Thymosin β4
The body's main actin-sequestering peptide: a 43-amino-acid molecule tested as eye drops for dry eye and corneal wounds, and not the same thing as the TB-500 sold online.
Thymosin β4 is made by almost every cell, where it holds actin in reserve until the cell needs to move or change shape. That role in cell movement made it a candidate for wound healing, and as eye drops (RGN-259) it has reached phase 3 trials. The TB-500 of the research-peptide market is usually a short synthetic fragment, not this molecule, though some listings use the name for both.
What is it?
Thymosin β4 was first isolated from thymus extracts, which is where the name comes from, but it turned out to be one of the most abundant peptides in the body and has nothing specific to do with the thymus. Inside cells it binds single actin molecules and keeps them from assembling into filaments, so it acts as a reserve the cell draws on when it needs to crawl, divide or change shape. Along with profilin it is one of the two main proteins that do this job, and platelets, which change shape within seconds, carry a great deal of it.
Because cell migration is the first step of wound repair, researchers began testing added thymosin β4 on injured tissue in the 1990s and 2000s. Reviews from that period describe faster healing of skin and corneal wounds in animals, new blood vessel growth and protection after heart injury, and a string of planned trials: topical gel for venous and pressure ulcers, intravenous dosing after heart attacks, and eye drops for the cornea. One of its breakdown products, the tetrapeptide Ac-SDKP from its first four amino acids, has biological effects of its own.
The human evidence is thin and mostly from the eye. A phase 1 study gave healthy volunteers up to 1,260 mg a day intravenously for 14 days without dose-limiting toxicity. A small phase 2 trial of 0.1% eye drops reported less discomfort and staining in severe dry eye, and an 18-patient phase 3 trial reported more complete healing of corneal defects in neurotrophic keratopathy. The developer, ReGenTree, ran phase 3 dry-eye trials that completed in 2018 and 2021; their results are not among the studies carded here.
Two cautions run through the literature. Thymosin β4 has been linked to metastasis in solid tumours, as the authors of a myeloma study note while reporting the opposite in that blood cancer, and a 2024 mouse study found it damaged the gut's mucus barrier. And TB-500, as our entry defines it, is a synthetic fragment of about seven amino acids (Ac-LKKTETQ), not the 43-amino-acid molecule these studies tested; some listings sell full-length thymosin β4 under the TB-500 name, which is part of the confusion. None of the findings here transfers to the fragment automatically.
Mechanism of Action
Thymosin β4's best-established job is holding actin monomers in reserve. The regenerative effects reported in animals, from blood-vessel growth to cell survival, are thought to follow from that and from signalling partners such as the EGF receptor, but most of them come from mouse and rat studies.
Actin reserve
It binds G-actin and keeps it from polymerising. Mice without the gene made platelets with less G-actin and more F-actin, which spread faster but activated poorly, formed weaker clots under flow and bled longer.
Cell migration and new vessels
In mice it was essential for coronary vessel development and made quiescent adult heart-surface (epicardial) cells grow out and differentiate into vessel cells. Its cleavage product Ac-SDKP boosted endothelial differentiation but could not replace it.
Repair signalling (EGFR)
In two mouse models of demyelination it increased new myelinating cells through the EGF receptor pathway; blocking that pathway abolished the effect in cultured cells.
Inflammation, with an exception
It reduced inflammatory damage in mouse skin and in rodent liver injury, lowering NF-κB and oxidative stress. But in mice with colitis it was raised, and giving it damaged the gut's mucus barrier by suppressing autophagy.
Published Research
Our evidence set holds 18 papers that name thymosin β4 in a form our citation gate recognises; 14 name it in their titles and have an abstract. Thirteen are carded below: four studies in people (two small eye-drop trials, a phase 1 safety study and a biomarker study), seven in animals and two reviews. The fourteenth, the design of a venous-ulcer trial with no results, is listed under References. Many more papers study thymosin β4 as a marker or a gene; the carded set is weighted toward what happens when it is given.
Two forms in mouse skin inflammation
Mouse skin immune cells make two forms of thymosin β4: the ubiquitous one and a splice variant limited to lymphoid tissue, which rose after activation. Tested as synthetic peptides in three models of skin inflammation (carrageenan, an irritant and an allergen), the lymphoid form was broader and usually stronger at reducing inflammation than the ubiquitous form.
Actin-sequestering protein that repairs tissue
A review of thymosin β4 as the major actin-sequestering molecule in cells and of its role in skin and corneal wound healing, proposing clinical uses in ulcers and in organs damaged by lack of oxygen, such as the heart after infarction and the brain after stroke, and in septic shock and multi-organ failure.
Heart vessels and epicardial progenitors, mice
In mice thymosin β4 was essential for every stage of coronary vessel development. Added to quiescent adult epicardial tissue, it stimulated outgrowth and differentiation into fibroblasts, smooth muscle and endothelial cells. Knocking it down in the heart cut levels of its cleavage product Ac-SDKP, which enhanced endothelial differentiation but did not rescue the knockout hearts.
