IGF-1 LR3: The Molecular Weight Nobody Can Source — and the Identity Test It Blocks

Every vendor page for IGF-1 LR3 quotes the same specification: 83 amino acids, molecular weight around 9,111 daltons. A full-text search of Europe PMC for the quoted extension sequence returns zero hits, and the 1992 paper that created the analogue describes something different.

That matters more than a footnote, because if you cannot establish a molecular weight you cannot verify identity by mass spectrometry — and for this compound, mass spectrometry is the only test that would tell you what is in the vial. IGF-1 LR3 sits in the growth hormone peptides category, alongside an approved parent drug with a serious label, and its IGF-1 LR3 compound page carries the platform's lowest purity index of any well-tested compound.

Why can't anyone source IGF-1 LR3's molecular weight?

IGF-1 LR3's quoted specification — 83 amino acids, a 13-residue N-terminal extension reading MFPAMPLSSLFVN, molecular weight around 9,111 daltons — has no source we could find. A full-text Europe PMC search for the literal string MFPAMPLSSLFVN returns zero hits across the indexed biomedical literature. The founding paper, Francis, Ross, Ballard et al. (Journal of Molecular Endocrinology, 1992, PMID 1378742), describes the first 11 amino acids of methionyl porcine growth hormone — eleven, not thirteen, which gives 81 residues rather than 83.

Not one paper in the indexed literature contains that sequence. The founding group's own words are that their analogues comprise "an IGF-I sequence and the first 11 amino acids of methionyl porcine growth hormone."

We are not saying the market specification is wrong. We are saying we could not find a source for it, and that the primary paper appears to disagree with it. Those are different statements, and the difference matters: a wrong number can be corrected, while an unsourceable number cannot be checked at all.

What is IGF-1 LR3, and how is it made?

IGF-1 LR3 is a recombinant fusion protein rather than a synthetic peptide, built from native human IGF-1 — a 70-residue single chain with three intramolecular disulfide bridges and a molecular weight of 7,649 Da, per FDA's mecasermin label — plus an N-terminal extension and a substitution at position 3, where native mature IGF-1 carries glutamate. Francis and colleagues expressed it in E. coli in 1992; a 2023 paper describes expression in Pichia pastoris.

The 7,649 Da figure is not our arithmetic. It is FDA's, from the approved label for mecasermin, and the mature sequence it refers to comes from UniProt P05019:

GPETLCGAELVDALQFVCGDRGFYFNKPTGYGSSSRRAPQTGIVDECCFRSCDLRRLEMYCAPLKPAKSA

That the product is recombinant has a practical consequence. A 2023 paper describes expressing both IGF-1 and LR3 IGF-1 in Pichia pastoris, and modern production still uses recombinant systems, because an 80-odd-residue disulfide-bonded protein sits at or beyond the edge of solid-phase peptide synthesis. Folding three correct disulfide bridges is a manufacturing problem, not a coupling problem.

The design rationale is sound and well established. Native IGF-1 circulating in blood is almost entirely bound to IGF binding proteins, principally IGFBP-3, which control its availability and clearance. The LR3 modifications are intended to reduce IGFBP binding so that more of the molecule stays free and active. That IGFBP-3 governs clearance is not speculation: FDA's mecasermin label states clearance of "0.04 L/hr/kg at 0.5 micrograms/mL of IGFBP-3, and 0.01 L/hr/kg at 3 micrograms/mL IGFBP-3" — a four-fold difference driven by binding protein alone.

What we could not verify: the exact residue count, the exact extension sequence, any molecular weight for LR3, and the widely quoted "20+ hour half-life." That last figure appears on our own compound page, and we could not source it independently. We are flagging it rather than repeating it.

What does a missing reference mass break?

A missing reference mass breaks the only test that identifies IGF-1 LR3. Mass spectrometry confirms identity by comparing a measured mass against an authoritative expected one, and for LR3 no such figure exists that we could verify — the usual comparator is the same unsourced 9,111 Da circulating on vendor pages. Three failures follow: a truncated or misfolded product can pass, native IGF-1 at 7,649 Da can substitute undetected, and HPLC purity measures homogeneity only.

That is a different and worse position than a compound whose mass is known but hard to measure. Here, a certificate of analysis reporting a mass has nothing authoritative to be checked against.

