TB-500 is not thymosin beta-4

Two molecules, one name

Thymosin beta-4 Medium is one of the most abundant peptides in mammalian cells. It is 43 amino acids long and its main job is to hold single actin molecules in reserve, the raw material cells use to move, divide and close wounds; the foundational biochemistry showed it binds actin monomers directly ¹. Here is its full sequence, with the seven residues at positions 17 to 23 marked:

SDKPDMAEIEKFDKSKLKKTETQEKNPLPSKETIEQEKQAGES

TB-500 Low is the name on vials sold online. When the Ghent doping-control laboratory analysed a product sold as TB-500, the peptide it identified was the acetylated fragment Ac-LKKTETQ, those seven residues and nothing else ². The Hong Kong Jockey Club's laboratory, writing about the same product for horse racing, described TB-500 as a synthetic version of an active region of thymosin beta-4 ³. The fragment's logic is that LKKTETQ is the actin-binding stretch, so it might keep the function at a fraction of the size and cost. Whether it does in a living body is the open question.

Whose evidence is it?

The human studies belong to the full protein. A randomised, placebo-controlled study gave healthy volunteers single and then 14 daily intravenous doses of synthetic thymosin beta-4 at 42, 140, 420 or 1,260 mg, with infrequent, mild adverse events ⁴. A double-blind, placebo-controlled dose-escalation study in Italy and Poland tested it topically on venous leg ulcers ⁵. The furthest it went was the eye: 0.1% thymosin beta-4 drops, known as RGN-259, reduced discomfort by 35.1% and corneal staining by 59.1% at day 56 in a nine-patient phase 2 study of severe dry eye ⁶, and in an 18-patient phase 3 trial in neurotrophic keratopathy, 6 of 10 treated eyes and 1 of 8 on placebo had complete healing at four weeks ⁷. A review by the same investigator traces that path from bench to bedside ⁸.

The fragment's literature is different in kind. The papers that name TB-500 are mostly about detecting it: methods for finding Ac-LKKTETQ and its breakdown products in urine and plasma, tested in horses and rats ^{2 3 9}. The rat work also screened the fragment and its metabolites for wound-healing activity in cell culture ⁹. We found no published human trial of the fragment.

What the confusion does to the claims

When a TB-500 product page cites "clinical trials", the trials are almost always thymosin beta-4 trials, of a molecule six times longer, given intravenously, topically or as eye drops. Three things do not carry over from one to the other. The dose: milligrams of a 43-residue protein are not milligrams of a 7-residue fragment, and no human dose of the fragment has been studied. The behaviour in the body: a short acetylated peptide is broken down and cleared differently, and in the rat study its main metabolite appeared within the first six hours while a longer one stayed detectable for up to 72 hours ⁹. And the safety record: the intravenous safety data and the eye-drop trials say nothing about a fragment injected under the skin for weeks.

The same caution applies to the cancer question. Reviews of thymosin beta-4 in heart repair note its involvement in tumour biology alongside its repair effects ¹⁰; whether the fragment shares that involvement has not been studied either way.

What is actually in the vial

The name does not reliably fix the contents. The question our Name Decoder tells buyers to ask is whether a TB-500 vial holds the short fragment or the full protein, and the two differ hugely in mass, about 889 daltons against about 4,963 ¹¹. A certificate of analysis should state which one and give a mass that matches; the Name Decoder lists the question, and the COA Decoder explains how to check the answer.

For sport the distinction does not matter: WADA's 2026 Prohibited List names "thymosin-β4 and its derivatives e.g. TB-500" under S2.3, as non-Specified substances banned at all times ¹². For everyone else it decides which evidence, if any, applies to what they are injecting.

Frequently asked questions

Is TB-500 the same as thymosin beta-4?

No. Thymosin beta-4 is a 43-amino-acid protein; the TB-500 identified in a product analysed by a doping-control laboratory was Ac-LKKTETQ, an acetylated seven-amino-acid fragment from positions 17 to 23. A label alone does not say which molecule a vial holds.

