scenarios, clinical literature does not authorize axis intervention as an aging, performance, or aesthetic strategy.

Editorial closing

The GH/IGF-1 axis is a well-characterized physiological system. The pharmacological classes acting on it are chemically distinct, enter at different points in the chain, and — crucially — have very different volumes of human clinical evidence.

Only tesamorelin has a robust phase 3 program and regulatory approval, in a narrow indication. Only recombinant GH has multiple registrations for confirmed deficiency. The rest exist in a zone where literature is restricted to small studies, in limited populations, with short follow-up. Magistral compounding under medical prescription is a legal route; what circulates outside that route is irregular.

pephealth neither recommends nor discourages axis interventions. The function of this guide is to describe, with transparency about what does and does not exist in the 2026 clinical literature, the terrain where the conversation between the person who researches and the person who prescribes needs to happen.