Thymosin beta-4 is a 43-amino-acid protein found in nearly every human cell, where it binds actin, and TB-500 is a synthetic seven-residue fragment of it, the acetylated sequence Ac-LKKTETQ. Most of the research people cite for TB-500 was done on the full protein, the FDA reported in 2026 that it found no human data at all on the fragment, and TB-500 is banned in sport by name.
The two names are used as if they were one compound. The published record treats them as two, and the difference decides what the evidence can support.
At a glance
| Thymosin beta-4 | TB-500 | |
|---|---|---|
| What it is | Naturally occurring 43-amino-acid protein | Synthetic 7-residue fragment, Ac-LKKTETQ (residues 17 to 23, acetylated) |
| Where it comes from | Isolated from calf thymus; sequenced in 1981 | A research-market product name, not an adopted drug name |
| Main role described | Actin sequestering; studied in wound and tissue repair | Proposed to mimic the parent protein's actin-binding region |
| Animal work | Rat and mouse wound models; mouse heart model | Very little; one mouse study used an unacetylated version of the sequence |
| Human trials | Phase 2 studies of topical and eye-drop formulations | None identified by the FDA |
| FDA approval | No thymosin beta-4 product approved | None |
| US compounding review | Not reviewed as a bulk substance | Advisory committee recommended it 8 to 6, one abstention, July 2026, against FDA staff advice |
| Sport | Prohibited (WADA S2) | Prohibited by name (WADA S2) |
What thymosin beta-4 is
Low, Hu and Goldstein published the complete sequence of thymosin beta-4 from calf thymus in 1981 (PNAS). Later work found it far outside the thymus. A 2010 review from the protein's main research groups (Sosne and colleagues, FASEB Journal) describes it as a small, ubiquitous protein that is the major actin-sequestering molecule in eukaryotic cells, found in essentially all cells and body fluids. Actin is the protein that forms much of a cell's internal scaffolding, and holding it in reserve is how thymosin beta-4 is thought to influence cell movement.
The same review maps several "active sites" onto the protein. A four-residue piece at the start, Ac-SDKP, is associated with reduced inflammation and fibrosis. A central stretch containing LKKTETQ, the actin-binding domain at residues 17 to 23, is associated with blood-vessel formation, wound healing and cell migration. That central stretch is the sequence TB-500 is built on.
What TB-500 is
TB-500 is a product name rather than a drug name. Esposito and colleagues at the Ghent doping-control laboratory (Drug Testing and Analysis, 2012) analyzed a product sold as TB-500 and identified its main ingredient as the N-terminal acetylated 17 to 23 fragment of thymosin beta-4, Ac-LKKTETQ. A Hong Kong Jockey Club laboratory reached the same conclusion the same year (Ho and colleagues, Journal of Chromatography A, 2012), describing TB-500 as a veterinary preparation and building a test to detect it in horses.
The FDA's July 2026 briefing document notes that "TB-500" is a common name that does not follow standard naming conventions, and that this creates a risk that the material in a container is not the substance a reader expects. A certificate for one TB-500 product says nothing about another.
The evidence, most of it on the parent protein
Animal work on the full protein. Malinda and colleagues (Journal of Investigative Dermatology, 1999) reported faster closure of full-thickness skin wounds in rats treated with thymosin beta-4. Philp and colleagues (Wound Repair and Regeneration, 2003) reported faster wound repair in diabetic and aged mice. Bock-Marquette and colleagues (Nature, 2004) reported improved heart function in mice after a coronary artery was tied off.
Animal work on the fragment. The closest thing to fragment data in that literature is in the Philp study, which also tested a synthetic seven-amino-acid peptide, LKKTETQ, and reported repair in aged mice comparable to the parent protein. That peptide was not acetylated, so it was not TB-500 as sold. The FDA's 2026 review makes the point directly: acetylation changes how a peptide behaves, so results for the unmodified sequence cannot simply be transferred.
Cell work on TB-500 itself. Rahaman and colleagues at the Korea Institute of Science and Technology (Journal of Chromatography B, 2024) studied how TB-500 breaks down in human serum, enzyme systems and rats. In cultured fibroblasts, TB-500 did not show significant wound-healing activity compared with a control, while one of its breakdown products, Ac-LKKTE, did. The authors suggested that wound-healing activity attributed to TB-500 may belong to that metabolite rather than the parent. That is a cell-culture finding and a hypothesis, not a clinical result.
