TB-500 (Thymosin Beta-4 Fragment): Mechanisms & Research

Few peptides have traveled a stranger path to scientific interest than TB-500. It started in racehorse barns, where trainers used it to speed recovery from tendon injuries. It spread through bodybuilding forums, where athletes swapped dosing protocols. And it drew the attention of cardiologists, who noticed that the parent molecule it comes from — Thymosin Beta-4 — could coax damaged heart tissue into repairing itself.

Today, TB-500 sits at the intersection of legitimate regenerative medicine research and a largely unregulated consumer market. The science behind it is genuinely interesting. The hype around it often outpaces the evidence. This guide separates one from the other.

Quick Facts

Property	Detail
Full name	Fequesetide (Ac-LKKTETQ)
Parent molecule	Thymosin Beta-4 (Tβ4)
Type	Synthetic heptapeptide (7 amino acids)
Amino acid sequence	Leu-Lys-Lys-Thr-Glu-Thr-Gln
Molecular weight	~889 g/mol (acetylated form)
Molecular formula	C₃₈H₆₈N₁₀O₁₄
Parent molecule weight	~4,963 Da (43 amino acids)
Primary mechanism	G-actin sequestration → cell migration
FDA status	Not approved for human use
WADA status	Prohibited (S2 category)
Research stage	Primarily preclinical; limited human trials on full-length Tβ4

What Is TB-500?

TB-500 is a synthetic peptide built from seven amino acids. Its formal name is fequesetide, though almost nobody uses that term outside of regulatory documents and anti-doping reports. It corresponds to amino acids 17 through 23 of a much larger, naturally occurring protein called Thymosin Beta-4 (Tβ4).

That seven-amino-acid stretch — LKKTETQ — is the active site responsible for Thymosin Beta-4's ability to bind actin, the structural protein that forms the internal scaffolding of every cell in your body. TB-500 is essentially a concentrated version of Tβ4's most functionally important region, with an acetyl group attached to its N-terminus for stability.

First isolated from the thymus gland in the 1960s, Thymosin Beta-4 itself is one of the most abundant peptides in the human body. It shows up in nearly every cell type and tissue — blood platelets, wound fluid, developing embryos, and tears. TB-500 was developed as a way to deliver the regenerative properties of that active site in a smaller, more stable package.

The peptide gained its earliest practical use in veterinary medicine, particularly in the horse racing industry, where it was used to treat tendon and ligament injuries in racehorses. From there, it migrated into human athletic and wellness circles, despite never receiving regulatory approval for use in people.

TB-500 vs. Thymosin Beta-4: The Relationship Explained

This distinction matters, and most online sources get it wrong. TB-500 and Thymosin Beta-4 are related but they are not the same molecule.

Thymosin Beta-4 is the full-length endogenous peptide: 43 amino acids, molecular weight around 4,963 Da, produced naturally by your body. It has multiple functional domains, each responsible for different biological activities:

Amino acids 1–4 (Ac-SDKP): Anti-inflammatory and anti-fibrotic activity
Amino acids 1–15: Anti-apoptotic (cell survival) and cytoprotective effects
Amino acids 17–23 (LKKTETQ): Actin binding, cell migration, angiogenesis, wound healing

TB-500 is a synthetic copy of just that third region — the LKKTETQ actin-binding motif — with an N-terminal acetyl group added for metabolic stability. At 7 amino acids and roughly 889 Da, it is about one-sixth the size of the parent molecule.

Here is the practical consequence: most published research has been conducted on full-length Thymosin Beta-4, not on TB-500 specifically. When you read about "TB-500 benefits" online, the evidence cited almost always comes from studies using the full 43-amino-acid peptide. The fragment retains the actin-binding and cell-migration properties, but it lacks the additional anti-inflammatory (Ac-SDKP) and anti-apoptotic domains that contribute to Tβ4's broader biological activity.

A 2024 study published in the Journal of Pharmaceutical and Biomedical Analysis added another wrinkle: researchers found that TB-500's wound-healing activity in cell culture may actually come from one of its metabolites, Ac-LKKTE, rather than from the parent TB-500 molecule itself. This finding, if confirmed, would mean the body breaks TB-500 down into a slightly shorter fragment that does the real work.

