Growth Hormone Secretagogue: A Literature Course

This course walks through the published literature on growth hormone secretagogues (GHS) — the class of endogenous and synthetic molecules that act at the growth hormone secretagogue receptor. Each module summarises what specific papers examined, in what model, and what the authors reported, then closes with the limits of that evidence. This page is for educational purposes only and is not medical advice; consult a licensed physician before making any health decision. Nothing here describes a protocol, and no paper summarised below should be read as a recommendation.

Module 1: What a growth hormone secretagogue is and how it has been studied

Definition and class

"Growth hormone secretagogue" is a pharmacological class label rather than a single molecule. It groups compounds that bind the growth hormone secretagogue receptor (GHSR), a G protein-coupled receptor whose ligand-binding architecture was characterised in a 2022 review describing the receptor's binding domains and how they accommodate different ligand chemotypes (PMID 35959447). The receptor is usually discussed in its GHSR-1a splice form, which is the isoform researchers measured in rat hypothalamus in an olanzapine study (PMID 35835020).

Origin of the class

The receptor was named before its natural ligand was identified, and comparative work has since placed the system deep in vertebrate evolution: a 2016 analysis reported diversification and coevolution of the ghrelin/growth hormone secretagogue receptor system across vertebrate lineages (PMID 27066235). That evolutionary framing matters for interpretation, because much of the mechanistic literature uses non-human species whose receptor sequences are related but not identical to the human form (PMID 27066235).

Forms encountered in the literature

• Endogenous peptide ligand: ghrelin, whose actions on the rat and human gastrointestinal tract were examined alongside receptor localisation in a 2003 neuroscience study (PMID 12890514).
• Synthetic peptides: growth hormone secretagogue peptide-6 (GHRP-6), which a 2021 study applied in tilapia to measure transcription of antimicrobial peptides (PMID 34592474).
• Non-peptide small molecules: MK-0677, detected in equine hair after oral administration in an anti-doping method paper (PMID 36354265), and macimorelin and anamorelin, described as two approved drugs in a 2025 molecular recognition study of GHSR (PMID 40542284).

Limits of the evidence — Module 1

The class label covers chemically dissimilar molecules studied in different species for different purposes. No paper in this set compared peptide and non-peptide secretagogues head to head in humans, and the evolutionary and structural papers describe receptor biology rather than any compound's clinical profile (PMID 35959447, PMID 27066235).

Module 2: Mechanism as described in the literature

Receptor-level description

The mechanistic starting point is ligand engagement at GHSR. Researchers reviewing binding domain characterisation reported that distinct receptor regions contribute to recognition of different ligands (PMID 35959447). A 2025 study extended that picture by examining how the receptor molecularly recognises the two approved drugs macimorelin and anamorelin (PMID 40542284). Together these papers describe binding and recognition, not downstream clinical consequences.

Where the receptor sits

Localisation studies have repeatedly asked where GHSR is expressed. The 2003 study mapped growth hormone secretagogue receptors in rat and human gastrointestinal tract and examined ghrelin's effects there (PMID 12890514). In the central nervous system, a 2024 mouse neuroanatomy study reported intersection between growth hormone secretagogue receptor and cannabinoid receptor type 1 expression across brain regions (PMID 39702649). In cardiac tissue, a 2021 report described regional differences in the ghrelin–growth hormone secretagogue receptor signalling system in human heart disease (PMID 33644732).

Receptor loss and receptor upregulation

Two complementary strategies appear in the literature. Loss-of-function work reported that growth hormone secretagogue receptor deficiency promoted lung cancer growth in a model system by affecting the Th17/Treg balance (PMID 34988205), and a separate study reported that mice lacking the growth hormone secretagogue receptor showed enhanced suppression of food intake when given liraglutide (PMID 40738311). On the upregulation side, the study in rats reported increased hypothalamic GHSR-1a during olanzapine treatment (PMID 35835020).

Limits of the evidence — Module 2

Knockout and expression findings describe what happens when the receptor is absent or more abundant; they do not establish what a given secretagogue would do when administered. The CB1 intersection work was anatomical mapping in mouse brain rather than a functional pharmacology experiment (PMID 39702649), and the cardiac findings were regional expression comparisons in human disease tissue (PMID 33644732).