A truncated or misfolded product may pass inspection. An 80-residue chain has roughly 80 coupling or expression steps and three disulfide bonds that must form in the correct pairing. Incorrectly folded IGF-1 has the same mass as correctly folded IGF-1 and is biologically inactive. Mass spectrometry cannot see disulfide pairing.

Native IGF-1 substituted for LR3 is a real possibility. Native IGF-1 at 7,649 Da is a substantial mass difference — easily detected if you know the LR3 target. Without a verified target, an MS result showing "approximately 9,000 Da" is not an identity confirmation.

HPLC purity tells you almost nothing here. It measures homogeneity, and a homogeneous sample of a misfolded protein returns a clean chromatogram. For the underlying methods, see mass spectrometry for peptides, what HPLC testing can and cannot tell you and red flags in peptide certificates of analysis.

What does the approved IGF-1 product's label actually say?

Mecasermin (INCRELEX) is FDA-approved recombinant human IGF-1 — BLA 021839, approved 30 August 2005, sponsored by Ipsen, with a terminal half-life of 5.8 hours after a single 0.12 mg/kg subcutaneous dose. Its label reports hypoglycaemia in 30 subjects (42%), five with severe episodes requiring assistance and four with hypoglycaemic seizures or loss of consciousness. It does not carry a boxed warning; hypoglycaemia is Warnings and Precautions section 5.1.

A correction to a claim we have seen widely repeated, including in our own earlier working notes: Increlex does not carry a boxed warning. We checked three independent label sources — the Ipsen label revised July 2025, the Eton Pharmaceuticals label revised October 2025, and the FDA-approved label PDF revised January 2019. Hypoglycaemia is Warnings and Precautions section 5.1, not a boxed warning.

That correction cuts both ways, because the actual numbers in section 5.1 are not reassuring:

"Hypoglycemia was reported by 30 subjects (42%) at least once during their course of therapy... Five subjects had severe hypoglycemia (requiring assistance and treatment) on one or more occasions and 4 subjects experienced hypoglycemic seizures/loss of consciousness."

Forty-two per cent hypoglycaemia, four seizures or losses of consciousness — under paediatric endocrinology supervision, with glucose monitoring, in patients selected for the indication.

Other label warnings: intracranial hypertension, lymphoid tissue hypertrophy (tonsillar hypertrophy in 15% of subjects in the first one to two years), slipped capital femoral epiphysis, progression of pre-existing scoliosis, hypersensitivity and anaphylaxis, and malignant neoplasia. Malignant neoplasia is a contraindication, not merely a warning. Neither IGF-1 LR3 nor IGF-1 DES is approved anywhere, and ClinicalTrials.gov returns zero registered studies for IGF-1 LR3.

Does the LR3 design make hypoglycaemia more likely?

Nobody knows, because no human pharmacokinetic or safety study of IGF-1 LR3 has ever been published. The concern is mechanistic: IGF-1 has substantial affinity for the insulin receptor, IGFBP-3 binding limits how much stays free — mecasermin's clearance runs 0.04 L/hr/kg at 0.5 micrograms/mL of IGFBP-3 versus 0.01 at 3 micrograms/mL — and LR3 is engineered specifically to evade that binding. We searched for published case reports of LR3 harm and found none.

We want to be equally clear that finding none is not the same as establishing that none exists. Case reports are published when a clinician recognises a compound, connects it to a presentation, and writes it up; an unapproved analogue self-administered outside medical supervision is close to invisible to that process.

More free IGF-1, for longer, is the entire point of the modification. What can be said is this: an approved IGF-1 product with normal binding-protein interaction produced hypoglycaemia in 42% of a monitored paediatric population, and LR3 is engineered to increase the free fraction. Whether that translates into greater or more prolonged hypoglycaemia risk in humans has never been studied.

Does IGF-1 cause cancer, and what does that literature actually measure?

The epidemiology measures endogenous circulating IGF-1 as a biomarker in observational cohorts; it cannot quantify risk from injecting an analogue. The anchor citation is Renehan, Zwahlen, Minder et al., The Lancet 2004;363:1346–1353, PMID 15110491. We are not quoting effect sizes from it, because we could not retrieve the abstract to verify them. What is directly relevant: FDA made malignant neoplasia a contraindication on the approved IGF-1 product.