Has TB-500 been tested in humans?

Not the fragment, as far as published studies show. The human studies people cite, intravenous safety, venous ulcers and eye drops, all used full-length thymosin beta-4.

What is TB4 frag?

A seller name for a short piece of thymosin beta-4, usually the actin-binding stretch LKKTETQ. The evidence for full-length thymosin beta-4 does not transfer to it automatically.

Is TB-500 banned in sport?

Yes. WADA's 2026 List names thymosin beta-4 and its derivatives, including TB-500, under S2.3, prohibited at all times.

Related on Grey Peptides

• TB-500
• Thymosin beta-4
• Do peptides show up on drug tests?

Sources

1. Yu, F. X., et al. (1993). Thymosin beta 10 and thymosin beta 4 are both actin monomer sequestering proteins. J Biol Chem, 268(1), 502-9. PMID: 8416954
2. Esposito, S., et al. (2012). Synthesis and characterization of the N-terminal acetylated 17-23 fragment of thymosin beta 4 identified in TB-500, a product suspected to possess doping potential. Drug Test Anal, 4(9), 733-8. PMID: 22962027 · doi:10.1002/dta.1402
3. Ho, E. N., et al. (2012). Doping control analysis of TB-500, a synthetic version of an active region of thymosin β₄, in equine urine and plasma by liquid chromatography-mass spectrometry. J Chromatogr A, 1265, 57-69. PMID: 23084823 · doi:10.1016/j.chroma.2012.09.043
4. Ruff, D., et al. (2010). A randomized, placebo-controlled, single and multiple dose study of intravenous thymosin beta4 in healthy volunteers. Ann N Y Acad Sci, 1194, 223-9. PMID: 20536472
5. Guarnera, G., et al. (2007). Thymosin beta-4 and venous ulcers: clinical remarks on a European prospective, randomized study on safety, tolerability, and enhancement on healing. Ann N Y Acad Sci, 1112, 407-12. PMID: 17495250
6. Sosne, G., et al. (2015). Thymosin β4 significantly improves signs and symptoms of severe dry eye in a phase 2 randomized trial. Cornea, 34(5), 491-6. PMID: 25826322
7. Sosne, G., et al. (2022). 0.1% RGN-259 (Thymosin ß4) Ophthalmic Solution Promotes Healing and Improves Comfort in Neurotrophic Keratopathy Patients in a Randomized, Placebo-Controlled, Double-Masked Phase III Clinical Trial. Int J Mol Sci, 24(1). PMID: 36613994
8. Sosne, G. (2018). Thymosin beta 4 and the eye: the journey from bench to bedside. Expert Opin Biol Ther, 18(sup1), 99-104. PMID: 30063853
9. Rahaman, K. A., et al. (2024). Simultaneous quantification of TB-500 and its metabolites in in-vitro experiments and rats by UHPLC-Q-Exactive orbitrap MS/MS and their screening by wound healing activities in-vitro. J Chromatogr B Analyt Technol Biomed Life Sci, 1235, 124033. PMID: 38382158 · doi:10.1016/j.jchromb.2024.124033
10. Bjørklund, G., et al. (2020). Thymosin β4: A Multi-Faceted Tissue Repair Stimulating Protein in Heart Injury. Curr Med Chem, 27(37), 6294-6305. PMID: 31333080
11. Grey Peptides dataset: tb-500 (LKKTETQ, molecular weight 889.0) and thymosin-beta-4 (43 residues, molecular weight 4,963.5) records; read October 2, 2026.
12. World Anti-Doping Agency. Prohibited List 2026, section S2.3; read October 2, 2026. WADA

Educational information, not medical advice. Each dose in this guide names its source, an approved label or a published study. None is a recommendation for you. An unapproved compound has no established safe or effective human dose, and products sold for “research use only” are not made or tested for people. Talk to a doctor before acting on anything on this site, including before you start, stop or change any medicine or dose.