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Natural protein (investigational drug)Human trialsIn clinical trials

Thymosin Beta 4

Thymosin beta 4 (full length, 43 amino acids)

Last reviewed October 27, 2026 · 24 sources · Written for general readers · Compare with another compound →

The bottom line

Thymosin beta 4 is a natural protein found in almost every cell, and it is not the same thing as TB-500, a tiny piece of it. Animal studies of wounds, heart injury and hair growth looked promising, but controlled trials in people with dry eye, eye ulcers, skin ulcers and bedsores mostly failed their main goals, a 2025 heart attack trial found no overall drop in heart damage, and it has never been tested for sports injuries. It is still an unapproved, experimental drug, and athletes are banned from using it.

Where the evidence stands

What it is

Thymosin beta 4 is a protein your body makes on its own. The working form is 43 amino acids long, with a small acetyl group capping the front end. Its main known job inside cells is to grab single actin molecules, the building blocks of the cell skeleton, and hold them so they do not link up into long fibers.[1]

The official drug name for this protein is timbetasin, and the drug version has the same 43 amino acid sequence as the natural one.[2] The company RegeneRx and its partners have tested it in people as a skin gel and as eye drops called RGN-259.[3][4][5]

It is often confused with TB-500. FDA states plainly that thymosin beta 4 and TB-500 are not the same substance. TB-500 is a short fragment of only seven amino acids.[6]

Name
Thymosin Beta 4 (Thymosin beta 4 (full length, 43 amino acids))
Other names
Tβ4, timbetasin, RGN-259 (eye drops)
Type
Natural protein (investigational drug)
Sequence
Ac-SDKPDMAEIEKFDKSKLKKTETQEKNPLPSKETIEQEKQAGES
CAS number
77591-33-4

What it does in the body

Thymosin beta 4 is a natural protein found inside almost every cell. Its main known job is to hold single actin units, the building blocks of the cell skeleton, so they do not link up into fibers, which helps control how cells change shape and move.[1] A small piece cut from its front end, called Ac-SDKP, acts as a separate signal that slows the growth of blood forming stem cells.[1]

This is the biology, area by area. A change measured in the body, like a higher blood level, does not always mean a person feels a benefit. The claims check below covers that.

Cells and tissue repair

Well established

By binding actin, it controls how the cell skeleton is built, which affects cell movement.[1] In lab tests it made skin cells called keratinocytes move faster, one step in closing a wound.[7]

Skin and wounds

Measured in people

In rats with deep skin wounds, new skin growth was 42 to 61 percent faster.[7] In people, a skin gel did not heal bedsores or leg vein ulcers better than placebo: 8 of 54 versus 3 of 18 healed bedsores, and 12 of 55 versus 4 of 17 healed ulcers.[3][4]

Eyes

Measured in people

In a 9 person trial of severe dry eye, eye drops cut eye discomfort by about 35 percent and surface damage by about 59 percent versus placebo drops.[8] Larger trials were less clear: a 72 person trial missed its main goals, and the roughly 700 person ARISE-3 trial found the same improvement on drops and placebo.[9][10]

Heart

Measured in people

In mice, it helped heart muscle cells survive after a blocked artery and reduced scarring.[11][12] In 96 people treated with a stent for a major heart attack, IV drips of a lab made version did not shrink the damaged area overall at 90 days compared with placebo. In the 43 who got their first drip within 8 hours, the damaged area was smaller, a subgroup result that needs confirming.[12]

Bones

Seen in animals

In mice with broken leg bones, the healing bone took 41 percent more force to break and was about 25 percent stiffer after treatment, with more new mineralized bone at three weeks.[13] No human bone study exists.

Hair

Seen in animals

In rats and mice, it stimulated hair growth by activating stem cells in hair follicles.[14] No human hair trial exists.

Absorption and safety in people

Measured in people

In 84 healthy Chinese volunteers given IV drips of a lab made version, it cleared at a steady rate and did not build up with repeated use, and side effects were mild to moderate.[15] In 40 US volunteers given IV doses, side effects were few and mild.[16]

How it works

Most of what is known about how it might help healing comes from cells and animals.

  • Inside cells it binds actin and controls how the cell skeleton is built, which affects how cells move.[1]
  • In lab tests it made skin cells called keratinocytes move faster, which is one step in closing a wound.[7]
  • In mouse heart cells it switched on survival signals and helped heart cells move and live longer after injury.[11]
  • A piece cut from its front end, called Ac-SDKP, is a separate signal that slows the growth of blood forming stem cells in bone marrow.[1]

These mechanisms are well studied in the lab, but they have not translated into clear benefits in human trials.

