What mass should a retatrutide COA show?

KEGG DRUG lists retatrutide as C221H342N46O68 with an average molecular weight of 4731.34 and an exact monoisotopic mass of 4728.47. A certificate should print the expected value, the measured value and which kind of mass each is. If the measured mass sits near 4813 or 4114, the vial more likely holds tirzepatide or semaglutide than retatrutide, whatever the label says.

How can I tell if my retatrutide is actually tirzepatide?

Only a mass measurement can tell. Both are white powders, both are 39 residues long, and both can give a clean, high HPLC purity. Their average masses differ by about 82 units, 4731.34 for retatrutide against 4813.53 for tirzepatide, which LC-MS separates easily. Ask for identity by mass spectrometry on a sample from your batch before anything else.

Is retatrutide an approved drug?

Not in the US. The FDA states that retatrutide is not a component of any FDA-approved drug and has not been found safe and effective for any condition, and its published research describes it as an investigational triple agonist. There is no originator product to compare a vial against, so identity, purity and amount testing on the vial itself is the only way to know what it contains.

Why doesn't PubChem show a mass for retatrutide?

At the time of review, a PubChem name search for retatrutide or LY3437943 returns no compound record. The structure is registered elsewhere: KEGG DRUG D12430 gives the formula and masses, and the FDA's Global Substance Registration System lists the full systematic name, including the three non-coded residues and the side chain. A certificate should cite which reference it compared against.

Does a high purity figure mean my retatrutide is good?

Not on its own. Purity says how much of the sample is the main molecule; it does not say which molecule that is. A tidy 99% peak could be tirzepatide in a retatrutide vial. Read the identity result first, then purity, then the measured amount against the label. The HPLC purity lesson covers what the percentage does and does not mean.

One of the 30 compound testing profiles

Retatrutide

Retatrutide is a 39-amino-acid peptide (C221H342N46O68, average mass 4731.34 g/mol) with a C20 fatty diacid on lysine 17; a test should confirm that mass, because the central risk is a better known relative such as tirzepatide or semaglutide sold under the retatrutide name.

Retatrutide (LY3437943) is an investigational triple agonist peptide at the glucagon, GIP and GLP-1 receptors (Coskun et al., Cell Metabolism, 2022), and the US FDA states it is not a component of any FDA-approved drug (FDA, unapproved GLP-1 drugs).

Test your retatrutide

Identity card

Formula: C221H342N46O68
Average mass: 4,731.34 g/mol
Monoisotopic mass: 4,728.47 Da
Length: 39 residues
Modifications: Aib (2-methylalanine) at positions 2 and 20, shown as X in the sequence; 2-methyl-L-leucine (alpha-methyl leucine) at position 13, written as L in the KEGG one-letter sequence; Lys17 carries a C20 fatty diacid (19-carboxynonadecanoyl) attached through a gamma-glutamic acid and one 2-[2-(2-aminoethoxy)ethoxy]acetyl linker; C-terminal amide on Ser39
Also sold as: Reta, LY3437943
Sequence: YXQGTFTSDYSILLDKKAQXAFIEYLLEGGPSSGAPPPS
Note: PubChem returns no compound record for the names retatrutide or LY3437943, so the formula, average mass (4731.34) and exact monoisotopic mass (4728.4718, recorded as 4728.47) are taken from KEGG DRUG D12430, which also gives the sequence. The modified residues and the side-chain linker are from the systematic name in the FDA Global Substance Registration System (UNII NOP2Y096GV).

Source: KEGG DRUG D12430, Retatrutide (USAN), Global Substance Registration System, Retatrutide (UNII NOP2Y096GV).

Compare the mass printed on a certificate with the values above. The mass checker explains charge states and which mass a lab usually reports.

What usually goes wrong with retatrutide. Documented failure modes, with sources.

Tirzepatide or semaglutide in a retatrutide vial

Retatrutide is not a component of any FDA-approved drug, and the FDA lists it among the drugs it has warned companies for selling with false "for research purposes" labels (FDA, unapproved GLP-1 drugs). With no approved product to compare against, substitution with a better known relative is the central risk. The masses are far apart: retatrutide 4731.34 (KEGG D12430), tirzepatide 4813.53 (Mounjaro prescribing information) and semaglutide 4113.58 (KEGG D10025), so any working mass spectrometer separates them.

Three non-coded residues that are easy to get wrong

The registered structure has Aib at positions 2 and 20 and 2-methyl-L-leucine at 13 (GSRS, retatrutide). KEGG's one-letter sequence writes the methylated residue as a plain L (KEGG D12430), so a reference mass built from that string alone comes out wrong. Each position needs its own non-standard building block, and a standard residue in its place gives a different mass. A certificate should state the reference mass it used and where it came from.

