Thymosin beta-4

Also called Tβ4, TB4, TB-4, RGN-259 (eye drops), RGN-137 (skin gel), RGN-352 (injection), NL005 (recombinant). Not the same as TB-500^{1, 7, 22}

Thymosin beta-4 is a natural 43-amino-acid protein found in almost every cell. TB-500, as FDA defines it, is a synthetic copy of a 7-amino-acid piece of it, and FDA says the two "are not the same substance."¹ Some websites use the two names interchangeably, so a label alone doesn't tell you which one a product claims to be.^{1, 29} Unlike TB-500, full-length thymosin beta-4 has been tested in people: as eye drops for dry eye and corneal ulcers, as a skin gel for wounds and by IV.^{7, 16, 20} Safety looked clean, but three dry-eye trials and two corneal ulcer trials missed their main goals, and two skin-ulcer trials found no significant healing benefit.^{7, 12, 33, 16, 17} A third corneal ulcer trial, SEER-2, has finished, with results due.¹³ No trial has tested it for muscle or tendon injuries. Because it is longer than 40 amino acids, it meets FDA's definition of a protein, and protein drugs can't be compounded under the usual pharmacy exemptions.^{3, 4} What is sold online as "TB4" is a research chemical.

Looking for the 7-amino-acid fragment? See our TB-500 guide. See every peptide's status in the regulatory tracker

At a glance

Where to get it

MoveWell sells no peptides and links only to legal options. All options

Molecular structure

A chain of 43 amino acids, too large to draw atom by atom here. This schematic shows the sequence. TB-500 copies residues 17 to 23 and adds an acetyl cap.

Residues 17 to 23, the stretch TB-500 copies

Download imageDownload with MoveWell branding

SDKPDMAEIEKFDKSKLKKTETQEKNPLPSKETIEQEKQAGES

Length

43 amino acids, acetylated at the start

Formula

C212H350N56O78S

Weight

4,963.5 g/mol

UniProt

P62328 (gene TMSB4X)

TB-500

Residues 17–23, Ac-LKKTETQ

Ac-SDKP

Residues 1–4

Sequence from UniProt P62328, with the starting methionine removed, as in the mature protein.² Formula and weight are our calculation from that sequence plus the N-terminal acetyl group. Outlined residues 1 to 4 form Ac-SDKP, a separate natural fragment.³²

What it is

Thymosin beta-4 was first isolated from calf thymus. FDA describes it as a "full-length (43-amino acid) peptide" that is "ubiquitously expressed" and found in blood, saliva, tears and wound fluid.¹ Inside cells it binds single actin units and stops them from joining into filaments.²

Two smaller pieces of it are sold or studied on their own:

• TB-500. A synthetic copy of residues 17 to 23 with an acetyl cap. FDA says websites "often use the terms interchangeably," but the two are different substances, and FDA found no study in which TB-500 was given to people.¹ Australia's anti-doping agency lists "TB-500" as another name for thymosin beta-4, and WADA bans both under one entry, so a vial labeled TB-500 may claim to be either.^{28, 29} See our TB-500 guide.
• Ac-SDKP. Residues 1 to 4, released naturally from the protein and broken down by the blood-pressure enzyme ACE.³²

US developer RegeneRx built three products on full-length thymosin beta-4: RGN-259 eye drops, RGN-137 skin gel and RGN-352 for injection.^{7, 19} Its eye-drop partner ReGenTree, a US subsidiary of Korea's HLB Therapeutics, runs the eye trials.¹³ In China, a recombinant version called NL005 is in heart-attack trials.^{22, 24}

How it works

Most of what's known comes from cells and animals.

• Actin and cell movement. By holding actin, it helps control cell shape and movement. FDA names residues 17 to 22 (LKKTET) as its actin-binding region.^{1, 2}
• Wound closure in rats. On the skin or by injection, it sped skin wound healing by 42% at day 4 and up to 61% at day 7, with more new blood vessels.²⁵
• Heart in mice. After a blocked heart artery, it improved heart-cell survival and heart function.²⁶
• Cornea. RegeneRx's eye program rests on its claimed ability to protect the cornea and help it heal.¹³

In people, an IV study in healthy volunteers found blood levels rose in line with the dose.²⁰ That tells us how it is handled, not whether it heals anything.

Evidence map

Each use is graded separately. These grades are for full-length thymosin beta-4. TB-500 has its own page and lower grades.

Scale: 5 FDA-approved, 4 strong human, 3 limited human, 2 animal or lab only, 1 anecdotal

Key studies

RegeneRx says its three dry-eye trials showed significant effects in some subgroups, but FDA said that was not enough to support approval.⁷ Most of these results come from sponsor press releases, not peer-reviewed papers.