What the research shows

We keep animal and human evidence apart. A result in animals is a reason to study something in people, not proof that it works in people.

Animal and lab studies

Strong early results in rats and mice

  • In rats with deep skin wounds, the protein put on the skin or injected sped up new skin growth by 42 percent at day 4 and up to 61 percent at day 7 compared with salt water, and wounds had more collagen and new blood vessels.[7]
  • In mice with a blocked heart artery, treatment improved heart function and helped heart muscle cells survive early after the injury.[11]
  • In rats and mice, it increased hair growth by activating the stem cells in hair follicles.[14]
  • In mice given melanoma cells that were made to produce extra thymosin beta 4, tumors grew more blood vessels and spread to the lungs more often.[19]
  • In mice with broken leg bones, treated bone took 41 percent more force to break, was about 25 percent stiffer and had 18 percent more new mineralized tissue at three weeks than with salt water.[13]
  • In mice whose blocked heart artery was reopened, a lab made human version given for a week prevented loss of heart function and scarring at 28 days, and it lowered a blood marker of heart strain.[12]

Limit: Animal wound and heart results looked strong, but the cancer spread finding is a warning sign, and none of these results proved true in people.

Human studies

Several controlled trials, mostly disappointing

  • Safety studies: 40 healthy volunteers in the US got the protein or a placebo by vein, first once and then daily for 14 days, with few, mild to moderate side effects.[16] In China, 84 healthy volunteers got IV drips of a lab made version or placebo for up to 10 days. Side effects were mild to moderate, there were no serious events, and the drug did not build up in the body.[15]
  • Bedsores: in a placebo controlled trial of 72 people treated for up to 84 days, 8 of 54 on the gel healed fully versus 3 of 18 on placebo, so about the same share. Nearly everyone in both groups had some side effect, most often urinary infections and fever, at similar rates.[3]
  • Leg vein ulcers: in a placebo controlled trial of 72 people in Italy and Poland, 12 of 55 on the gel healed by day 84 versus 4 of 17 on placebo, again about the same. Itching and redness were among the reported side effects.[4]
  • Dry eye: in the largest trial, ARISE-3, about 700 people used the eye drops or placebo drops. After about two weeks, eye surface damage and eye discomfort improved by the same amount in both groups. Stinging or pain when putting in drops was a bit more common with the real drug, about 7 percent versus 5 percent.[10]
  • Dry eye, smaller trials (2015): in 9 people with severe dry eye, the drops for 28 days cut eye discomfort by 35 percent and eye surface staining by 59 percent versus placebo. In 72 people with moderate to severe dry eye, the main goals were missed, though discomfort during a dry air test was 27 percent lower. No side effects were seen in the 72 person trial.[8][9]
  • Eye ulcers that would not heal (neurotrophic keratopathy): in a small trial stopped early with only 18 people, 6 of 10 on the drops healed fully at about four weeks versus 1 of 8 on placebo. A later European trial, SEER-3, did not beat placebo on its main goal.[5][18]
  • A larger trial in the United States and Europe, SEER-2, finished seeing patients in August 2026, and results were expected in November 2026.[20]
  • Heart attack (2025): 96 people who had a stent placed for a major heart attack got IV drips of a lab made version or placebo. At 90 days the area of damaged heart did not differ significantly overall. It was smaller only in the 43 people whose first drip came within 8 hours, a subgroup result.[12]
  • No human study has tested it for sports injuries or hair loss.[6]

Limit: Hundreds of people have been studied, but the main goals were mostly missed, and the hopeful results came from very small trials or subgroups.

Claims check

The most common claims about this compound, and what studies actually found. Verdicts rate the evidence, not whether you should use it.

Speeds wound healing

No benefit in trials so far

In rats, it sped new skin growth by 42 to 61 percent.[7] But in people with bedsores treated with a gel for up to 84 days, 8 of 54 healed fully versus 3 of 18 on placebo, about the same share.[3] In people with leg vein ulcers, 12 of 55 healed versus 4 of 17 on placebo, again about the same.[4]

Heals tendon, muscle and sports injuries

Only in animals

No human study has tested the full protein, or its fragment TB-500, for tendon, ligament, muscle or bone injuries. FDA says it has not identified human exposure data for TB-500 at all.[17][6] In mice with broken leg bones, treated bones healed stronger, taking 41 percent more force to break.[13]

