A 43-amino-acid protein in nearly every cell, tested as eye drops, skin gel and IV infusion. Large dry eye trials missed their main goals.
Also called Thymosin beta 4, Thymosin β4, Tβ4, TB4, Tb4, TMSB4X, Fx (actin-sequestering peptide), RGN-259, RGN-137, RGN-352, NL005
When Thymosin Beta-4's status or evidence changes, it's in the next morning's email.
Thymosin beta-4 is a natural protein of 43 amino acids, found inside almost every human cell. It was first isolated from calf thymus, but its main job is mechanical: it holds onto single actin units so a cell has a ready supply when it needs to move or change shape. It also leaves cells after injury, and in animals it sped skin, eye and heart repair. TB-500 is a different molecule: a synthetic 7-amino-acid piece (Ac-LKKTETQ, positions 17 to 23) sold on the gray market. FDA states the two are not the same substance, and the human trial results below were all done with the full protein, never with TB-500.
The full protein has been through more human testing than most peptides people ask about, and most of those trials did not show a benefit. RegeneRx and its partners ran eye drops (RGN-259), a skin gel (RGN-137) and an IV form (RGN-352). Three late-stage dry eye trials enrolled 317, 601 and 700 people; the two with posted results show no difference from placebo on their primary endpoints, and the company announced in March 2021 that ARISE-3 did not meet its primary outcome measures. In neurotrophic keratopathy, a hard-to-heal corneal wound, the small SEER-1 trial fully healed the corneal defect in 6 of 10 treated patients vs 1 of 8 on placebo (p = 0.0656, not statistically significant), and a 70-patient phase 3, SEER-2, was recruiting in 2026. Skin gel trials in leg ulcers, pressure ulcers and epidermolysis bullosa were safe but did not prove healing.
The IV form was given to 40 healthy volunteers at up to 1,260 mg a day for 14 days with no dose-limiting toxicity. RegeneRx's heart attack trial was withdrawn, but a Chinese company, Beijing Northland Biotech, has run phase 2 heart attack trials of a recombinant version (NL005) and posted a 189-patient phase 2 in May 2026. Thymosin beta-4 is not approved anywhere, and WADA bans 'thymosin-beta-4 and its derivatives' at all times.
What it binds to, and what happens inside the body after that, step by step.
Inside cells, thymosin beta-4 binds single actin units one to one and stops them from joining into filaments, keeping a reserve that cells draw on to move. In resting platelets most unpolymerized actin is held this way. Released after injury, it promoted cell migration, new blood vessels and reduced inflammation in animal wounds, and in mouse hearts it activated integrin-linked kinase and the survival kinase Akt after a heart attack. Which of these effects explain any human result is not established.
Human results and animal results are kept apart below. A result in rats tells you a mechanism is plausible. It does not tell you what happens in a person.
Most of thymosin beta-4's randomized human trials did not show a benefit on their main goals. The A grade reflects that there are multiple randomized trials, not that they proved it works: three phase 3 dry eye trials did not show a primary-endpoint benefit, skin gel trials were too small to prove healing, and the neurotrophic keratopathy result narrowly missed significance in 18 patients. Safety has looked like placebo across eye, skin and IV studies. None of these data apply to TB-500.
700 adults with dry eye, 0.1% eye drops four times daily for 14 days
Did not meet its primary endpoints; corneal staining change was -0.41 vs -0.46 on placebo. RegeneRx reported that ocular grittiness, a secondary endpoint, improved (p = 0.0104 at 1 week); this figure is not in the posted registry results.
Read the study601 adults with dry eye, four times daily for 28 days
Posted results show ocular discomfort change of 0.07 vs -0.04 and staining change of 0.07 vs -0.01, no benefit over placebo on either primary endpoint.
Read the study18 patients with stage 2 or 3 neurotrophic keratopathy, eye drops 5 times daily for 4 weeks
Complete healing at 4 weeks in 6 of 10 treated vs 1 of 8 on placebo (p = 0.0656); the trial closed early for a business decision.
Read the study40 healthy adults in 4 cohorts, 42 to 1,260 mg IV daily for 14 days
No dose-limiting toxicity or serious adverse events; adverse events were infrequent and mild or moderate.
Read the study73 patients with venous leg ulcers at 8 European sites, topical gel
Safety was comparable to placebo; the authors said 0.03% gel may speed healing, with about 25% healed within 3 months.
Read the study72 adults with moderate to severe dry eye, 28 days
Both primary endpoints missed; discomfort during a dry-air challenge was 27% lower than placebo, a secondary endpoint.
Read the studyMice after coronary artery ligation; embryonic and postnatal heart cells
Treatment raised integrin-linked kinase and Akt activity, improved early heart cell survival and improved heart function.
Read the studyWhen a new Thymosin Beta-4 study in people is published, the daily email covers it.
These are questions the published research has not answered.
No product containing Thymosin Beta-4 is approved as a medicine by the FDA.
No regulator has approved thymosin beta-4. SEER-2 (NCT05555589), a phase 3 of RGN-259 eye drops in about 70 patients with neurotrophic keratopathy at sites in the US, Italy, Poland and Spain, was recruiting in 2026, sponsored by ReGenTree, a joint venture of RegeneRx and HLB Therapeutics. Beijing Northland Biotech posted a 189-patient phase 2 of recombinant thymosin beta-4 (NL005) for acute myocardial infarction (NCT07586865) in May 2026, not yet recruiting. WADA S2.3 names 'Thymosin-ß4 and its derivatives e.g. TB-500', prohibited in and out of competition.
When the FDA or WADA changes Thymosin Beta-4's status, it's in the next morning's email.
No human half-life has been published for Thymosin Beta-4, so we do not draw a clearance curve. Half-lives quoted online for it are usually estimates from animal data.
This table records the doses that trials and approved labels used, with the source for each row. It is a record of research, not a recommendation, and it is open to free members.
Which symptoms are expected and which mean stop and get help: side effects and red flags.
Tap a claim to see what the studies found.
Thymosin beta-4 is a 43-amino-acid natural protein. TB-500 is a synthetic acetylated 7-amino-acid fragment, Ac-LKKTETQ. FDA's 2026 briefing states they are not the same substance.
Phase 3 ARISE-3 did not meet its primary outcome measures, and ARISE-2's posted primary results show no difference from placebo. Benefits appeared only in secondary or post-hoc analyses.
The heart repair findings are from mice. The planned US heart attack trial was withdrawn, and Chinese phase 2 trials have not published results.
Gel trials in 73 venous ulcer and 72 pressure ulcer patients were safe and suggested faster healing in those who healed, but neither proved a benefit. An epidermolysis bullosa trial ended with 8 vs 5 healed, not significant.
These pairings come up often. Almost none have been tested together in a trial, and the evidence note on each says so plainly.
Sold for tendon and joint injuries, often with 'TB4' and 'TB-500' used interchangeably, on the theory that one drives cell migration and the other blood vessel growth.
EvidenceNo controlled human study has tested the combination. The full protein's human data are eye drops, gel and IV infusions for other conditions, not injections for injuries.
No published consumer stability data. Trial eye drops were a preservative-free sterile solution.
No published stability data for injectable solutions.
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