By goal

Pick your goal to see what the evidence supports.

Sources: FDA,¹ HHS Appendix B,⁵ RegeneRx.⁷

How it's used

We don't give a dose. No dose has been approved for any use.

In trials it was given as eye drops, a gel on wounds or an IV infusion.^{7, 16, 20} Online it is sold as a powder for injection under the skin, a route no published efficacy trial has tested. We report this as information, not instruction.

Side effects and who should avoid it

• In trials, mild. In an IV study of 40 healthy adults, some given placebo, side effects were "infrequent and mild or moderate," with no serious events.²⁰ In the largest dry-eye trial, the most common eye side effect was mild pain when the drops went in.¹¹ The skin gel trials reported no drug-related serious side effects.^{16, 17}
• Cancer, in theory. No study shows it causes cancer. But it is often highly expressed in some tumors, and in breast cancer samples higher levels went with spread to lymph nodes. That link is a correlation only.²⁷ It also promotes new blood vessels in animals.²⁵ People with cancer should talk to their oncologist.
• Online products. Research vials have no FDA quality checks, and FDA found identity problems even in material submitted for TB-500's review.^{1, 31}
• Unknowns. No published trial has tested repeated injections under the skin for months.
• Who should avoid it. Tested athletes, since it is banned at all times.²⁸ Anyone pregnant or breastfeeding, since there is no safety data.

Compared with

Sources: FDA,¹ HHS Appendix B,⁵ WADA.²⁸ See our TB-500 and BPC-157 guides, and BPC-157 vs TB-500.

Stacks

The "Wolverine" stack pairs BPC-157 with TB-500, not with full-length thymosin beta-4, and some sellers swap the names. We found no study of thymosin beta-4 combined with BPC-157. Any blend is outside the HHS policy, which allows one active ingredient per product.⁵ See our Wolverine stack guide.

Myths vs evidence

All options

MoveWell sells no peptides. Checked November 1, 2026

Clinical trials

The SEER-2 trial of RGN-259 eye drops for neurotrophic keratopathy ran at US and European sites and has finished.^{13, 15} A European corneal trial, SEER-3, missed its main goal in 2025.³³ The sponsor says it is preparing another dry-eye trial, ARISE-4.¹³ Search ClinicalTrials.gov for "thymosin beta 4" or "RGN-259" for open studies.

Licensed compounding, with a prescription

None. Since March 23, 2020, a chain of more than 40 amino acids counts as a protein, and FDA treats protein drugs as biological products.³ FDA says biological products "are not eligible for the exemptions for compounded drugs under sections 503A and 503B."⁴ Thymosin beta-4 is also not one of the six peptides in the Oct 2026 HHS policy.⁵ In 2022 FDA framed its feedback on the dry-eye program around a biologics license application, the approval route for biological products.⁷ Telehealth pages offering "TB4" have no lawful product to fill it with, so we don't link them.

Research use only products

We don't list sellers. In August 2026 FDA warned five peptide sellers that a "research use only" label doesn't excuse products clearly meant for people.³¹

Legal status history

Newest first.

1. Current statusNov 1, 2026Not approved in the US or, as far as we found, anywhere else. Investigational in eye trials. Not compoundable. Banned in sport.^{4, 13, 28}
2. Next · Nov 2026SEER-2 topline results, which the sponsor expected about two to three months after the last patient visit.^{13, 14}
3. Oct 28, 2026HHS peptide policy covers TB-500 acetate by injection. Full-length thymosin beta-4 is not included.⁵
4. Aug 26, 2026Sponsor announces SEER-2's last patient visit. It says the trial ran at 50 sites in the US, Italy, Spain and Poland.¹³
5. Apr 22, 2026FDA's compounding list names only the TB-500 fragment, as "nominated but withdrawn." Full-length thymosin beta-4 does not appear.⁶
6. Jun 2025SEER-3, a European phase 3 trial in neurotrophic keratopathy, misses its primary endpoint.^{33, 34}
7. Apr 2023SEER-2 starts dosing.¹⁵
8. Mar 2021ARISE-3, the third dry-eye phase 3, misses its primary outcomes.¹¹
9. Mar 23, 2020FDA's protein definition takes effect: over 40 amino acids is a biological product, outside compounding exemptions.^{3, 4}
10. Jan 2016Sport's top court bans 34 Essendon players for thymosin beta-4 use.³⁰

See every peptide's status in the regulatory tracker.

FAQ

Related guides

References