TB-500 and thymosin beta-4: what each name refers to and what the research covers

TB-500 vs thymosin beta-4: two different molecules#

Thymosin beta-4 (Tβ4) is a naturally occurring protein of 43 amino acids. TB-500 is a much shorter synthetic peptide. In its 2026 briefing document FDA defines TB-500 as a seven-amino-acid fragment of thymosin beta-4, Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln, corresponding to residues 17 to 23 of the protein with an acetyl group added to the first amino acid, and states plainly that "thymosin beta-4 and TB-500 are not the same substance".^[1]

The name causes most of the confusion. FDA notes that TB-500 is a common name, not an official nonproprietary name, and that different salts and derivatives are sold under it.^[1] The fragment itself was characterized in 2012 by an anti-doping laboratory that analyzed a product sold as TB-500. Using high-resolution mass spectrometry, the authors found that it did not contain full-length thymosin beta-4 but the acetylated 17-23 heptapeptide, which they described as the acetyl derivative of the major actin-binding site of thymosin beta-4. They also noted that this acetylated form is not described among the body's own forms of the protein, which lets testing laboratories tell it apart from natural thymosin beta-4.^[2]

The practical point for anyone reading a label or a paper: "TB-500" and "thymosin beta-4" can refer to molecules that differ more than sixfold in size, so the name alone does not identify the material. The sequence and the measured mass do. The vial on the TB-500 product page is the acetylated 17-23 fragment, Ac-LKKTETQ.

TB-500 sequence, molecular weight and CAS number#

FDA notes that some suppliers use the free-base CAS number for the acetate salt.^[1] A Certificate of Analysis should therefore say which form a lot is, rather than leaving it to be inferred from the CAS number.

What the thymosin beta-4 literature studied#

Almost everything usually quoted for TB-500 comes from research on the full 43-residue protein. That literature is real and comes from several independent groups, but it is about a different molecule.

Actin sequestration in human white blood cells#

Actin is the protein that forms the internal scaffolding of cells. Cassimeris and colleagues measured thymosin beta-4 in resting human neutrophils (a type of white blood cell) and found that it appeared to be the major protein holding actin in its single-unit, unpolymerized form. When the cells were stimulated, the amount of actin bound to thymosin beta-4 fell, consistent with actin being released to build filaments. This was a cell and biochemistry study.^[3] The 17-23 region that TB-500 copies is part of the actin-binding site.^[2]

Skin wounds in rats#

Malinda and colleagues made full-thickness skin wounds in rats and applied thymosin beta-4 to the wounds or gave it systemically. They measured re-epithelialization, wound contraction, collagen deposition and new blood vessel formation, and reported increases in each compared with saline controls. In a separate cell assay, thymosin beta-4 increased the migration of keratinocytes (skin cells).^[4]

Heart injury in mice#

Bock-Marquette and colleagues studied thymosin beta-4 in cultured heart cells and in adult mice after the coronary artery was tied off to cause a heart attack. In cells, they reported more migration and survival, linked to activation of the enzymes ILK and Akt. In mice treated after the injury, they measured heart function by echocardiography, scar size and early cell death, and reported better function, smaller scars and less early cell death than in untreated mice.^[5]

A 2012 review by researchers involved in this work summarized thymosin beta-4 as a peptide that binds actin and promotes cell migration, and described clinical trials then ongoing or planned in skin wounds, corneal injury, and heart and nervous system injury.^[6]

Human trials of thymosin beta-4 (not TB-500)#

The human studies all used full-length thymosin beta-4 in eye-drop or topical gel formulations developed by RegeneRx Biopharmaceuticals and its partners. Registered examples include:

• Venous stasis ulcers (NCT00832091): a Phase 2, randomized, placebo-controlled, quadruple-masked trial of a thymosin beta-4 topical gel against placebo gel in 72 participants, sponsored by RegeneRx with sigma-tau, completed in January 2009, with results posted on the registry in 2010.^[7]
• Dry eye, Phase 2: a single-center, double-masked, placebo-controlled trial of a thymosin beta-4 eye drop (RGN-259) in 72 people, using a controlled adverse environment model. Neither co-primary endpoint, ocular discomfort and inferior corneal staining, differed significantly from placebo in the primary analysis; some secondary measures, including central and superior corneal staining, did.^[8]
• Dry eye, Phase 3 (ARISE-3, NCT03937882): a multicenter, randomized, double-masked, vehicle-controlled trial sponsored by ReGenTree, planned to randomize about 700 adults with dry eye. The primary endpoint in the protocol was change in inferior corneal staining at day 15.^[9]

None of these trials tested TB-500, and their findings cannot be transferred to it. They are evidence about a 43-residue protein in specific eye and skin formulations, nothing more.

