TB-500 vs Thymosin Beta-4: The Ac-LKKTETQ Fragment and How Doping Labs Identified It

The name suggests a whole protein, but the product sold as TB-500 is, by the analysis of anti-doping chemists, built around a seven-residue fragment. Ho and colleagues describe the key ingredient as the peptide LKKTETQ with artificial acetylation of the N-terminus, a synthetic version of a short segment of thymosin beta-4 (Ho 2012). This Vial Post explainer separates the parent protein from the fragment, explains how laboratories identified and tracked it, and summarizes its FDA and WADA status.

Nothing below is guidance on use in people or animals. The Vial Post covers TB-500 strictly as a subject of laboratory and literature research.

Quick Facts

• Active ingredient: Ac-LKKTETQ, the N-terminal acetylated 17-23 fragment of human thymosin beta-4 (Esposito 2012).
• PubChem record: CID 62707662, synonyms TB-500 and Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln-OH; formula C38H68N10O14, molecular weight 889.0 g/mol, CAS 885340-08-9 (PubChem).
• Parent protein: thymosin beta-4, encoded by the human TMSB4X gene; UniProt P62328 (UniProt).
• Regulatory note: FDA lists "Thymosin beta-4, fragment (LKKTETQ), also known as TB-500" among bulk substances whose compounding nominations were withdrawn (FDA bulk substances page).
• Sport: WADA's 2026 list names "Thymosin-ß4 and its derivatives e.g. TB-500" under S2.3 growth factors and growth factor modulators (WADA 2026).

Parent Protein vs Fragment

It helps to put the two side by side.

Thymosin beta-4 (the protein), per UniProt: - The sequence entry is 44 residues long. The initiator methionine is removed, leaving the mature chain at positions 2 to 44, which begins with an N-acetylserine. - UniProt's function note says it plays an important role in organizing the cytoskeleton, and that it binds and sequesters actin monomers (G-actin), inhibiting actin polymerization. - Positions 2 to 5 of the same chain are annotated as a separate peptide, the hemoregulatory peptide AcSDKP.

TB-500 (the fragment): - Seven residues, LKKTETQ, sitting at positions 17 to 23 of the mature protein (Esposito 2012; Ho 2012), capped with an acetyl group on the N-terminus (Ho 2012). - Ho and colleagues describe segment 17-23 as the active site within thymosin beta-4 responsible for actin binding, cell migration and wound healing in the protein (Ho 2012). That is a statement about the parent protein's biology, not a demonstrated property of the vial.

A quick arithmetic check: summing the seven residues, adding one water for the free termini and adding an acetyl group gives C38H68N10O14, the same formula PubChem lists for TB-500 (PubChem).

How Doping Laboratories Pinned It Down

Most of the hard chemistry on TB-500 comes from anti-doping work:

• Identification (Ghent, 2012). The DoCoLab group detected and identified Ac-LKKTETQ in TB-500 by high-performance liquid chromatography with high-resolution mass spectrometry on an Orbitrap Exactive instrument. They also made Ac-LKKTETQ by solid-phase peptide synthesis and proposed a triple-quadrupole LC-MS approach for plasma and urine (Esposito 2012).
• Equine testing (Hong Kong, 2012). The Hong Kong Jockey Club racing laboratory identified likely metabolites from in vitro studies, then isolated the parent peptide and metabolites from horse urine and plasma with ion-exchange solid-phase extraction before LC/MS. N-acetylated LKKTETQ could be detected and confirmed at 0.02 ng/mL in plasma and 0.01 ng/mL in urine. The authors describe this as the first identification of TB-500 and its metabolites in post-administration samples from horses (Ho 2012).
• Multi-peptide screening (2013). A follow-up method extracted seven peptides from horse plasma with one mixed-mode anion-exchange cartridge, including GHRP-2, ipamorelin, CJC-1295 and N-acetylated LKKTETQ. Limits of detection were estimated below 50 pg/mL, and the TB-500 metabolite N-acetylated LK was detected alongside the parent (Kwok 2013).
• In vitro metabolism models (2015). Ghent researchers incubated TB-500 and other small peptide hormones with human liver microsomes, S9 fraction and serum. All showed endopeptidase and exopeptidase activity, and several metabolites were detected for each peptide (Esposito 2015).

FDA and WADA Status

• FDA. TB-500 appears in FDA's table of nominations withdrawn from category 2. FDA cites possible immunogenicity for certain routes of administration due to aggregation and peptide-related impurities, says it has not identified any human exposure data on drug products containing the fragment, and says it lacks important safety information, including whether it would cause harm if given to humans (FDA bulk substances page).
• WADA. The 2026 list, in effect from 1 January 2026, includes thymosin-ß4 and its derivatives, such as TB-500, among substances prohibited at all times (WADA 2026).

See our report on the interim compounding policy for the regulatory backdrop.

Questions Readers Ask

Is TB-500 the same as thymosin beta-4? No. Thymosin beta-4 is the full protein (UniProt P62328). TB-500's key ingredient is Ac-LKKTETQ, a seven-residue fragment of it (Ho 2012; Esposito 2012).

What is the molecular weight of TB-500? PubChem lists 889.0 g/mol for Ac-LKKTETQ (PubChem).

Can TB-500 be detected in testing? Yes. Published LC-MS methods detect N-acetylated LKKTETQ and its metabolites in equine plasma and urine (Ho 2012; Kwok 2013).

A Note for the Lab

Given the gap between the name and the molecule, the most useful single document is a Certificate of Analysis showing an observed mass consistent with Ac-LKKTETQ, not full-length thymosin beta-4. For laboratory work, Thymosin Beta 4 (TB500) research material from Dynamite Research Peptides is supplied with a batch Certificate of Analysis.

For research and educational use only. This article is not medical advice and does not describe dosing or human use.