Module 3: Reported outcomes by study

The table below lists the models, endpoints and reported results from the papers in this course. No entry should be read as a benefit claim; each is a description of what one study measured.

The human endpoint in context

The single human administration study in this set examined hypogonadal men and reported that growth hormone secretagogue treatment raised serum IGF-1 (PMID 28830317). IGF-1 is a biochemical marker along the GH axis; the study title and abstract scope describe that laboratory change and not body composition, strength, longevity or any symptomatic outcome (PMID 28830317).

Non-mammalian and immunological endpoints

The tilapia work sits outside the growth-hormone framing entirely: researchers reported that GHRP-6 enhanced transcription of oreochromicins and antimicrobial activity in the fish (PMID 34592474). The lung cancer study similarly measured immune balance rather than growth hormone, reporting that receptor deficiency affected the Th17/Treg ratio alongside tumour growth (PMID 34988205).

Limits of the evidence — Module 3

These are single studies in heterogeneous models with different compounds and endpoints. Results in tilapia (PMID 34592474) and in genetically modified mice (PMID 40738311) do not transfer to human physiology, and a marker change such as serum IGF-1 (PMID 28830317) is not evidence of a clinical outcome.

Module 4: Growth Hormone Secretagogue Side Effects: What Studies Report

What the cited papers did and did not tabulate

Adverse-event reporting is the thinnest part of this evidence base. The human study in hypogonadal men was framed around a laboratory endpoint — serum IGF-1 — and its published scope centres on that measurement rather than on a safety table (PMID 28830317). The equine paper was an analytical detection method describing recovery of MK-0677 from hair after oral administration, not a tolerability study (PMID 36354265). Readers looking for a systematic adverse-event profile will not find one in these sources.

Safety-adjacent findings researchers did report

Several papers reported biology that is relevant to safety discussion without being adverse events of administration. Researchers reported that growth hormone secretagogue receptor deficiency promoted lung cancer growth through effects on the Th17/Treg balance, linking the receptor pathway to tumour immunology in that model (PMID 34988205). A separate report described regional differences in ghrelin–GHSR signalling in human heart disease, placing the pathway in diseased cardiac tissue (PMID 33644732). In rodents, the study of antipsychotic treatment reported increased hypothalamic GHSR-1a during olanzapine administration, a change discussed in the context of appetite regulation (PMID 35835020).

Appetite and feeding signals

Because the receptor is embedded in feeding circuitry, food-intake changes appear throughout the literature as endpoints rather than complaints. The study in receptor-deficient mice reported enhanced suppression of food intake with liraglutide, indicating crosstalk between the two pathways (PMID 40738311), and gastrointestinal receptor mapping with ghrelin effects describes a peripheral arm of the same system (PMID 12890514).

Limits of the evidence — Module 4

None of the papers in this set constitutes a controlled safety trial of a growth hormone secretagogue in humans. Knockout findings describe consequences of receptor absence, not of receptor stimulation (PMID 34988205, PMID 40738311), and no long-term human follow-up appears in this literature. Absence of reported harm in a paper that never measured harm is not evidence of safety.

Module 5: Pharmacokinetics where data exist

Administration routes documented

Route information in this set is sparse but explicit where present. MK-0677 was given orally to horses and subsequently detected in hair, which the authors used to build a detection window for equine testing (PMID 36354265). That paper is an analytical-chemistry contribution: it establishes that the compound and its traces can be recovered from a keratinised matrix after oral dosing rather than characterising plasma half-life (PMID 36354265).

What structural work implies and does not measure

The molecular recognition study of macimorelin and anamorelin at GHSR described how the receptor engages these two approved drugs at the structural level (PMID 40542284), and the binding domain review described the receptor regions involved in ligand interaction (PMID 35959447). Binding and recognition data describe affinity and geometry, not absorption, distribution, metabolism or elimination.

Limits of the evidence — Module 5

No paper in this verified set reported human plasma concentration curves, bioavailability percentages, half-life values or clearance routes for any growth hormone secretagogue. The only in-vivo disposition data are from horses and concern hair detection after oral administration (PMID 36354265). Species differences in the receptor system, which comparative work has documented across vertebrates, add further uncertainty to any cross-species extrapolation (PMID 27066235).