Anyone citing specific odds ratios from that paper should be asked for the source.

The framing matters more than the numbers in any case. People with naturally higher circulating IGF-1 differ from people injecting an IGFBP-evading analogue in every relevant way — age, body composition, growth hormone status, insulin sensitivity, and the shape of their IGF-1 exposure over a day. A biomarker association in a cohort is not a dose-response measurement of an injected drug, and it cannot be converted into one.

What is a regulator's judgement on the class, applied to a supervised paediatric indication, is the contraindication on the approved product.

Which IGF-1 LR3 claims survive the evidence?

Three of ten survive. IGF-1 LR3 is a genuine IGF-1 analogue designed for reduced IGFBP binding, IGF-1 itself is approved as mecasermin, and the compound is prohibited in sport under WADA S2.3 — those hold. The 83-amino-acid, 9,111 Da specification is unsourceable, the 20-plus-hour half-life is unverified, human testing has never happened, and two widely repeated claims — that Increlex carries a boxed warning and that hypoglycaemia is minor — are false.

Note what the middle rows have in common. They are not claims we disproved; they are claims with nothing behind them, which is the harder position for a buyer, because there is no correction to apply.

Where do regulators and anti-doping bodies place IGF-1 LR3?

IGF-1, IGF-1 LR3 and IGF-1 DES appear on none of FDA's compounding lists. We enumerated both tables on the page current 22 April 2026 in full — 14 substances in active Category 2, 17 in the nominated-but-withdrawn table — and no member of the IGF-1 family is on either. That absence is not clearance. WADA is explicit: section S2.3 prohibits insulin-like growth factor 1 (IGF-1, mecasermin) and its analogues at all times.

We enumerated both tables on the page current 22 April 2026 because absence from a list is only meaningful if you have read the list. Because mecasermin is an approved product, plain IGF-1's status under the component-of-an-approved-drug prong is arguable; LR3 and DES are distinct molecules and are not components of any approved drug, so they satisfy none of the three statutory routes for 503A compounding.

WADA names this class explicitly. Section S2.3, "Growth factors and growth factor modulators," prohibits, in its own words: "Insulin-like growth factor 1 (IGF-1, mecasermin) and its analogues" and "Mechano growth factors (MGFs)," alongside FGFs, HGF, PDGF, thymosin-β4 and VEGF. Prohibited at all times. "And its analogues" captures LR3 and DES without ambiguity — this is not an S0 catch-all argument, it is a named class.

LR3 and DES are also established doping-control analytes. A 2022 method paper on probing for peptidic drugs in blood samples names "long-R3-IGF-I, R3-IGF-I and Des 1–3-IGF-I" specifically. Testing laboratories are looking for these compounds by name, which is a materially different situation from being covered by a category. See do peptides show up on drug tests.

What do 125 lab tests and 42 shops show?

The Purity Index records IGF-1 LR3 at 98.51% average across 125 tests (verified August 2026), from ILS Laboratories, Kovera, Bioviridian and Liquilabs, across 220 verified shops — the lowest purity index of any well-tested compound on this platform. Individual results run 94.29% to 99.90%. Quantity variance runs +1% to +72%, with one vendor's June 2026 sample testing 72% over label on a compound whose signature adverse effect is hypoglycaemia.

A 72% overage is not generosity. It is direct evidence that fill quantity is uncontrolled, and on a hypoglycaemic agent a 72% dosing error has an obvious mechanism for harm.

IGF-1 LR3 price data shows 42 shops in stock, median $75.00/mg, lowest $30.00/mg on a 1 mg vial (verified August 2026). Those figures exclude shipping, taxes and customs, coupon codes, bulk tiers, multi-vial kits and account-gated pricing.

Note the price scale: $75 per milligram against $3–5 for typical synthetic peptides. That is what a recombinant, disulfide-folded protein costs to make properly — and it is also a strong incentive to cut corners. Those figures sit inside a platform tracking 530 shops and 11,852 independent lab tests across 118 peptides, with 1,283 community reviews (verified August 2026).

Compare vendors on the IGF-1 LR3 compound page, and read where to buy IGF-1 LR3 with purity and identity checks before ordering.

How does IGF-1 LR3 compare with mecasermin, IGF-1 DES and PEG-MGF?

