Does growth hormone act directly on tissues?

Partly, but many effects attributed to it are mediated by IGF-1, which the liver produces in response to it.

Why is growth hormone high in Laron syndrome?

IGF-1 is the feedback signal that suppresses it. Without IGF-1 production, that brake is absent.

How does this relate to GHRH analogues?

They act at the top of the loop by stimulating pituitary release; IGF-1 analogues act at the bottom by supplying the mediator directly.

IGF-1 LR3

Two Hormones, One Feedback Loop

JMWritten & reviewed by Jack Muncaster · Founder, UK PeptidesLast reviewed 2026-08-233 cited sources

Growth hormone acts on the liver, which produces IGF-1, and circulating IGF-1 feeds back to suppress further growth hormone release. Many effects attributed to growth hormone are mediated by IGF-1 rather than by growth hormone acting directly.

Key facts

Sequence: Pituitary GH → liver → IGF-1
Feedback: IGF-1 suppresses GH release
In Laron syndrome: GH high, IGF-1 very low
Why: Feedback arm is absent
Liver as endocrine organ: Rhyu 2021 (PMID 34904032)
Related on this site: GHRH analogues and secretagogues

The sequence of events

The pituitary releases growth hormone in pulses. Growth hormone acts on hepatocytes, which produce IGF-1 and release it into circulation. IGF-1 then acts on peripheral tissues and also returns to the hypothalamus and pituitary, where it suppresses further growth hormone release. It is a classical endocrine loop with the liver as the middle step.

Why the intermediary matters

Many effects historically attributed to growth hormone are produced by IGF-1 rather than by growth hormone acting on the tissue directly. Growth hormone is in that sense partly an instruction to the liver, and the liver's product does much of the work — which is why the two cannot be treated as interchangeable.

Research material referenced

IGF-1 LR3 1mg — third-party HPLC tested

View — £37.99

What Laron syndrome does to the loop

It breaks the middle step. Growth hormone is produced and released normally but cannot be received, so IGF-1 production fails. Because IGF-1 is the feedback signal, its absence removes the brake and growth hormone rises further. The result is the characteristic combination of high growth hormone with very low IGF-1 — a pattern that identifies the condition.

The liver as an endocrine organ

Rhyu and colleagues reviewed newly discovered endocrine functions of the liver in the World Journal of Hepatology in 2021. IGF-1 production is the long-established example, and it is a useful corrective to thinking of the liver purely as a metabolic and detoxifying organ — it is also a hormone-secreting tissue responding to a pituitary signal.

Why this connects to other compounds in this catalogue

Growth hormone releasing hormone analogues and growth hormone secretagogues act at the top of this loop, stimulating pituitary release. IGF-1 and its analogues act at the bottom, supplying the downstream mediator directly. They engage the same axis from opposite ends, and the feedback arm means an intervention at either end affects the other.

Why direct supply bypasses the regulation

Stimulating the pituitary produces IGF-1 through a loop that includes its own suppression — more IGF-1 means less growth hormone means less IGF-1. Supplying IGF-1 directly enters below that control point, so the feedback that would normally limit it is not engaged in the same way. That is a structural difference between acting at the top of an axis and acting at the bottom.

Quick reference

	Normal	Laron syndrome
Growth hormone	Pulsatile, regulated	High
GH receptor	Functional	Defective
IGF-1	Produced by liver	Very low
Feedback on GH	Present	Absent

Extended research context

The IGF-1 LR3 deep dive

Deep dive: the design that runs backwards from everything else here

Almost every engineered compound in this catalogue solves the same problem in the same direction. Semaglutide attaches a C18 diacid, retatrutide and tirzepatide a C20, all to bind serum albumin and extend duration. CJC-1295's DAC goes further and binds albumin covalently at Cys34. The strategy throughout is to add binding, so the molecule survives longer in circulation. IGF-1 LR3 does the reverse. Its two modifications - arginine replacing glutamate at position 3, and a thirteen-residue N-terminal extension - exist to REMOVE binding, specifically to the IGF binding proteins that sequester the overwhelming majority of circulating IGF-1 in a ternary complex with IGFBP-3 and the acid-labile subunit. Same bound-free equilibrium, engineered in the opposite direction. The reason is that the two families face opposite constraints: an incretin peptide is cleared too fast to be useful, while IGF-1 is held too tightly to be available. Baxter's 1994 review in Hormone Research is the standard account of just how tightly.