Module 6: Regulatory status stated factually

Approved products

Not every growth hormone secretagogue occupies the same regulatory category. The 2025 structural paper explicitly described macimorelin and anamorelin as two approved drugs when analysing their recognition by the growth hormone secretagogue receptor (PMID 40542284). Approval status varies by jurisdiction and by indication, and the paper's framing refers to their status as marketed medicines rather than endorsing any particular use.

Research-use-only material and unapproved compounds

Other molecules in this class have no marketing authorisation for human therapeutic use and circulate as research chemicals labelled research use only (RUO). RUO labelling denotes material that is not manufactured, tested or released under standards required for human administration. MK-0677 appears in the literature as an analyte of interest to testing laboratories, with a published method for detecting it in equine hair after oral administration (PMID 36354265). GHRP-6, the peptide used in the tilapia experiment, appears in that paper as an experimental reagent in an animal model (PMID 34592474).

Compounding

In the United States, compounded preparations are made by pharmacies or outsourcing facilities under sections of the Federal Food, Drug, and Cosmetic Act rather than through new-drug approval, and eligibility of a given substance for compounding depends on regulatory lists and determinations that change over time. Anti-doping and animal-testing contexts operate under separate rule sets, as illustrated by the equine hair detection method developed for racing testing (PMID 36354265). This section is general regulatory information and is not legal advice.

Limits of the evidence — Module 6

Regulatory categories are jurisdiction-specific and time-sensitive; a compound's status in one country says nothing about another. The scientific papers cited here describe pharmacology and analytical detection, and only one of them comments on approval status at all (PMID 40542284).

What the studies did not test

Across this set, several questions remain unaddressed:

• Long-term human outcomes. The human study reported a serum IGF-1 change in hypogonadal men and did not follow participants for clinical endpoints (PMID 28830317).
• Comparative efficacy. No paper here compared peptide secretagogues such as GHRP-6 (PMID 34592474) against non-peptide agents such as MK-0677 (PMID 36354265) in a shared model.
• Human pharmacokinetics. Disposition data in this set come from horses rather than people (PMID 36354265).
• Oncological and cardiac consequences of administration. The cancer and heart findings were receptor-deficiency and tissue-expression studies, not administration trials (PMID 34988205, PMID 33644732).
• Behavioural or neurological function. The brain work mapped receptor overlap with cannabinoid receptor type 1 in mice without testing behaviour after secretagogue administration (PMID 39702649).

Readers evaluating claims about this class can use these gaps as a checklist: if a statement goes beyond what a cited paper measured, the citation does not support it. Questions about individual health, medication interactions or laboratory testing belong with a licensed physician.

References

• Binding Domain Characterization of Growth Hormone Secretagogue Receptor (Journal of Translational Internal Medicine, 2022)
• Growth hormone secretagogue receptor and cannabinoid receptor type 1 intersection in the mouse brain (Brain Structure & Function, 2024)
• Growth hormone secretagogue peptide-6 enhances oreochromicins transcription and antimicrobial activity in tilapia (Oreochromis sp.) (Fish & Shellfish Immunology, 2021)
• Growth Hormone Secretagogue Treatment in Hypogonadal Men Raises Serum Insulin-Like Growth Factor-1 Levels (American Journal of Men's Health, 2017)
• Detection of the growth hormone secretagogue MK-0677 in equine hair following oral administration (Drug Testing and Analysis, 2023)
• Molecular recognition of two approved drugs Macimorelin and Anamorelin by the growth hormone secretagogue receptor (Acta Pharmacologica Sinica, 2025)
• Growth hormone secretagogue receptor deficiency promotes lung cancer growth by affecting the Th17/Treg balance (Annals of Translational Medicine, 2021)
• Diversification and coevolution of the ghrelin/growth hormone secretagogue receptor system in vertebrates (Ecology and Evolution, 2016)
• Regional Differences in the Ghrelin-Growth Hormone Secretagogue Receptor Signalling System in Human Heart Disease (CJC Open, 2021)
• Liraglutide induces enhanced suppression of food intake in mice lacking the growth hormone secretagogue receptor (Molecular and Cellular Endocrinology, 2025)
• Growth hormone secretagogue receptors in rat and human gastrointestinal tract and the effects of ghrelin (Neuroscience, 2003)
• Increased growth hormone secretagogue receptor-1a (GHSR-1a) in hypothalamus during olanzapine treatment in rats (Psychoneuroendocrinology, 2022)