Structure, function and clinical programmes
A review of thymosin β4's cellular effects, including cell migration, vessel formation, cell survival, stem-cell differentiation and modulation of cytokines and proteases, as the rationale for dermal, corneal and cardiac trials, with emphasis on its development for heart attack treated with percutaneous coronary intervention.
Intravenous safety study in 40 volunteers
Four cohorts of 10 healthy volunteers received a single intravenous dose of placebo or 42, 140, 420 or 1,260 mg of synthetic thymosin β4, then the same dose daily for 14 days. Adverse events were infrequent and mild or moderate, with no dose-limiting toxicity and no serious events. Exposure rose in proportion to dose and the half-life lengthened at higher doses.
Multiple myeloma
In a large patient population, myeloma cells expressed less thymosin β4 than normal plasma cells, and patients with higher expression had longer event-free and overall survival. Overexpressing it in mouse myeloma cells slowed their growth, raised their sensitivity to apoptosis and prolonged survival in mice. The authors contrast this with its link to metastasis in solid tumours.
Eye drops for severe dry eye (phase 2)
Nine patients with severe dry eye, some from graft-versus-host disease, used 0.1% thymosin β4 drops (RGN-259) or vehicle six times a day for 28 days. At day 56, four weeks after stopping, the treated eyes had 35.1% less ocular discomfort and 59.1% less corneal staining than vehicle, with longer tear break-up time and more tear volume. The drops were well tolerated.
Remyelination in two mouse models
In mice with experimental autoimmune encephalomyelitis, daily thymosin β4 for 30 days improved neurological scores, increased new myelinating oligodendrocytes and reduced axonal damage, and the gain in mature oligodendrocytes tracked the functional gain (r = 0.73). It also increased remyelination after a cuprizone diet. Blocking the EGF receptor abolished the effect in cultured progenitor cells.
Liver injury and fibrosis, mice and rats
Carbon tetrachloride raised liver enzymes, oxidative damage, inflammatory and fibrotic signals and collagen in mice and rats. Added thymosin β4 significantly attenuated all of these changes and lowered NF-κB activation, which the authors attribute to reduced oxidative stress and inflammation.
Blood levels in fatty liver disease
In 24 patients with simple fatty liver and 21 with steatohepatitis (NASH), serum thymosin β4 was lower in NASH (2.98 vs 5.12 mg/L) and fell as liver inflammation and oxidative damage rose. A cut-off of 3.94 mg/L detected NASH with 85.7% sensitivity and 79.2% specificity, better than the liver enzyme ALT. This measured the body's own peptide; nothing was given.
Platelets without it, mice
Mice lacking the thymosin β4 gene had fewer, slightly larger platelets and faulty platelet formation. Their platelets held less G-actin and more F-actin, spread and retracted clots faster, but activated poorly through the collagen receptor GPVI, formed weaker aggregates under flow, were protected from occlusive arterial thrombosis and bled longer.
Eye drops for neurotrophic keratopathy (phase 3, 18 patients)
In a randomised, placebo-controlled, double-masked phase 3 trial (as its title describes it), eighteen patients with stage 2 or 3 neurotrophic keratopathy and persistent corneal defects used 0.1% RGN-259 or placebo. At four weeks, 6 of 10 treated and 1 of 8 placebo patients had healed completely (p = 0.066); by day 43 the difference in healing was significant (p = 0.036) with no recurrences on treatment, and comfort measures improved. No significant adverse effects were seen.
Gut mucus barrier in colitis, mice
In mice with chemically induced colitis, thymosin β4 levels rose while the protective mucus layer thinned. Injecting it into healthy mice damaged the mucus barrier, cut mucin-2 production, disrupted tight junctions and suppressed autophagy, and the same effects appeared in human colon tissue and colon cells. The authors propose it as a marker of barrier defects.
Dosing Data
Thymosin β4 has no approved dose. In people it has been given intravenously in one safety study, at 42 to 1,260 mg a day for 14 days, and as 0.1% eye drops in trials. Other programmes, such as a topical gel for venous ulcers, were designed but their results are not in our evidence set.
Educational information, not medical advice. Each dose above is tagged with its source: an approved label, a published human study, animal data, or amounts reported by users. None is a recommendation for you. An unapproved compound has no established safe or effective human dose, and products sold for “research use only” are not made or tested for people. Talk to a doctor before acting on anything on this site, including before you start, stop or change any medicine or dose. Full medical disclaimer.
Side Effects & Contraindications
Reported Side Effects
The human studies were small and short, so this list reflects how little has been measured, not proven safety.
Contraindications & Cautions
No trial defined who should avoid thymosin β4. These cautions follow from the studies above.
Legal Status
Thymosin β4 is not approved as a medicine anywhere. It has been developed under clinical trial registrations, mainly as eye drops.
Frequently Asked Questions
?Is thymosin β4 the same as TB-500?
Not usually. Thymosin β4 is the natural 43-amino-acid peptide the studies on this page tested. TB-500, as our entry defines it, is a synthetic fragment of about seven amino acids from its middle (Ac-LKKTETQ), although some listings sell full-length thymosin β4 under that name. Findings for one do not transfer automatically to the other.