Repairs the heart after a heart attack

Mixed results

In mice with a blocked heart artery, treatment improved heart function and helped heart muscle cells survive.[11][12] In a placebo controlled trial of 96 people treated with a stent for a major heart attack, IV drips of a lab made version did not significantly shrink the damaged heart area overall at 90 days. Only a subgroup of 43 people who got the first drip within 8 hours had smaller damage, which needs a new trial to confirm.[12]

Heals the eye surface and dry eye

Mixed results

In a 9 person trial of severe dry eye, eye drops cut discomfort by 35 percent and eye surface damage by 59 percent compared with placebo drops.[8] In a 72 person trial, the main goals were missed, though some later measures improved.[9] In the largest trial, about 700 people with dry eye improved the same on the drops and on placebo.[10] In a small trial stopped early, 6 of 10 people with eye ulcers that would not heal recovered fully on the drops versus 1 of 8 on placebo, but a later European trial missed its main goal.[5][18]

Regrows hair

Only in animals

In rats and mice, thymosin beta 4 stimulated hair growth and activated hair follicle stem cells.[14] No human trial has tested it for hair loss.

Known risks and side effects

  • Possible cancer concern. In mice, extra thymosin beta 4 made melanoma tumors grow more blood vessels and spread more to the lungs. Whether this matters for people is unknown.[19]
  • No proven benefit. Placebo controlled trials in dry eye, bedsores and leg ulcers found it worked about the same as placebo on the main goals.[3][4][10]
  • Side effects reported in trials. In the big dry eye trial, stinging or pain on putting in drops affected about 7 percent on the drug versus 5 percent on placebo.[10] In the skin gel trials, itching and redness were reported, and IV use in healthy volunteers caused few, mild to moderate side effects with no serious events.[4][16][15]
  • Banned in sport. The World Anti Doping Agency bans thymosin beta 4 and its pieces, such as TB-500, at all times, in and out of competition, and the 2027 list keeps that ban.[21][22]
  • Unregulated online products. No thymosin beta 4 product is approved, and FDA's compounding lists do not include the full protein, so anything sold online has no FDA check on what is really inside, how pure it is, or whether it is sterile.[6][23]

Regulatory status

Today: Thymosin beta 4 is not approved anywhere in the United States. Its eye drop form is an investigational drug for neurotrophic keratitis, a rare corneal disease, with FDA orphan drug status for that use, and results from its latest phase 3 trial were expected in November 2026.[20] It is not on FDA's compounding category lists.[23] The October 2026 HHS compounding policy covers only the short TB-500 fragment, not the full protein.[24] WADA bans it at all times, and the 2027 list names thymosin beta 4 and its derivatives under growth factors.[21][22]

  1. Results posted for the phase 2 leg ulcer trial showed similar healing on the gel and on placebo.[4]
  2. The ARISE-3 dry eye trial of about 700 people was completed. Its main measures did not differ between drug and placebo.[10]
  3. The European SEER-3 eye ulcer trial was reported to have missed its main goal.[18]
  4. A 96 person heart attack trial of a lab made version is published, with no overall reduction in heart damage.[12]
  5. Patient visits ended in the SEER-2 eye ulcer trial, with results expected in November 2026.[20]
  6. HHS issues a compounding enforcement policy that covers TB-500 acetate but not full thymosin beta 4.[24]
  7. The 2027 WADA list takes effect and still bans thymosin beta 4 and its derivatives.[22]