Human trials of the full protein. Thymosin beta-4 reached phase 2 in topical and eye-drop formulations. In a European study of 73 patients with venous leg ulcers (Guarnera and colleagues, Annals of the New York Academy of Sciences, 2010), safety was comparable to placebo and the authors described efficacy results as suggestive. In a single-center dry-eye trial that randomized 72 adults (Sosne and Ousler, Clinical Ophthalmology, 2015), neither primary endpoint differed significantly from placebo, while several secondary measures did. No thymosin beta-4 product has been approved by the FDA.
Human data on TB-500. None on file with the agency. The FDA's July 2026 review found no information in the medical literature in which TB-500 was given to patients for any condition, no nonclinical toxicity studies, and no human exposure data by any route.
Regulatory status
TB-500 is not approved by the FDA for any use. It had been placed in Category 2 of the FDA's interim compounding policy, the category for substances that may present significant safety risks, and was removed in April 2026 after its nomination was withdrawn. The FDA's safety summary still lists it among withdrawn substances and says the agency lacks important information about whether it would cause harm in humans.
At the July 23, 2026 meeting of the Pharmacy Compounding Advisory Committee, FDA staff proposed against adding TB-500 to the 503A bulks list for its nominated use, wound healing. The committee voted 8 to 6, with one abstention, to recommend it. That vote is advisory. It is not an approval, and we could find no proposed or final rule adding TB-500 to the list as of this writing. Our guide to FDA approval and our 503A explainer cover the difference.
Is TB-500 banned?
Yes, in sport. The World Anti-Doping Agency's 2026 Prohibited List bans, at all times, "Thymosin-β4 and its derivatives e.g. TB-500" under section S2.3, growth factors and growth factor modulators. Anti-doping laboratories have published detection methods for the fragment and its breakdown products in human and equine samples.
Outside sport, "banned" is the wrong word for US law. TB-500 is an unapproved substance, and FDA enforcement in this market has focused on how products are marketed and to whom. Our piece on whether research peptides are legal sets out how that works.
For the side-by-side with the compound it is most often paired with, see BPC-157 vs TB-500, and for the wider class, our review of the repair peptides.
Frequently asked questions
What is thymosin beta-4?
A naturally occurring 43-amino-acid protein present in nearly all human cells and body fluids. Its best-established role is binding and sequestering actin, and it has been studied in wound, eye and heart repair models.
Is TB-500 the same as thymosin beta-4?
No. TB-500 is a seven-residue acetylated fragment of thymosin beta-4, Ac-LKKTETQ. Most of the published research describes the full protein, and the FDA found no human data on the fragment.
Is TB-500 banned?
In sport, yes: WADA prohibits thymosin beta-4 and its derivatives, naming TB-500, at all times under S2. In the United States it is not an approved drug, and its compounding status is still under review after the July 2026 advisory vote.
Has TB-500 been tested in humans?
We found no published human trial of TB-500. The human trials in this area tested formulations of the full thymosin beta-4 protein, and none led to an FDA approval.
Sources:
- Low, Hu and Goldstein, PNAS, 1981 (sequence of thymosin beta-4)
- Sosne et al., FASEB J, 2010 (active sites of thymosin beta-4, review)
- Esposito et al., Drug Test Anal, 2012 (Ac-LKKTETQ identified in TB-500)
- Ho et al., J Chromatogr A, 2012 (TB-500 detection in horses)
- Malinda et al., J Invest Dermatol, 1999 (rat wound model)
- Philp et al., Wound Repair Regen, 2003 (diabetic and aged mice; unacetylated LKKTETQ)
- Bock-Marquette et al., Nature, 2004 (mouse cardiac model)
- Rahaman et al., J Chromatogr B, 2024 (TB-500 metabolism; fibroblast wound assay)
- Guarnera et al., Ann N Y Acad Sci, 2010 (phase 2, venous leg ulcers)
- Sosne and Ousler, Clin Ophthalmol, 2015 (phase 2, dry eye, 72 adults)
- FDA briefing document, TB-500-related bulk drug substances, PCAC, July 23 to 24, 2026
- FDA, Certain bulk drug substances for use in compounding that may present significant safety risks
- McDermott, PCAC backs majority of peptides in two-day public meeting (vote tallies)
- WADA 2026 Prohibited List, as published in the Dutch treaty series (S2.3)
No compound discussed in this review is FDA-approved. Nothing above is dosing guidance or a treatment claim. Research use only.
For research use only · Not for human or veterinary use · No research compound discussed here is FDA-approved for any indication