TB-500: A Fragment Carrying a Larger Protein's Reputation

The short version

TB-500 is a small synthetic peptide — just seven amino acids — with the sequence Ac-LKKTETQ. That sequence is the actin-binding region of a larger natural protein called thymosin beta-4. Actin is part of the internal skeleton cells use to hold their shape and to crawl toward a wound, so the parent protein is closely involved in cell movement, healing, and new blood-vessel growth ^[10].

The single most important thing to know about TB-500 is a naming gap. In commerce and in anti-doping labs, "TB-500" means the short seven-amino-acid fragment. But most of the published effectiveness research was conducted with full-length thymosin beta-4 — which is about five times larger ^[8]. It is not established that the small fragment reproduces what the whole protein does. TB-500 is not an approved medicine, it is banned in sport, and this page reports doses only as they appear in the studies — never as a recommendation.

What it is

TB-500 is a synthetic, N-terminally acetylated heptapeptide (a seven-amino-acid peptide with a small acetyl cap on one end), sequence Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln. This LKKTETQ stretch corresponds to residues 17-23 of thymosin beta-4 (Tβ4), a 43-amino-acid protein encoded by the TMSB4X gene, and it is the conserved actin-binding region shared across the beta-thymosin family.

The distinction matters throughout this page: the fragment sold and detected as "TB-500" weighs about 889 daltons, while full-length Tβ4 is about 4,963 daltons. Wherever a study used the full protein rather than the fragment, this page flags it — because that is exactly where TB-500 marketing tends to borrow a bigger molecule's data ^[8].

How it works

Full-length thymosin beta-4 is the body's major intracellular G-actin sequestering protein. "G-actin" is the free, single-unit form of actin; "sequestering" means Tβ4 holds those units in reserve so they can be called up when a cell needs to change shape or crawl somewhere. A 2-angstrom X-ray crystal structure established that Tβ4 forms a 1:1 complex with G-actin and caps both ends of the monomer, preventing assembly — the structural basis for its actin-buffering role ^[12].

Why does that matter for repair? By governing the actin skeleton, Tβ4 (and its LKKTETQ motif) is associated with faster cell migration, new blood-vessel growth, reduced scarring, anti-inflammatory signaling, and recruitment of progenitor cells into damaged tissue — consolidated across skin-wound, corneal, cardiac and CNS models ^[10]. Whether the isolated seven-amino-acid fragment reproduces all of that at doses used in peptide research has not been shown in controlled human trials ^[10].

What the research shows

Structural basis. The crystallography work established the precise 1:1 actin-capping mechanism ^[12], and a multi-model review consolidated Tβ4's actin-binding, pro-migratory, anti-scarring, anti-inflammatory and angiogenic functions as the rationale for clinical development ^[10].

Human safety (full-length protein). In a randomized, placebo-controlled Phase 1 study, synthetic thymosin beta-4 was given intravenously to 40 healthy volunteers — single dose then daily for 14 days at 42, 140, 420 or 1260 mg. It was well tolerated, with only infrequent mild-to-moderate adverse events, no dose-limiting toxicities, no serious adverse events, and dose-proportional pharmacokinetics ^[11]. Crucial note: this used full-length Tβ4, not the TB-500 fragment.

Animal dose-response. In rats with an embolic stroke model, intraperitoneal thymosin beta-4 (2, 12 or 18 mg/kg starting 24 hours after stroke, then every three days) improved neurological function at 2 and 12 mg/kg, but not at 18 mg/kg — a non-monotonic result where higher was not better ^[9].

Broader safety context. A 2026 Sports Medicine review naming TB-500/thymosin beta-4 among unapproved peptides concluded that while animal-model results are often favorable, rigorous human safety data are scarce, potential for serious harm exists, and these compounds operate largely outside regulatory oversight ^[8].

Reported effects, cautions & safety

The cautions for TB-500 are concrete and several are specific to this compound:

• Identity confusion. "TB-500" is the fragment, but most efficacy data come from full-length Tβ4; efficacy claims for the fragment in humans are unproven ^[8]. No completed controlled clinical trials of the TB-500 fragment exist for any indication.
• Tumor and angiogenesis signal. Thymosin beta-4 is overexpressed in several cancers and is implicated in metastasis and tumor blood-vessel growth; the same pro-migratory, pro-angiogenic properties that aid repair could, theoretically, support tumor progression ^[10].
• Mixed preclinical results. In dystrophin-deficient mice, chronic Tβ4 increased regenerating fibers but did not improve muscle strength, cardiac function or fibrosis; a porcine study found no benefit against myocardial ischemia-reperfusion injury ^[8][10].
• Non-monotonic dosing. The stroke study found 18 mg/kg helped less than 12 mg/kg — higher is not necessarily better, which undermines loading-protocol rationales ^[9].
• Regulatory status. TB-500 is WADA-prohibited and is classified as a prescription medicine in some jurisdictions; it has appeared as a doping agent in racehorses, with dedicated LC-MS detection assays developed ^[8].

No community-anecdote reports are included in this site's source material for TB-500 as a single compound; the points above are drawn from the cited literature.

Where it fits in recovery research

TB-500 occupies a particular position here: a compound whose mechanism — actin regulation driving cell migration — is well described at the protein level, but whose evidence as the actual fragment sold is the thinnest of the four singles ^[8]. Where BPC-157 leads with angiogenesis and GHK-Cu leads with matrix-building and human topical data, TB-500's story is really thymosin beta-4's story, borrowed. That makes it the clearest example of why this field rewards careful reading. It also appears as one half of the Wolverine blend and one component of KLOW. See how all six compare on the compare page.