Read This First
This page is not built on clinical evidence. It is built on the regulatory record, the trial registry and peptide chemistry. What thymosin beta-4 does in people is covered on the library entry; this page is about what is actually in a vial labelled with its name, which is usually something else.
Thymosin beta-4 is a 43-amino-acid protein that exists as a characterised drug only inside clinical trials run by its developers. Nobody sells that product. What is sold as "thymosin beta-4," "TB4" or "Tβ4" by peptide vendors is, in almost every case, TB-500, a seven-residue synthetic fragment, and the two are graded, regulated and tested as different things.1
Legal Status
Where Does Thymosin Beta-4 Stand?
Investigational everywhere, approved nowhere. It has been in registered trials since 2006, including three Phase 3 trials for dry eye that missed their endpoints and a Phase 3 programme for neurotrophic keratopathy that is still running.23 Investigational status grants no access outside a trial.
In the United States it is not a component of an approved drug, has no monograph and is not on the 503A bulks list. It has never been nominated for compounding: the FDA's Category 2 list names "thymosin beta-4, fragment (LKKTETQ), also known as TB-500," not the protein, among substances nominated and withdrawn, and the July 2026 advisory vote concerned TB-500 for wound healing.14 No pharmacy has a lawful basis on which to compound either.
Is It Prohibited in Sport?
Yes. Thymosin beta-4 and its derivatives are named under WADA section S2 as prohibited growth factors, and the substance was at the centre of the 2016 Court of Arbitration for Sport decision banning 34 Essendon players.56 That case is worth a sourcing page's attention for one reason: the players were found to have used a substance whose origin, identity and even administration none of them could establish. The tribunal reasoned from circumstantial evidence about what had been ordered and injected, because no one involved had verified what was in the vials.
Product Identity
What Is Being Sold?
| Label says | What it almost always is | How to tell |
|---|---|---|
| TB-500, Thymosin beta-4 fragment, TB4 fragment | Ac-LKKTETQ, the seven-residue synthetic | Mass around 890 daltons on a certificate |
| Thymosin beta-4, TB4, Tβ4, "full length" | Usually still TB-500; occasionally a longer synthetic of uncertain fidelity | The full protein has a mass around 4,963 daltons; a certificate showing anything near 890 is the fragment |
| RGN-259, RGN-137, NL005 | Developer product codes; not for sale anywhere | A vendor using these codes is borrowing a name |
The distinction is not academic. The 43-residue protein is long for routine solid-phase synthesis, and each of 42 coupling steps is an opportunity for a deletion. A vendor genuinely supplying full-length thymosin beta-4 would need to demonstrate it by mass and, ideally, by sequence; a vendor supplying the fragment and calling it thymosin beta-4 is describing a molecule with a research record it does not have.
Why Does the Molecule Matter to the Evidence?
Because every human trial used the protein. The intravenous safety study, the wound gels, the eye drops and the Chinese cardiac programme all administered 43-residue thymosin beta-4.78 The fragment has one trial, recruiting since February 2026, with no results.9 A product page for a vial of TB-500 that cites the ARISE or SEER trials is citing a different molecule, a different route and a different organ.
There is also the metabolite question. A 2024 study found that TB-500's wound-healing activity in cell culture may belong to a breakdown product rather than the fragment itself, which if true means even the fragment is a precursor to the active agent.10 The ONPEPS TB-500 sourcing page covers the fragment market in detail, including salt forms and certificate reading; this page does not repeat it.
Purity Testing Versus Identity Testing
| Test | Question it answers | Question it cannot answer |
|---|---|---|
| HPLC purity | What proportion of the sample is one main peak | Whether that peak is the 43-residue protein, the 7-residue fragment or a truncated intermediate |
| Mass spectrometry identity | Whether the mass matches; this is the test that separates protein from fragment | How much of the sample it represents; whether deletion sequences co-elute |
| Endotoxin assay | Whether bacterial cell-wall fragments are present | Identity or content |
| Sequence confirmation | The order of residues | Almost never offered for a 43-residue product |
For this compound the single most useful number on a certificate is the observed mass, because it answers the question the label is most likely to be wrong about.
Certificates of Analysis
What a Certificate Is
A laboratory report on one batch. Read it for the laboratory's independence from the seller, the lot number and whether it matches the vial, the date, the substance named, the observed mass, and which tests were actually run rather than mentioned.
What It Cannot Establish
That the vial in hand came from the batch tested. Its value rests on a chain of custody the buyer cannot inspect. A certificate is a PDF, and nothing about the format prevents reuse or fabrication; the FDA has documented fabricated labels on gray-market peptide products.11
"Research Use Only"
Does the Label Change Anything?
No. It is a liability position adopted by the seller, not a regulatory category, and the FDA has held in warning letters that it does not survive product copy establishing human use.12 A page describing tendon repair, injury recovery and a loading-phase dosing schedule, followed by a line that the vial is not for human consumption, has supplied the agency's argument.
The Route Problem
Why Does It Matter How the Trials Gave It?