IGF-1 LR3 sits in a four-compound group with one approved drug at its centre and three unapproved analogues around it, all named in the same WADA section. Mecasermin has an approval and a paediatric trial record; IGF-1 LR3, IGF-1 DES and PEG-MGF have zero registered human studies between them, and FDA found no human exposure data for PEG-MGF by any route. All four fall under WADA S2.3.

The growth-factor group has an unusual profile: a real approved drug at the centre with a serious safety label, surrounded by unapproved analogues with no human data, all of them explicitly named in the same WADA section. The approved product is the only member with numbers attached to it, which is why this article keeps returning to its label.

The trial that would settle this

The trial that would settle IGF-1 LR3 cannot start until the molecule is characterised. Publishing the actual sequence and mass of the commercial product is the first requirement, because nobody can currently verify what LR3 is. After that: a Phase 1 with human pharmacokinetics and glucose monitoring, direct measurement of the free IGF-1 fraction, mecasermin as comparator at equivalent free-IGF-1 exposure, and confirmed disulfide pairing on every batch.

The first row blocks the rest. You cannot run a trial of a molecule whose specification is not established, and no sponsor has published one.

Frequently Asked Questions

What is IGF-1 LR3?

A recombinant analogue of human IGF-1, designed to evade IGF binding proteins so that more of the molecule stays free and active. It was developed by Francis and colleagues in 1992 as an E. coli fusion protein. It is not a synthetic peptide.

Why can't you give its molecular weight?

Because we could not source one. A full-text search of Europe PMC for the extension sequence quoted on vendor pages returns zero results, and the founding 1992 paper describes 11 amino acids of methionyl porcine growth hormone rather than the 13 that circulate in market specifications. Without a verified reference mass, mass spectrometry cannot confirm identity.

Is IGF-1 approved?

Recombinant human IGF-1 is, as mecasermin (Increlex, BLA 021839, approved 2005), for growth failure in children with severe primary IGF-1 deficiency. IGF-1 LR3 and IGF-1 DES are different molecules and are approved nowhere.

Does Increlex have a boxed warning?

No. We checked three label versions. Hypoglycaemia is Warnings and Precautions section 5.1. The numbers there are still significant: 42% of subjects experienced hypoglycaemia, five had severe episodes requiring assistance, and four had seizures or loss of consciousness.

Does IGF-1 cause cancer?

The epidemiological literature associates higher endogenous circulating IGF-1 with cancer risk in observational cohorts. That is a biomarker association, not a measurement of what injecting an analogue does. What is directly relevant is that FDA made malignant neoplasia a contraindication on the approved IGF-1 product.

Is IGF-1 LR3 banned in sport?

Yes, explicitly. WADA section S2.3 prohibits "insulin-like growth factor 1 (IGF-1, mecasermin) and its analogues" at all times. LR3 and DES are also named analytes in published doping-control detection methods.

Where IGF-1 LR3 sits

IGF-1 LR3 is the only compound on this platform whose molecular weight we could not establish, and that is the finding rather than a footnote. The design is legitimate — Francis and colleagues built a real analogue for a documented reason — but the market specification traces to nothing we could find, no human study has ever been registered, purity runs down to 94.29%, and one fill tested 72% over label.

The IGFBP-evasion rationale is supported by the approved product's own clearance data. There is nothing fraudulent about the concept, and that is worth saying plainly before the rest.

But the half-life figure on our own page is unsourced, the founding paper appears to disagree with the specification everyone quotes, and all of it sits on a molecule whose parent drug produces hypoglycaemia in 42% of monitored paediatric patients — sold at $75 a milligram to people with no glucose monitoring at all.

Browse the growth hormone peptides category, or our complete peptide guide with 118 compounds (verified August 2026). For reconstitution and per-injection volume, use the peptide dosing calculator alongside the reconstitution calculator. Compare shops through independent lab tests and community-verified shop reviews.

This article is for educational and informational purposes only and does not constitute medical advice. Peptides discussed may be investigational compounds not approved by the FDA (or equivalent regulators in your jurisdiction) for human use. Always consult a qualified healthcare provider before using any peptide or research compound. Peptigrity is an independent review platform and does not sell, endorse, or recommend specific products or vendors.