Scientifically reviewed by Dr Stephan Hansberg
Thymosin beta-4 vs TB-500: what is the difference? (2026)
Thymosin beta-4 is a natural 43-amino-acid peptide; TB-500 is a synthetic 7-amino-acid acetylated fragment of it. A side-by-side comparison of identity, research, trials and regulation, why the two are confused, and which Celyfe pens contain TB-500.
Key takeaways
- •Thymosin beta-4 is the full natural peptide: 43 amino acids, N-acetylated, about 4963 g/mol.
- •TB-500, as described by the FDA and PubChem, is Ac-LKKTETQ: seven amino acids matching residues 17 to 23 of thymosin beta-4, about 889 g/mol.
- •Most published research, and all of the late-stage clinical work, concerns full-length thymosin beta-4, not TB-500.
- •Regulators treat them as related but separate: Australia schedules each by name, WADA bans thymosin-beta-4 and its derivatives including TB-500, and the FDA reviewed TB-500 at its July 2026 compounding meeting.
- •Celyfe's WOLVERINE, GLOW and KLOW pens contain TB-500 15mg, not full-length thymosin beta-4.
The short answer
Thymosin beta-4 and TB-500 are not the same molecule. Thymosin beta-4 is a naturally occurring 43-amino-acid peptide found in most human cells, while TB-500 is a synthetic seven-amino-acid peptide, Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln, that copies residues 17 to 23 of thymosin beta-4, the stretch that contains its actin-binding motif.
TB-500 is therefore a fragment derived from thymosin beta-4, about one sixth of its mass. Research findings for the full peptide cannot be assumed to apply to the fragment, and vice versa.
Side by side
Identifiers checked against PubChem, UniProt and the FDA's July 2026 TB-500 briefing document.
| Property | Thymosin beta-4 | TB-500 |
|---|---|---|
| What it is | Natural peptide encoded by the TMSB4X gene | Synthetic fragment of thymosin beta-4 |
| Length | 43 amino acids | 7 amino acids |
| Sequence | Ac-SDKPDMAEIEKFDKSKLKKTETQEKNPLPSKETIEQEKQAGES | Ac-LKKTETQ (Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln-OH), residues 17 to 23 |
| Molecular formula | C212H350N56O78S | C38H68N10O14 |
| Molar mass | approx. 4963 g/mol | 889.0 g/mol (free base); 949.1 g/mol listed by FDA for the acetate |
| CAS number | 77591-33-4 | 885340-08-9 (free base) |
| PubChem CID | 16132341 | 62707662 |
| FDA UNII | Not listed here | QHK6Z47GTG |
| Registered human trials | Yes: RGN-259 eye drops (Phase 3), recombinant NL005 (Phase 2) | None we could verify |
| FDA | Not approved; investigational products only | Not approved; reviewed at PCAC July 2026 |
| AU TGA | Schedule 4, named (THYMOSIN BETA 4) | Schedule 4, named (TB500) |
| WADA 2026 | S2.3, prohibited at all times | S2.3, named as a derivative, prohibited at all times |
The key differences
Size and identity. At about 4963 g/mol versus 889 g/mol, the two are easy to tell apart by mass spectrometry. TB-500 also lacks the methionine at position 6 of the full peptide, so it does not form the sulfoxide variant described for thymosin beta-4.
What each covers. TB-500 contains the LKKTETQ stretch that reviews of thymosin beta-4 identify as part of its actin-binding region (Sosne et al., 2010). It does not contain other regions of the parent, such as the N-terminal Ac-SDKP tetrapeptide, which is studied separately (Conte et al., 2015). Older biochemical work by Hannappel and colleagues (1993) compared the actin-sequestering ability of thymosin beta-4 with fragments of it, which is why researchers do not treat fragments as interchangeable with the whole peptide.
Evidence base. The cell and animal literature and all of the registered late-stage clinical programmes (for example ARISE-2 NCT02974907, ARISE-3 NCT03937882, SEER-2 NCT05555589) use full-length thymosin beta-4. The FDA's July 2026 briefing on TB-500 described non-existent information on its use in humans.