Because none of them gave it the way the market does. The human safety record is for intravenous infusion of the protein in a clinical unit, topical gel on wounds, and eye drops.7 No trial of thymosin beta-4 has used subcutaneous injection, which is the only way anyone outside a trial uses it. Subcutaneous administration of an impure or aggregated peptide is the setting in which immunogenicity becomes a question, and for this compound and this route there is no human data at all.
Storage and Handling
What Is Known
No manufacturer-validated storage specification exists for any product a consumer can buy. The developers' trial formulations, a gel and an ophthalmic solution, have stability data that belong to those formulations and are not published for consumer use. What can be stated is general peptide chemistry: lyophilised powder is more stable than solution; heat and repeated freeze-thaw degrade peptides; a 43-residue protein in solution is more vulnerable to aggregation and proteolysis than a short fragment. Every specific duration circulating online is extrapolated.
Multi-Dose Vials
The FDA's guidance for compounded injectables is to discard a multi-dose vial within 28 days of first use regardless of what remains or what the supplier says, and to use a new sterile needle and syringe for every dose.11
Signs to Stop Rather Than Proceed
A certificate whose observed mass does not correspond to the molecule on the label
Powder that does not dissolve cleanly, or a solution that turns cloudy or develops particles on standing, which in a long peptide suggests aggregation
Injection-site reactions that grow with successive doses, the pattern of an immune response
Any product using a developer's code name, which no developer supplies to the public
Community Language
Thymosin beta-4, TB4, Tβ4
The 43-residue protein. In the peptide market, usually a label on the fragment.
TB-500
Ac-LKKTETQ, the seven-residue synthetic fragment. The product actually sold. Graded D by ONPEPS against the protein's B.
RGN-259, RGN-137, NL005
Developer codes for the ophthalmic solution, the dermal gel and the Chinese recombinant injectable. Trial products; not for sale.
Wolverine stack
Community name for TB-500 combined with BPC-157. The one controlled animal comparison found no additive benefit; the ONPEPS stack article covers it.
Certificate of analysis
A laboratory report on one batch. It describes the batch tested, not necessarily the vial received, and for this compound the mass on it is the number that matters.
Related ONPEPS Coverage
Thymosin beta-4 library entry: the trials, the missed endpoints and the keratopathy signal
Thymosin beta-4 cost and access: a drug that exists only inside its trials
TB-500 sourcing and quality: COA verification, salt forms and supply risks
Sources
US Food and Drug Administration. Category 2 of the Bulk Substances Nominated Under Sections 503A or 503B of the Federal Food, Drug, and Cosmetic Act. Entry for thymosin beta-4 fragment (LKKTETQ). Content current as of 22 April 2026. fda.gov
ClinicalTrials.gov. NCT03937882. ARISE-3. Sponsor ReGenTree, LLC. Phase 3, 700 participants, completed, results posted. Record retrieved 16 September 2026. clinicaltrials.gov/study/NCT03937882
ClinicalTrials.gov. NCT05555589. SEER-2. Sponsor ReGenTree, LLC. Phase 3, recruiting. Record retrieved 16 September 2026. clinicaltrials.gov/study/NCT05555589
US Food and Drug Administration. July 23-24, 2026: Meeting of the Pharmacy Compounding Advisory Committee. Docket FDA-2025-N-6895. fda.gov
United States Anti-Doping Agency. 2018 Prohibited List: Summary of Major Changes. Archived advisory. usada.org
ABC News (Australia). Essendon supplements saga: players banned for 12 months after WADA appeal upheld by Court of Arbitration for Sport. 12 January 2016. abc.net.au
Ruff D, Crockford D, Girardi G, Zhang Y. A randomized, placebo-controlled, single and multiple dose study of intravenous thymosin beta4 in healthy volunteers. Ann N Y Acad Sci. 2010;1194:223-229. Paywalled. pubmed.ncbi.nlm.nih.gov/20536472
Sosne G, Kleinman HK, Springs C, Gross RH, Sung J, Kang S. 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. 2022;24(1):554. Open access. pubmed.ncbi.nlm.nih.gov/36613994
ClinicalTrials.gov. NCT07487363. TB-500 (Thymosin Beta 4 17-23 Fragment) for Cardiovascular Biomarkers in Stable ASCVD. Sponsor Hudson Biotech. Phase 1/2, recruiting. Record retrieved 16 September 2026. clinicaltrials.gov/study/NCT07487363
Rahaman KA, Muresan AR, Min H, Son J, Han HS, Kang MJ, Kwon OS. 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. 2024;1235:124033. Open access. pubmed.ncbi.nlm.nih.gov/38382158
US Food and Drug Administration. FDA's Concerns with Unapproved GLP-1 Drugs Used for Weight Loss. Drug Alerts and Statements. Cited for the agency's general guidance on multi-dose vials and fabricated labels. fda.gov
US Food and Drug Administration. Warning Letter: Gram Peptides. MARCS-CMS 721806, 31 March 2026. fda.gov
Not Medical Advice
Thymosin beta-4 holds no marketing authorisation for human use in any jurisdiction. If you are already using a product obtained outside a pharmacy, the route back to oversight is a clinician who knows what you have taken, and the first thing to establish is which molecule that was.
Regulatory and registry sources verified 16 September 2026. Trial status and the FDA's compounding categories change frequently. Re-verify before relying on any statement here.