For a deeper look at the full-length peptide and all its functional regions, see the complete Thymosin Beta-4 research profile.

How TB-500 Works: Mechanism of Action

The Actin Connection

To understand TB-500, you need to understand actin. Actin is the most abundant protein inside most human cells. It forms filaments — long chains — that act as the cell's internal skeleton, giving cells their shape and enabling them to move, divide, and contract.

Actin exists in two states. Globular actin (G-actin) is the unassembled monomer form — individual building blocks floating around inside the cell. Filamentous actin (F-actin) is the assembled form — those building blocks snapped together into structural chains. The constant assembly and disassembly of these filaments is what allows a cell to crawl toward an injury site, divide to produce new tissue, or contract during muscle movement.

Thymosin Beta-4 — and by extension, TB-500 — is the body's primary G-actin sequestering protein. It binds to G-actin monomers in a 1:1 ratio, preventing them from polymerizing into filaments prematurely. In most cell types, Tβ4 sequesters 40 to 50 percent of the total G-actin pool. This creates a ready reserve: when a cell receives a migration signal (say, from an injury), it releases the sequestered actin, which rapidly assembles into filaments that push the cell forward.

Think of it like a warehouse full of building materials. TB-500 keeps those materials organized and ready. When a construction signal arrives, everything is immediately available for rapid assembly.

Beyond Actin: Downstream Effects

The actin-sequestering mechanism triggers a cascade of downstream effects:

Cell migration. By controlling when and where actin polymerizes, TB-500 helps cells migrate toward injury sites more efficiently. Research using human umbilical vein endothelial cells showed that both full-length Tβ4 and the LKKTETQ fragment promote cell migration at concentrations around 50 nM, with near-identical activity.

Angiogenesis. TB-500 promotes the growth of new blood vessels. This is one of its most well-documented effects and also one of its most debated — blood vessel growth is beneficial for healing but raises theoretical concerns about tumor growth (more on that in the safety section).

ILK/Akt signaling. TB-500 activates integrin-linked kinase (ILK), which in turn activates Akt — a protein kinase with wide-ranging effects on cell survival, growth, and motility. This pathway is central to how TB-500 protects cells from dying after injury.

Matrix metalloproteinase activation. TB-500 promotes the activity of enzymes (MMPs) that break down the extracellular matrix surrounding cells. This sounds destructive, but it is necessary: cells need to clear a path through surrounding tissue to reach injury sites, and the breakdown of matrix molecules can release additional chemical signals that guide repair.

An Intrinsically Disordered Protein

Structurally, Thymosin Beta-4 is what biochemists call an "intrinsically unstructured protein" (IUP). In solution, it has almost no fixed three-dimensional shape — at most, about six residues form a brief alpha helix. The rest of the molecule is floppy and disordered.

This is not a defect. It is a feature. Because Tβ4 only acquires a defined shape when it binds to actin (or potentially other partners), it can interact with multiple different proteins depending on the context. Researchers describe this as "moonlighting" — the same molecule performing different jobs in different situations. It may explain why Thymosin Beta-4 appears to affect so many different biological processes, from wound healing to cardiac repair to neuroprotection.

Research Evidence by Category

Wound Healing and Tissue Repair

This is the area with the strongest and most consistent evidence.

In a foundational 2003 mouse study, topical application of Thymosin Beta-4 accelerated wound healing in healthy, diabetic, and aged mice. Wounds treated with Tβ4 showed faster re-epithelialization, increased blood vessel density, reduced inflammation, and earlier tissue maturation. Treated wounds also healed with less scarring than controls.

A particularly notable finding: in aged mice, which normally heal slowly, Tβ4 administration brought healing rates up to levels typically seen in younger animals. Even more striking, researchers demonstrated that the LKKTETQ fragment alone — essentially TB-500 — could reproduce this age-related healing improvement.

The mechanisms behind wound healing involve several of TB-500's downstream effects working together: cell migration brings keratinocytes and fibroblasts to the wound site, angiogenesis provides blood supply, and matrix metalloproteinase activity remodels the tissue as it heals. Tβ4 also promotes collagen deposition, which is the structural foundation of repaired skin.