What has been studied on TB-500 itself#

Direct data on the fragment are thin. FDA's 2026 review found the following:^[1]

• One cell wound-closure study in which TB-500 was not significantly different from vehicle, while a shorter breakdown product, acetylated LKKTE, produced a small but significant effect. FDA noted the study did not test a range of concentrations.
• Pharmacokinetic and metabolite studies in horses and rats, done mainly to support anti-doping detection.
• No acute, repeat-dose, genetic, reproductive or carcinogenicity toxicology studies.
• No human studies of any kind: no clinical trials, no pharmacokinetic data and no case reports.

Since then, one rat study has tested it. Biçer and colleagues repaired the Achilles tendon in 32 rats split into control, BPC-157, TB-500 and combined groups. At four weeks the TB-500 group had a significantly higher load to failure than controls and lower scores on two tendon histology scales (Bonar and Movin, on which lower scores indicate tissue closer to normal tendon); the combination added nothing beyond either peptide alone. The authors called the study exploratory.^[10] Its abstract describes TB-500 as synthetic thymosin beta-4 without giving a sequence, a small example of why the naming matters.

Regulatory status of TB-500#

• Approval: FDA found no approved TB-500 product in the United States, the other countries it checked or the EU, and the fragment is not a component of any FDA-approved drug.^[1]
• FDA compounding safety-risk list: FDA's page (current as of April 22, 2026) lists "Thymosin beta-4, fragment (LKKTETQ), also known as TB-500" among substances nominated but withdrawn. It says compounded drugs containing the fragment may pose a risk for immunogenicity, that FDA has not identified any human exposure data, and that FDA lacks important information about its safety.^[11]
• 503A bulks list review: FDA's July 2026 briefing concluded that TB-500 free base and acetate are not well characterized physically and chemically and proposed not adding either to the list of substances pharmacies may compound.^[1] On July 23, 2026 the advisory committee voted 8 to 6, with one abstention, to recommend adding TB-500. The vote is nonbinding; formal rulemaking is required.^[12]
• Sport: WADA's 2026 Prohibited List names "Thymosin-β4 and its derivatives e.g. TB-500" under S2.3, Growth Factors and Growth Factor Modulators, prohibited at all times.^[13]

BPC-157 vs TB-500: how the two molecules compare#

BPC-157 and TB-500 are often sold and discussed together, and one vial on our site, the BPC-157 + TB-500 Blend, contains both. As molecules they have little in common.

The evidence bases differ in kind. BPC-157 has a long publication list, but most of it comes from one research centre, and independent laboratories have contributed only a handful of studies.^[14][15] TB-500 has almost no literature of its own; the large thymosin beta-4 literature belongs to the parent protein. The only study that tested the two together is the 2026 rat tendon study above, which found no added effect from combining them.^[10] Our BPC-157 research summary covers that peptide in detail, and the Wolverine stack explainer covers the marketing name often given to the pair.

What a TB-500 Certificate of Analysis should show#

For TB-500, the identity check matters more than usual, because the name does not settle which molecule is in the vial. A mass spectrum showing a mass consistent with the acetylated 17-23 fragment (889.01 g/mol for the free base) confirms the fragment; a mass near 4,963 would indicate full-length thymosin beta-4. FDA observed that the one example free-base Certificate of Analysis it found tested appearance, identity and purity, but not assay, impurities, endotoxin or aggregates.^[1] A useful COA should show:

• The lot number, matching the vial.
• Identity by mass spectrometry against the expected mass for the stated form.
• Purity by HPLC, with the chromatogram.
• The form: free base or acetate.
• The testing laboratory and the date.

Our guides to reading a Certificate of Analysis and to common purity report misreadings cover each item, and the COA library lists every Renuvia lot and its current testing status.