Sources

  1. UniProt Consortium. UniProtKB entry P62328, Thymosin beta-4 (TMSB4X, human). UniProt P62328
  2. U.S. Food and Drug Administration and NCATS. Global Substance Registration System: timbetasin (UNII 2D5MRE3SSY) and thymosin beta-4 records. FDA GSRS
  3. RegeneRx Biopharmaceuticals. A Randomized, Double-Blind, Placebo-Controlled, Dose Response Study of the Safety and Efficacy of Thymosin Beta 4 in the Treatment of Patients With Pressure Ulcers. ClinicalTrials.gov NCT00382174 (results posted). NCT00382174
  4. RegeneRx Biopharmaceuticals. Study of Thymosin Beta 4 in Patients With Venous Stasis Ulcers. ClinicalTrials.gov NCT00832091 (results posted March 2010). NCT00832091
  5. ReGenTree LLC. Phase 3 study of RGN-259 ophthalmic solution for neurotrophic keratopathy (SEER-1). ClinicalTrials.gov NCT02600429 (results posted). NCT02600429
  6. U.S. Food and Drug Administration. FDA Briefing Document for TB-500-Related Bulk Drug Substances. Pharmacy Compounding Advisory Committee Meeting, July 23-24, 2026 (memo dated May 15, 2026). FDA briefing document
  7. Malinda KM, Kleinman HK, Sidhu GS, Mani H, Banaudha K, Maheshwari RK, Goldstein AL. Thymosin beta4 accelerates wound healing. J Invest Dermatol. 1999;113(3):364-368. PMID 10469335
  8. Sosne G, Dunn SP, Kim C. Thymosin beta4 significantly improves signs and symptoms of severe dry eye in a phase 2 randomized trial. Cornea. 2015;34(5):491-496. PMID 25826322
  9. Sosne G, Ousler GW. Thymosin beta 4 ophthalmic solution for dry eye: a randomized, placebo-controlled, Phase II clinical trial conducted using the controlled adverse environment (CAE) model. Clin Ophthalmol. 2015;9:877-884. PMID 26056426
  10. ReGenTree LLC. A Multi-Center, Randomized, Double Masked, Placebo Controlled Clinical Study to Assess the Safety and Efficacy of RGN-259 Ophthalmic Solutions for the Treatment of Dry Eye (ARISE-3). ClinicalTrials.gov NCT03937882 (results posted). NCT03937882
  11. Bock-Marquette I, Saxena A, White MD, DiMaio JM, Srivastava D. Thymosin beta4 activates integrin-linked kinase and promotes cardiac cell migration, survival and cardiac repair. Nature. 2004;432(7016):466-472. Nature 2004
  12. Zhang Y, Dong Q, Bian X, et al. Recombinant human thymosin beta 4 improves ischemic cardiac dysfunction in mice and patients with acute ST-segment elevation myocardial infarction after reperfusion. Cardiovasc Res. 2025;121(17):2747-2758. PMID 41229390
  13. Brady RD, Grills BL, Schuijers JA, et al. Thymosin beta4 administration enhances fracture healing in mice. J Orthop Res. 2014;32(10):1277-1282. PMID 25042765
  14. Philp D, Nguyen M, Scheremeta B, et al. Thymosin beta4 increases hair growth by activation of hair follicle stem cells. FASEB J. 2004;18(2):385-387. PMID 14657002
  15. Wang X, Liu L, Qi L, et al. A first-in-human, randomized, double-blind, single- and multiple-dose, phase I study of recombinant human thymosin beta4 in healthy Chinese volunteers. J Cell Mol Med. 2021;25(17):8222-8228. PMID 34346165
  16. Ruff D, Crockford D, Girardi G, Zhang Y. A randomized, placebo-controlled, single and multiple dose study of intravenous thymosin beta4 in healthy volunteers. Ann N Y Acad Sci. 2010;1194:223-229. DOI 10.1111/j.1749-6632.2010.05474.x
  17. U.S. Food and Drug Administration. Certain Bulk Drug Substances for Use in Compounding that May Present Significant Safety Risks. Content current as of April 22, 2026. FDA safety risks page
  18. Harp MD. HLB Therapeutics misses primary endpoint in phase 3 SEER-3 trial of RGN-259. Ophthalmology Times. July 1, 2025. News report
  19. Cha HJ, et al. Role of thymosin beta4 in tumor metastasis and angiogenesis. J Natl Cancer Inst. 2003;95(22):1674-1680. PMID 14625258
  20. EDAILY. HLB Therapeutics completes last patient visit in global phase 3 SEER-2 trial of RGN-259. August 26, 2026. News report
  21. World Anti-Doping Agency. World Anti-Doping Code International Standard: Prohibited List 2026. Effective 1 January 2026. WADA 2026 List, S2
  22. World Anti-Doping Agency. International Standard: The 2027 Prohibited List. Comes into effect 1 January 2027. WADA 2027 List, S2.3
  23. U.S. Food and Drug Administration. Bulk Drug Substances Nominated for Use in Compounding Under Section 503A of the FD&C Act (category lists). Updated May 14, 2026. FDA 503A category lists
  24. U.S. Department of Health and Human Services. HHS Enforcement Discretion Policy Regarding 503A Compounding Using Certain Peptide Bulk Drug Substances, Appendices A and B. October 2026. HHS policy, Appendix B
Medical disclaimer

This page is educational information only. It is not medical advice and does not replace care from a licensed medical provider. Do not use this page to decide whether to take any substance. Talk to a licensed medical provider first.

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