Human clinical trials on wound healing (using full-length Tβ4) have shown mixed results. Phase 2 trials in patients with pressure ulcers, venous stasis ulcers, and epidermolysis bullosa showed trends toward faster healing — a mean healing time of 22 days versus 57 days for placebo in one trial — but the results did not always reach statistical significance. The peptide was consistently found to be safe and well-tolerated.

Cardiac Repair

Some of the most exciting Thymosin Beta-4 research involves the heart.

In a landmark series of experiments, researchers at University College London discovered that systemic administration of Tβ4 could reactivate embryonic developmental programs in the adult mouse heart. After coronary artery ligation (an induced heart attack), Tβ4-treated mice showed reduced infarct size, improved cardiac function, and increased blood vessel growth within the damaged tissue. The peptide both protected existing heart muscle cells from dying and activated resident cardiac progenitor cells — essentially coaxing the heart to repair itself using mechanisms it last employed during embryonic development.

What made this finding remarkable was that the degree of improvement was similar whether Tβ4 was delivered directly into the heart or given systemically via injection, suggesting the peptide naturally found its way to damaged cardiac tissue through the bloodstream.

Subsequent studies in rodent and canine models confirmed significant increases in ejection fraction (a measure of how efficiently the heart pumps), normalization of heart size, and increased vascular density in Tβ4-treated animals.

The human data is still limited. A Phase I clinical trial in 40 healthy volunteers found that intravenous Tβ4 at doses up to 1,260 mg daily for 14 days was well-tolerated with no dose-limiting toxicity — an important first step toward cardiac applications. A first-in-human study in Chinese volunteers using recombinant human Tβ4 (NL005) confirmed safety across a range of doses. Clinical trials specifically targeting acute myocardial infarction have been proposed based on these safety results.

Neurological Protection and Repair

Thymosin Beta-4 is naturally expressed throughout the mammalian brain — in the hippocampus, cerebral cortex, amygdala, dentate gyrus, and in some microglia. Its presence in neural tissue is not incidental. It plays active roles in synapse formation, neuronal migration, axon growth, and the structural changes that underlie learning and memory.

Traumatic brain injury (TBI): In rat models, Tβ4 administered 6 hours after induced brain injury reduced cortical lesion volume, prevented hippocampal cell loss, and improved functional recovery. These benefits appeared to involve both direct neuroprotection (preventing cells from dying) and neurorestorative effects (promoting repair of damaged circuits).

Spinal cord injury: Rats treated with Tβ4 after mild spinal cord compression showed markedly improved behavioral scores compared to saline-treated controls. The treated animals had significantly more surviving neurons and oligodendrocytes — the cells that produce myelin, the insulating sheath around nerve fibers. Tβ4 also protected spinal cord neural stem cells from oxidative stress through the TLR4/MyD88 signaling pathway.

Stroke: In a rat embolic stroke model, Tβ4 given 24 hours after the event improved neurological outcomes, with significant functional improvement visible by day 14. Interestingly, researchers found that Tβ4 acted as a neurorestorative agent in young rats but as a neuroprotectant in aged rats — suggesting its mechanism shifts depending on the biological context.

Multiple sclerosis models: Tβ4 promoted differentiation of oligodendrocyte progenitor cells, which produce myelin. This resulted in improved myelination and axonal repair in experimental autoimmune encephalomyelitis (EAE), the standard animal model for multiple sclerosis.

Alzheimer's disease: Overexpression of Tβ4 in APP/PS1 mice (a genetic Alzheimer's model) reduced brain amyloid-beta accumulation, reversed unhealthy states of microglia and astrocytes, and improved cognitive behavior.

For comparison with other peptides being studied for neuroprotective properties, see the research profiles for Semax and Selank.

Eye Health

The ophthalmic research on Thymosin Beta-4 is among the most clinically advanced, thanks largely to the work of RegeneRx Biopharmaceuticals and their drug RGN-259 (a topical eye drop containing Tβ4).

A Phase 2 randomized clinical trial in 72 subjects with moderate-to-severe dry eye tested 0.1% Tβ4 eye drops against placebo over 28 days. While the primary endpoints did not reach significance, the treatment group showed a 27% reduction in discomfort scores (P=0.0244) and statistically significant improvements in central and superior corneal staining (P=0.0075 and P=0.0210, respectively). No adverse events were reported.

