TB-500 vs thymosin β4
TB-500 usually refers to a short synthetic fragment of thymosin β4, a 43-residue actin-binding protein, but the name is applied inconsistently. This page compares the two and explains why the conflation matters for identity testing, evidence interpretation and regulatory review.
Thymosin β4 is a naturally occurring 43-residue peptide encoded by the TMSB4X gene and found in most human cells, where it binds monomeric actin. TB-500 is a research-catalog name rather than a defined pharmacopeial substance. It is most often used for the acetylated heptapeptide Ac-LKKTETQ, corresponding to the actin-binding region around residues 17–23 of thymosin β4, but it has also been applied to other fragments and to the full-length peptide.
The distinction matters because the two molecules have very different sizes and masses, and because the clinical literature concerns full-length thymosin β4 formulations, not the fragment. When the FDA's compounding advisory committee reviewed TB-500 in July 2026, FDA flagged variable sequences and noted that no direct human studies of TB-500 itself exist.
Side-by-side
| Attribute | TB-500 | Thymosin β4 |
|---|---|---|
| Origin | Synthetic; derived from thymosin β4 | Endogenous; encoded by TMSB4X; research material synthetic or recombinant |
| Length | Commonly 7 residues (Ac-LKKTETQ); varies by source | 43 residues |
| Terminal chemistry | Synthetic N-acetyl group on the fragment; C-terminus varies by source | N-acetylated serine (begins Ac-SDKP); free C-terminus |
| Oxidation-prone residues | None in Ac-LKKTETQ | One methionine near the N-terminus |
| Proposed targets | Actin-binding motif; activity inferred from the parent protein | G-actin sequestration; cell migration and wound-repair pathways |
| Evidence level | Preclinical; no direct human studies | Late-clinical trials of specific formulations; none approved |
| FDA / compounding | Not FDA-approved; PCAC voted 8-6-1 in July 2026 to recommend 503A bulk listing, no final FDA action | Not FDA-approved; not part of the July 2026 PCAC vote; check current FDA lists |
| WADA | Prohibited under S2 (thymosin β4 fragments) | Prohibited under S2 |
| Analytical pitfalls | Which sequence is present; acetylation and C-terminal form; mislabeled full-length material | Methionine oxidation; truncations; aggregation; recombinant host impurities if not synthetic |
| Storage | Lyophilized at -20 °C, protected from light and moisture | Lyophilized at -20 °C, protected from light and moisture; avoid freeze-thaw |
Fragment vs full-length molecule
Thymosin β4 is a 43-residue peptide that is largely unstructured in solution and becomes ordered when bound to actin. The LKKTETQ stretch near its center is part of the actin-binding region, and researchers synthesized this short sequence to study whether it could reproduce some activities of the full protein in cell and animal models. Sharing a motif does not make the fragment equivalent to the whole molecule: other regions of thymosin β4, including the N-terminal Ac-SDKP sequence, have their own reported activities that the fragment lacks.
Why the conflation matters for identity testing
Because one label covers different molecules, the expected mass of a lot labeled TB-500 depends on which molecule the producer intended. The heptapeptide and full-length thymosin β4 differ by several thousand daltons, so a mass spectrum confirms one and rules out the other only if the certificate states which sequence it was compared against. A certificate that reports a correct mass for one form cannot be used to confirm the other. Acetylation state and C-terminal form also change the expected mass of the fragment. Use the sequence analyzer to compute both.
Why it matters for reading the evidence
Most clinical work, including ophthalmic and topical trials, used formulations of full-length thymosin β4, and even those trials produced mixed results without leading to an approved product. Animal and cell findings on thymosin β4 are also more extensive than those on the fragment. Statements that attribute thymosin β4 trial data to TB-500 therefore overstate the evidence. The FDA made this point during the 2026 compounding review, noting that human thymosin β4 studies are not evidence for the fragment.
Regulatory and sport status
Neither molecule is approved by the FDA for any use. The July 2026 PCAC vote of 8-6-1 recommending 503A bulk listing concerned TB-500, against the recommendation of FDA's own reviewers; the vote is advisory and requires notice-and-comment rulemaking before any change takes effect. For sport, WADA treats the two together: thymosin β4 and its fragments, including TB-500, are prohibited under class S2.
Frequently asked questions
Is TB-500 the same as thymosin β4?
Not usually. TB-500 most often refers to the synthetic acetylated fragment Ac-LKKTETQ, a seven-residue piece of the 43-residue thymosin β4 sequence. Some sources use the name for full-length thymosin β4 or other fragments. Because the label is inconsistent, a certificate of analysis should state exactly which sequence was confirmed by mass spectrometry.
Do thymosin β4 clinical trials apply to TB-500?
No. Human trials were conducted with specific formulations of full-length thymosin β4, and their results were mixed. A short fragment lacks most of the parent sequence and may behave differently in absorption, stability and activity. The FDA noted in 2026 that there are no direct human studies of TB-500 itself and that thymosin β4 data are not evidence for the fragment.
How can a lab tell which molecule is in a TB-500 vial?
Mass spectrometry is the most direct test, because the heptapeptide and full-length thymosin β4 have very different masses. The certificate should report the observed mass alongside the expected mass for a named sequence, including its acetylation and C-terminal form. HPLC purity alone cannot distinguish the two, since either could appear as a single clean peak.
Full entries
- TB-500
TB-500 is a research name most often applied to a short synthetic N-acetylated fragment of thymosin β4, Ac-LKKTETQ, but it is also used loosely for full-length thymosin β4. There are no direct human studies of the fragment.
- Thymosin β4
Thymosin β4 is a naturally occurring 43-amino-acid peptide that binds monomeric actin and is abundant in many cell types. It has been studied in animal models of tissue repair and in human trials of ophthalmic and topical formulations, and is not approved.
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