?Has thymosin β4 been tested in people?
Yes, but in small numbers: a 40-person intravenous safety study, a 9-patient phase 2 dry-eye trial and an 18-patient phase 3 trial in neurotrophic keratopathy, and phase 3 dry-eye trials whose results are not in our evidence set. It has not been tested as an injection for muscle, tendon or general healing.
?What did the eye-drop trials find?
In severe dry eye, 0.1% drops reduced discomfort by 35% and corneal staining by 59% against vehicle at day 56. In neurotrophic keratopathy, 6 of 10 treated patients healed completely at four weeks against 1 of 8 on placebo, a difference that became significant by day 43.
?Could thymosin β4 affect cancer?
The evidence points both ways. It has been linked to metastasis in solid tumours, while in multiple myeloma lower levels went with worse survival and adding it slowed myeloma cells in mice. No study has tested whether taking it changes cancer risk in people.
?What is Ac-SDKP?
A tetrapeptide cut from thymosin β4's first four amino acids, acetylated serine-aspartate-lysine-proline. In a 2007 mouse study it boosted the formation of blood-vessel cells but could not replace thymosin β4 itself.
?Is thymosin β4 approved anywhere?
No. As of September 27, 2026 it is investigational: its eye drops have been through phase 3 trials in dry eye, and a neurotrophic keratopathy trial was listed as recruiting.
References
- [Animal] Girardi M, et al. "Anti-inflammatory effects in the skin of thymosin-beta4 splice-variants." Immunology, 2003;109(1):1-7. PMID: 12709011.
- [Review] Goldstein AL, et al. "Thymosin beta4: actin-sequestering protein moonlights to repair injured tissues." Trends Mol Med, 2005;11(9):421-9. PMID: 16099219.
- [Animal] Smart N, et al. "Thymosin beta4 induces adult epicardial progenitor mobilization and neovascularization." Nature, 2007;445(7124):177-82. PMID: 17108969.
- [Review] Crockford D, et al. "Thymosin beta4: structure, function, and biological properties supporting current and future clinical applications." Ann N Y Acad Sci, 2010;1194:179-89. PMID: 20536467.
- [Human] Ruff D, et al. "A randomized, placebo-controlled, single and multiple dose study of intravenous thymosin beta4 in healthy volunteers." Ann N Y Acad Sci, 2010;1194:223-9. PMID: 20536472.
- [Animal] Caers J, et al. "Thymosin beta4 in multiple myeloma: friend or foe." Ann N Y Acad Sci, 2010;1194:125-9. PMID: 20536459.
- [Human] Sosne G, et al. "Thymosin β4 significantly improves signs and symptoms of severe dry eye in a phase 2 randomized trial." Cornea, 2015;34(5):491-6. PMID: 25826322.
- [Animal] Zhang J, et al. "Thymosin beta4 promotes oligodendrogenesis in the demyelinating central nervous system." Neurobiol Dis, 2016;88:85-95. PMID: 26805386.
- [Animal] Li X, et al. "Effects of exogenous thymosin β4 on carbon tetrachloride-induced liver injury and fibrosis." Sci Rep, 2017;7(1):5872. PMID: 28724974.
- [Human] Jiang Y, et al. "Serum thymosin beta4 as a noninvasive biomarker in patients with nonalcoholic steatohepatitis." Rev Esp Enferm Dig, 2018;110(1):19-24. PMID: 29271227.
- [Animal] Scheller I, et al. "Thymosin β4 is essential for thrombus formation by controlling the G-actin/F-actin equilibrium in platelets." Haematologica, 2022;107(12):2846-2858. PMID: 34348450.
- [Human] Sosne G, et al. "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). PMID: 36613994.
- [Animal] Hao M, et al. "Upregulated Tβ4 expression in inflammatory bowel disease impairs the intestinal mucus barrier by inhibiting autophagy in mice." Exp Cell Res, 2024;434(1):113871. PMID: 38049080.
- [pubmed] Guarnera G, et al. "Thymosin beta-4 and venous ulcers: clinical remarks on a European prospective, randomized study on safety, tolerability, and enhancement on healing." Ann N Y Acad Sci, 2007;1112:407-12. PMID: 17495250.
- [clinical-trial] ReGenTree. "Assessment of the Safety and Efficacy of RGN-259 Ophthalmic Solutions for Dry Eye Syndrome" (phase 3, thymosin β4 eye drops). ClinicalTrials.gov NCT03937882; completed October 7, 2021.
Sources & Citations
Every citation on this page is PubMed-indexed and has been checked against its record, except one ClinicalTrials.gov registration, read at the registry. One paper is listed without a card: the design of a venous-ulcer trial that reports no results. Grey Peptides sells nothing and links to no vendor.
Medical Disclaimer: This content is for educational and informational purposes only. It is not medical advice, and nothing here is an instruction to obtain or use any compound. Talk to a licensed clinician about your own situation.