For neurotrophic keratitis — a serious condition where the cornea loses sensation and fails to heal — a Phase 3 clinical trial found that 60% of patients treated with RGN-259 achieved complete corneal healing. The FDA granted RGN-259 Orphan Drug status for this condition in 2013, and additional Phase 3 trials are underway.

Musculoskeletal Recovery

TB-500's original use case — and still one of the most common reasons people seek it out — is recovery from muscle, tendon, and ligament injuries.

In a 6-month mouse study, Tβ4 improved skeletal muscle fiber regeneration in dystrophin-deficient mice (a model for Duchenne muscular dystrophy). The peptide's effects on actin dynamics appear directly relevant here: by helping cells migrate to damaged areas and by reducing scar tissue formation, Tβ4 may create conditions that favor functional muscle regeneration over fibrotic scarring.

Studies in experimental models of skeletal muscle injury have shown accelerated healing, reduced fibrosis, and improved functional recovery with Tβ4 treatment. Its effects on connective tissue — promoting collagen remodeling in tendons and ligaments — add another dimension to its potential for musculoskeletal applications.

This is the area where TB-500 is most commonly compared to BPC-157, another peptide studied for tissue repair. The comparison is covered in more detail below.

Hair Growth

Thymosin Beta-4 has a documented effect on hair follicle stem cells. In mouse models, Tβ4 accelerated hair growth by promoting the migration and differentiation of hair follicle stem cells. The active region responsible for this effect maps to the same LKKTETQ actin-binding domain that TB-500 represents.

While this research is still early — no controlled human trials have been published specifically on TB-500 or Tβ4 for hair loss — the mechanism makes biological sense. Hair follicle cycling depends on stem cell migration and differentiation, both of which are processes that Tβ4 directly influences.

Human Clinical Trial Data

The human evidence for Thymosin Beta-4 (not TB-500 specifically) comes from a handful of trials:

Phase I safety studies have been uniformly positive. The 2010 trial in 40 healthy volunteers tested IV doses from 42 to 1,260 mg daily for 14 days. Adverse events were infrequent and mild. No dose-limiting toxicity was found. A 2021 Chinese Phase I trial of recombinant Tβ4 (NL005) at doses from 0.05 to 25.0 μg/kg confirmed safety and tolerability.

Phase II wound healing trials in patients with pressure ulcers, stasis ulcers, and epidermolysis bullosa showed trends toward faster healing, but most results did not reach statistical significance in the primary endpoints.

Phase II/III ophthalmic trials using RGN-259 showed statistically significant improvements in dry eye symptoms and corneal staining, plus a 60% complete healing rate in neurotrophic keratitis.

Cardiac trials remain in early stages, with safety data supporting further development.

The overall picture: Tβ4 has a clean safety record in humans across multiple trials. Its efficacy has been most convincingly demonstrated in eye conditions, with wound healing and cardiac applications still awaiting larger, more definitive studies.

Safety Profile and Side Effects

What the Clinical Data Shows

Across published human trials, Thymosin Beta-4 has been consistently well-tolerated. A comprehensive toxicology assessment found no significant adverse effects in rodent models at doses up to 100 mg/kg. The 2010 Phase I human trial — specifically designed to find safety problems — found none, even at the highest tested dose of 1,260 mg IV daily for two weeks.

Commonly Reported Side Effects

Based on clinical trials and anecdotal reports, the most frequently mentioned side effects are mild and transient:

Redness, swelling, or tenderness at injection sites
Mild fatigue or lethargy, usually in the first week
Occasional headaches
Temporary changes in sleep patterns

The Angiogenesis Question

The most discussed theoretical concern involves TB-500's pro-angiogenic properties. New blood vessel growth is beneficial for wound healing and tissue repair. But angiogenesis also plays a role in tumor growth — cancers need blood supply to expand.

The current evidence: preclinical studies have not shown tumor-promoting effects in standard carcinogenicity assays. There is no human evidence linking Tβ4 or TB-500 to cancer development. However, most researchers and clinicians recommend against using TB-500 in anyone with active, untreated malignancies until more definitive safety data becomes available. This is a reasonable precaution, not a demonstrated risk.

For context, this same theoretical concern applies to other pro-angiogenic peptides and growth factors, including GHK-Cu. It is worth discussing with a physician, but it should not be treated as equivalent to a proven risk.