Deep dive: a certificate written for the wrong failure mode

This is a recombinant protein, and it fails differently from everything else on this site. A synthetic peptide accumulates deletion sequences, because each coupling is slightly less than complete - at 99% efficiency a 39-mer finishes around 68% full-length, and those impurities differ by one residue's mass and show up in a mass spectrum. Ribosomal synthesis does not fail that way at all; a host cell builds the protein correctly or not at all. What contaminates the product instead is everything else the cell contained: host cell proteins, host DNA, and in bacterial systems endotoxin, which is not a protein, does not appear in a protein purity figure, and needs its own assay entirely. Layered on top is a failure mode with no peptide equivalent. This protein has three disulfide bonds, meaning six cysteines that can pair fifteen ways of which one is correct - and Milner and colleagues showed in the Biochemical Journal in 1995 that B-domain mutations influence oxidative folding, which is directly relevant since position 3 sits in the B-domain. A misfolded isomer has identical mass, identical composition, and is invisible to mass spectrometry. A certificate reading exactly like a synthetic peptide's is testing for problems this production route does not have while missing the ones it does.

Deep dive: an approved relative, and why that raises the risk rather than lowering it

Unusually for this catalogue, a licensed IGF-1 medicine exists. Mecasermin, marketed as Increlex, is recombinant human IGF-1 with an FDA label and a subcutaneous route, used in severe primary IGF-1 deficiency - Petriczko and colleagues reported on it in 2019 and Denaite and colleagues in Frontiers in Pediatrics in 2024. That is a real approval with real clinical evidence. It covers the native seventy-residue hormone. LR3 is an eighty-three-residue engineered analogue whose defining feature is deliberately altered binding behaviour, holds no marketing authorisation anywhere, and has essentially no primary literature under its own name - searches for IGF-1 LR3 and Long R3 IGF-1 return nothing. A nearby approval invites the inference that the analogue is somehow covered by it, and that inference is the most consequential error available in this category. Separately and unconditionally: IGF-1 and its analogues sit in WADA class S2, prohibited at all times, with the list extending to related substances and mimetics precisely so that structural modification creates no exemption.

Research applications

▸Cell culture supplementation where secreted IGFBPs confound dosing
▸IGF binding protein interaction and affinity studies
▸Recombinant protein expression and purification methodology
▸Disulfide folding and oxidative refolding research
▸Comparative work on engineered versus native growth factors
▸Anti-doping analytical method development

Handling checklist

✓Expect no PubChem record - proteins are indexed in UniProt (native IGF-1 is P05019)
✓Read the certificate for host cell protein, host DNA and endotoxin, not deletion sequences
✓Understand that mass spectrometry cannot detect a misfolded disulfide isomer
✓Treat loss of fold as the principal failure mode - it leaves mass unchanged
✓Avoid shaking and foaming; interfaces unfold proteins and aggregation is irreversible
✓Use low-binding consumables; adsorption is significant at microgram quantities
✓Aliquot before freezing - freeze-thaw is worse for a folded protein than a short peptide

Common research-handling mistakes

Learnt from thousands of researcher orders across our UK labs.

✗ Reading mecasermin's approval as covering IGF-1 LR3

Fix: Increlex is native 70-residue rhIGF-1. LR3 is an 83-residue engineered analogue with altered binding, unlicensed everywhere. The approval does not transfer.

✗ Expecting a synthetic-peptide certificate for a recombinant protein

Fix: There are no deletion sequences here. The relevant tests are host cell protein, host DNA, endotoxin and something addressing conformation.

✗ Treating a clean mass spectrum as proof the protein is intact

Fix: A misfolded disulfide isomer has identical mass and composition. Only an activity or conformational assay addresses folding.

✗ Assuming absence from PubChem is a red flag

Fix: It is expected for a protein of this size - PubChem indexes small molecules, UniProt indexes proteins. This is unlike retatrutide, where absence is genuinely odd.

✗ Reading the cell-culture rationale as transferable

Fix: Bypassing IGFBP regulation is useful in a dish, where it is an experimental nuisance. In an organism the same system regulates a growth factor acting on nearly every tissue.

Continue researching

Peer-reviewed guides, comparators and matched reference materials.

Frequently asked questions

Does growth hormone act directly on tissues?: Partly, but many effects attributed to it are mediated by IGF-1, which the liver produces in response to it.
Why is growth hormone high in Laron syndrome?: IGF-1 is the feedback signal that suppresses it. Without IGF-1 production, that brake is absent.
How does this relate to GHRH analogues?: They act at the top of the loop by stimulating pituitary release; IGF-1 analogues act at the bottom by supplying the mediator directly.

Primary sources & clinical trials

Peer-reviewed research and registered trials from PubMed, ClinicalTrials.gov, PubChem, FDA and NIH. All links open in a new tab and point to the primary source, so every claim can be verified at origin.

Written and reviewed by

Jack Muncaster · Founder, UK Peptides

Jack founded UK Peptides in Manchester after repeatedly receiving research compounds with missing or recycled paperwork. He is responsible for supplier selection, batch release decisions and the content published in this research library. Every article here is sourced to primary literature and every product page to a signed third-party certificate.

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