Why they are commonly confused
TB-500 is very often sold and discussed as TB-500 (thymosin beta-4), which suggests they are the same thing. Ho and colleagues, writing on doping control in 2012, described TB-500 as a synthetic version of an active region of thymosin beta-4, and WADA's list groups them together as thymosin-beta-4 and its derivatives, which reinforces the association.
The FDA also found identity data being mixed up. Its 2026 TB-500 briefing noted that the nomination named TB-500 free base while the accompanying certificate was for TB-500 acetate, and that several supplier websites use the free-base CAS number for the acetate. Those are two different substances in regulatory terms, and neither is full-length thymosin beta-4.
Which Celyfe pens contain which
No Celyfe pen contains full-length thymosin beta-4. Three pens contain TB-500 at 15mg per 3ml cartridge, identified in our product data as C38H68N10O14 (889.0 g/mol, CAS 885340-08-9), the 17 to 23 fragment:
| Pen | Composition | Certificate status |
|---|---|---|
| WOLVERINE (/products/wolverine-pen) | BPC-157 15mg + TB-500 15mg | Batch C26071, Analiza Bialek, HPLC, 99% purity, 2 October 2026, in /coa |
| GLOW (/products/glow-pen) | GHK-Cu 75mg + BPC-157 15mg + TB-500 15mg | Being re-issued; listed in /coa when published |
| KLOW (/products/klow-pen) | GHK-Cu 75mg + BPC-157 15mg + TB-500 15mg + KPV 15mg | Being re-issued; listed in /coa when published |
Regulatory notes for 2026
At the FDA's 23 July 2026 Pharmacy Compounding Advisory Committee, FDA staff proposed that neither TB-500 free base nor TB-500 acetate be added to the 503A bulks list, citing a lack of characterisation data, no historical use in compounding, no information on use in humans and a risk of immunogenicity for injectable peptides. The committee voted 8-6 with one abstention to recommend adding TB-500. The vote is advisory and rulemaking was pending as of October 2026.
In Australia both are listed separately in Schedule 4 of the Poisons Standard (October 2026 instrument). For athletes, the WADA 2026 List prohibits thymosin-beta-4 and its derivatives, e.g. TB-500, at all times under S2.3.
Sources
Primary and official sources checked October 2026:
- PubChem: Thymosin beta-4, CID 16132341; TB-500, CID 62707662 (pubchem.ncbi.nlm.nih.gov)
- UniProt: P62328, Thymosin beta-4, Homo sapiens (uniprot.org)
- FDA: Briefing document for TB-500-related bulk drug substances (TB-500 free base and TB-500 acetate), Pharmacy Compounding Advisory Committee, 23-24 July 2026 (fda.gov)
- Hannappel E et al. Actin-sequestering ability of thymosin beta 4, thymosin beta 4 fragments, and thymosin beta 4-like peptides as assessed by the DNase I inhibition assay. Biol Chem Hoppe Seyler, 1993. PMID 8471179
- Sosne G et al. Biological activities of thymosin beta4 defined by active sites in short peptide sequences. FASEB J, 2010. PMID 20179146
- Ho EN et al. Doping control analysis of TB-500, a synthetic version of an active region of thymosin beta-4, in equine urine and plasma. J Chromatogr A, 2012. PMID 23084823
- Conte E et al. Thymosin beta-4 and its N-terminal fragment Ac-SDKP in human lung fibroblasts and a mouse model. Expert Opin Biol Ther, 2015. PMID 26098610
- Young JD et al. Thymosin beta 4 sulfoxide generated by monocytes. Nat Med, 1999. PMID 10581087
- ClinicalTrials.gov records NCT02974907, NCT03937882, NCT05555589, NCT05984134
- AJMC: FDA panel backs 6 peptides for compounding, 31 July 2026
- Therapeutic Goods (Poisons Standard, October 2026) Instrument 2026, F2026L01327 (legislation.gov.au)
- WADA: Prohibited List 2026, S2.3, in force 1 January 2026 (wada-ama.org)
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