Quality and Contamination Risks

Because TB-500 is not manufactured under pharmaceutical-grade regulation for consumer use, product quality varies enormously. Without FDA oversight, there is no guarantee of purity, accurate dosing, or absence of contaminants. This is arguably the biggest practical safety concern — not the peptide itself, but what else might be in the vial.

Legal and Regulatory Status

FDA

TB-500 is not approved by the FDA for any human indication. It is classified as a Category 2 bulk drug substance — meaning the FDA has identified it as a "Substance with Safety Concerns" and has prohibited it from being compounded by pharmacies for human use. It cannot be legally prescribed by physicians or dispensed by pharmacists.

Products sold online are marketed under "research use only" labels. In 2025, the FDA began sending warning letters to companies that used social media influencers to imply therapeutic benefits for research-only peptides.

Advocacy groups including the Alliance for Pharmacy Compounding (APC) have petitioned the FDA to reconsider its classification, but changing the status would require new, high-quality human safety data submitted through a formal Investigational New Drug (IND) application.

WADA and Sports

TB-500 is explicitly banned by the World Anti-Doping Agency under category S2: Peptide Hormones, Growth Factors, Related Substances, and Mimetics. The prohibited list specifically names "Thymosin-β4 and its derivatives e.g. TB-500." This ban applies at all times — in and out of competition.

The United States Anti-Doping Agency (USADA) and the NCAA also classify TB-500 as a prohibited substance. Athletes in any drug-tested sport should consider it entirely off-limits.

International and State-Level Regulation

Several U.S. states, including New York, California, Illinois, and Texas, have passed laws restricting the sale of performance-enhancing supplements to minors, with peptides grouped into these categories. Online vendors in these states face fines if they cannot verify buyer age.

Personal possession of TB-500 is generally not a criminal offense in the United States — it is not a controlled substance. But selling it for human consumption is illegal, and the gap between "research use" and actual human use is a gray area that enforcement agencies are increasingly willing to challenge.

For related regulatory context, the immune-modulating peptide Thymosin Alpha-1 — derived from a different thymosin — has a notably different regulatory path, with approved pharmaceutical formulations in some countries.

Research Dosing Protocols

Important disclaimer: No dosing protocol for TB-500 has been approved by any regulatory body for human use. The following information is drawn from published research protocols and veterinary applications, and is provided for educational purposes only.

In published animal research, dosing has varied widely depending on the species, condition being studied, and route of administration:

Wound healing studies (mice): Topical Tβ4 applied directly to wounds or injected intraperitoneally
Cardiac studies (mice): Systemic IP injection, typically beginning before or shortly after induced injury
Phase I human safety trials: IV doses ranging from 42 to 1,260 mg of synthetic Tβ4 daily

In veterinary and athletic contexts, commonly reported protocols (not endorsed or validated by clinical trials) typically describe:

Loading phase: 2 to 2.5 mg administered subcutaneously twice weekly for 4 to 6 weeks
Maintenance phase: 2 mg once every one to two weeks
Cycle duration: 4 to 6 weeks for acute injuries; 8 to 12 weeks for chronic conditions

TB-500 injections do not need to be administered at the specific injury site. Because the peptide distributes systemically, subcutaneous injection at any convenient location is the most common approach described in non-clinical contexts.

TB-500 and BPC-157: The "Wolverine Stack"

TB-500 is frequently discussed alongside BPC-157 (Body Protection Compound-157), another regenerative peptide. The combination has been nicknamed the "Wolverine Stack" in athletic communities, a reference to the Marvel character's rapid healing abilities.

The rationale for combining them is based on their different — and potentially complementary — mechanisms:

Feature	TB-500	BPC-157
Scope	Systemic (whole-body)	More localized
Primary mechanism	Actin regulation, cell migration	Angiogenesis, nitric oxide pathways
Dosing range	Milligrams per week	Micrograms per day
Best studied for	Muscle, cardiac, neurological repair	Tendons, ligaments, gut healing
Angiogenesis pathway	Promotes VEGF expression	Upregulates VEGFR-2

The VEGF angle is particularly interesting from a scientific perspective. TB-500 increases production of vascular endothelial growth factor (VEGF), the signal that tells the body to build new blood vessels. BPC-157 upregulates VEGFR-2, the receptor that receives that signal. In theory, one amplifies the message while the other amplifies the receiver.