Synthesis by-products along a 39-residue chain

Solid-phase synthesis leaves deletion sequences from incomplete Fmoc removal, insertion sequences from excess reagent, racemised residues and protecting-group adducts (D'Hondt et al., 2014). In follow-on and compounded semaglutide and liraglutide, LC-MS found amino acid deletions and additions absent from the originators, plus unidentified impurities (Kopp et al., Pharmaceutical Research, 2026). Retatrutide is a lipidated incretin-class chain of the same size, so these are the impurity classes to look for: small peaks beside the main HPLC peak, named by their masses.

Glutamine deamidation and the lipid side chain

Retatrutide has glutamine at positions 3 and 19 and no methionine, cysteine, tryptophan or asparagine (KEGG D12430), which removes the main oxidation sites: methionine first, then tryptophan (Butré et al., Molecules, 2023). Deamidation, slower at glutamine than at asparagine, shifts the mass by only +0.984 Da, which a low-resolution intact mass check can miss. A missing or incomplete side chain on Lys17 changes the mass substantially, so mass spectrometry catches it readily.

What we check in Retatrutide. Silent, with captions.

Which tests matter most

LC-MSAnswers the main question: is this retatrutide at 4731.34, or tirzepatide or semaglutide at a very different mass?
HRMSAccurate mass near the 4728.47 monoisotopic value, and the isotope pattern, separate deamidated and side-chain variants from the intact molecule.
HPLC-UVGives the purity figure and shows deletion or insertion impurities as extra peaks, once identity has been confirmed.
Peptide mappingLocates a modification or a missing residue when the intact mass is close to but not equal to the reference.

Reading retatrutide results

Read the mass line first. A retatrutide certificate should name the expected mass (4731.34 average or 4728.47 monoisotopic, from KEGG D12430) and the measured mass. A value near 4813 suggests tirzepatide and one near 4114 suggests semaglutide. The identity lesson explains how the deconvoluted mass is read, and the mass checker computes expected masses from a sequence.

An amount test reports milligrams of retatrutide against a reference standard. For an investigational peptide with no pharmacopoeial or approved reference product, ask which standard the laboratory used and how that standard was itself identified, ideally by mass and amino acid analysis; the amount result is only as good as that standard.

Public Tested.ph results

No public retatrutide records yet

Each public retatrutide record will appear here with its identity, HPLC purity and measured amount against the label, once the first vial is tested.

FIG 13Amount against the label

Questions about testing retatrutide

What mass should a retatrutide COA show?: KEGG DRUG lists retatrutide as C221H342N46O68 with an average molecular weight of 4731.34 and an exact monoisotopic mass of 4728.47. A certificate should print the expected value, the measured value and which kind of mass each is. If the measured mass sits near 4813 or 4114, the vial more likely holds tirzepatide or semaglutide than retatrutide, whatever the label says.
How can I tell if my retatrutide is actually tirzepatide?: Only a mass measurement can tell. Both are white powders, both are 39 residues long, and both can give a clean, high HPLC purity. Their average masses differ by about 82 units, 4731.34 for retatrutide against 4813.53 for tirzepatide, which LC-MS separates easily. Ask for identity by mass spectrometry on a sample from your batch before anything else.
Is retatrutide an approved drug?: Not in the US. The FDA states that retatrutide is not a component of any FDA-approved drug and has not been found safe and effective for any condition, and its published research describes it as an investigational triple agonist. There is no originator product to compare a vial against, so identity, purity and amount testing on the vial itself is the only way to know what it contains.
Why doesn't PubChem show a mass for retatrutide?: At the time of review, a PubChem name search for retatrutide or LY3437943 returns no compound record. The structure is registered elsewhere: KEGG DRUG D12430 gives the formula and masses, and the FDA's Global Substance Registration System lists the full systematic name, including the three non-coded residues and the side chain. A certificate should cite which reference it compared against.
Does a high purity figure mean my retatrutide is good?: Not on its own. Purity says how much of the sample is the main molecule; it does not say which molecule that is. A tidy 99% peak could be tirzepatide in a retatrutide vial. Read the identity result first, then purity, then the measured amount against the label. The HPLC purity lesson covers what the percentage does and does not mean.

Sources

Coskun T et al. LY3437943, a novel triple glucagon, GIP, and GLP-1 receptor agonist. Cell Metab. 2022;34(9):1234-1247, PubMed
FDA's Concerns with Unapproved GLP-1 Drugs Used for Weight Loss, US Food and Drug Administration
KEGG DRUG D12430, Retatrutide, Kanehisa Laboratories
KEGG DRUG D10025, Semaglutide, Kanehisa Laboratories
D'Hondt M et al. Related impurities in peptide medicines. J Pharm Biomed Anal. 2014;101:2-30, PubMed
Kopp KL et al. Impurities and Potential Immunogenicity Associated With Follow-on and Compounded GLP-1 Receptor Agonists. Pharm Res. 2026;43(8):2861-2886, Pharmaceutical Research (PMC)
Mounjaro (tirzepatide) prescribing information, Eli Lilly and Company
Butré CI et al. Interlaboratory Evaluation of a User-Friendly Benchtop Mass Spectrometer. Molecules. 2023;28(6):2855, Molecules (PMC)

Last reviewed 6 Oct 2026.