Whether this translates to meaningfully better outcomes than either peptide alone is not yet established in controlled research. The combination has not been tested in any published clinical trial. The popularity of the stack is based largely on anecdotal reports and mechanistic reasoning.

Frequently Asked Questions

Is TB-500 the same as Thymosin Beta-4?

No. TB-500 is a synthetic fragment — 7 amino acids — corresponding to the actin-binding region (amino acids 17–23) of the full 43-amino-acid Thymosin Beta-4 protein. Many vendors market TB-500 as "Thymosin Beta-4," which is inaccurate. The fragment retains certain properties of the parent molecule but lacks other functional domains.

Is TB-500 legal to buy?

In the United States, TB-500 can be purchased labeled "for research use only." It is not legal to sell for human consumption, not approved by the FDA, and not available by prescription. It is also banned in all competitive sports governed by WADA, USADA, or the NCAA.

What are the main risks?

The peptide itself appears well-tolerated based on available clinical data from trials using full-length Tβ4. The primary practical risk is product quality — without pharmaceutical oversight, purity and dosing accuracy are not guaranteed. The primary theoretical risk is the promotion of angiogenesis in people with undetected cancers, though no evidence of this has been observed.

How is TB-500 different from other healing peptides?

TB-500 works through actin regulation and systemic cell migration. BPC-157 works more locally through nitric oxide and growth factor pathways. GHK-Cu operates through copper-dependent mechanisms that influence gene expression. LL-37 is an antimicrobial peptide with wound-healing properties driven by immune modulation. Each has different strengths depending on the application.

Does TB-500 need to be injected at the injury site?

No. TB-500 distributes systemically after subcutaneous injection. In animal studies, systemic administration produced similar results to local injection — including in cardiac studies, where systemic Tβ4 delivery matched direct cardiac injection in effectiveness.

Can TB-500 promote cancer?

There is no clinical or preclinical evidence that TB-500 or Tβ4 causes cancer. Standard carcinogenicity assays have not shown tumor-promoting effects. However, because TB-500 promotes angiogenesis, which tumors depend on for growth, most researchers recommend avoiding it in anyone with active malignancies as a precaution.

How long does TB-500 take to work?

In animal research, early cellular responses appear within 1 to 2 weeks, with substantial tissue changes typically requiring 4 to 6 weeks. Biomechanical improvements in tissue strength continue developing through 8 to 12 weeks in most injury models.

The Bottom Line

TB-500 represents a genuinely interesting molecule in regenerative medicine research. Its parent compound, Thymosin Beta-4, has a solid foundation of preclinical evidence across wound healing, cardiac repair, neuroprotection, and ocular health — and a clean safety profile in the human trials conducted so far.

But the gap between research promise and clinical reality remains wide. Most evidence comes from animal models. The human trials that exist are small and often missed their primary endpoints, even when secondary outcomes looked promising. The ophthalmic research is the furthest along, with Phase 3 trials underway for neurotrophic keratitis — but that involves the full-length Thymosin Beta-4 protein, not the TB-500 fragment.

For the fragment specifically, the evidence is thinner still. A 2024 study suggesting that TB-500's wound-healing activity may actually come from its metabolite Ac-LKKTE raises important questions about whether the molecule people are buying is even the active agent, or whether it is simply a precursor that the body converts into the real therapeutic compound.

What is not in question is the biological significance of the Thymosin Beta-4 pathway. The body's own use of this peptide — in wound healing, in embryonic development, in maintaining the actin cytoskeleton — is well-established. The challenge is translating that biology into validated, safe, and effective human therapeutics.

Until large-scale clinical trials deliver clearer answers, TB-500 remains in a frustrating middle ground: too scientifically interesting to dismiss, too clinically unproven to endorse. Anyone considering its use should do so under medical supervision, with realistic expectations, and with a clear understanding of both the potential and the limitations of the current evidence.

For a broader perspective on peptides being studied in regenerative and longevity research, see the profiles for Epitalon and DSIP.

This article is for educational purposes only. TB-500 is not approved by the FDA for human use. Nothing in this article should be interpreted as medical advice or an endorsement of any product. Consult a qualified healthcare provider before making any decisions about peptide use.