$ cat tb-500/mechanism.md
TB-500 is the synthetic Ac-LKKTETQ fragment of thymosin beta-4, read through its actin-binding mechanism.
Seven amino acids, residues 17 to 23 of a 43-residue parent protein. This is a sourced reading of what the studies measured — and a standing flag that most efficacy data are on the full-length protein, not the fragment.
What is TB-500?
TB-500 is the synthetic, N-acetylated heptapeptide Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln-OH — written Ac-LKKTETQ. It corresponds to residues 17-23 of thymosin beta-4 (Tβ4), the 43-amino-acid protein that is the body's principal actin-sequestering peptide [9]. That short stretch, LKKTETQ, is the conserved actin-binding region of the beta-thymosins [8]. The molecule weighs roughly 889 Da and carries the formula C38H68N10O14.
The name carries a load it does not quite earn. "TB" points at Thymosin Beta-4; "500" is a product and research designation (sold alongside the veterinary label TB1000), not an official chemical name. So when a page says "TB-500," the verifiable identity is a seven-residue fragment in commerce and in the anti-doping literature — while the bulk of the efficacy research that gets cited under that name was run on the full-length protein. This site keeps those two things on separate lines, because the difference is the most important fact about the molecule.
The parent protein is present in nearly every human cell and is released by platelets and macrophages at sites of injury [5]. TB-500 itself is a synthetic construct, not a species the body makes. What the isolated 7-mer reproduces of the full protein's biology, at the doses peptide research uses, is not settled in controlled human trials [12].
TB-500 as a research peptide: Ac-LKKTETQ vs full-length thymosin beta-4
TB-500 is a research peptide, supplied as a lyophilized powder for laboratory use and reconstituted before study. As a chemical, it is unambiguous: the Ac-LKKTETQ heptapeptide, ~889 Da, formula C38H68N10O14, the actin-binding fragment of Tβ4.
The parent protein is a different object. Full-length thymosin beta-4 (gene TMSB4X, UniProt P62328) is 43 residues and ~4963 Da — more than five times the mass [9]. A synthetic peptide containing Tβ4's actin-binding domain has reproduced aspects of the protein's wound-healing activity in animal models [7], which is the strongest reason to think the fragment matters. But "reproduced aspects in mice" is not "carries the full protein's effects in humans," and the site holds that line throughout. Where a cited finding used full-length Tβ4 rather than the 7-mer, it is marked. The full account of the fragment vs the parent protein sits on the mechanism page.
Why belabor the distinction? Because it is where most writing on this molecule goes wrong. A reader searching TB-500 peptide will find pages citing dramatic wound-healing and cardiac-repair numbers — almost all of them generated with the full-length protein — presented as if they describe the fragment in the vial. The structural overlap makes that slippage easy and the missing 36 residues make it unsafe. This digest logs the species and the molecule for every figure so the slippage has nowhere to hide.
One more separation worth stating: Tβ4's N-terminus can be cleaved to Ac-SDKP, a small peptide with its own anti-fibrotic and angiogenic activity. Ac-SDKP comes from the other end of the protein and is not produced by the C-terminal-region TB-500 fragment — so claims resting on Ac-SDKP do not transfer to TB-500.
How does TB-500 differ from BPC-157?
TB-500 and BPC-157 get grouped as "recovery peptides," but chemically they share nothing. TB-500 is the Ac-LKKTETQ fragment of thymosin beta-4 and acts on the actin cytoskeleton — binding monomeric actin and regulating the unpolymerized pool [1]. BPC-157 is a separate 15-amino-acid peptide originally derived from gastric juice, with a different proposed mechanism. Neither is an FDA-approved drug, and neither has a completed controlled human efficacy trial for the recovery uses they are marketed for. A 2026 Sports Medicine review lists both among unapproved peptides where animal-model repair signals look favorable but rigorous human safety data are scarce [12].
The two are sometimes discussed together because the rationale overlaps — tissue repair, angiogenesis, cell migration — not because the molecules are related. This site covers TB-500 only.
Is TB-500 a steroid?
No. TB-500 is a synthetic peptide — a short chain of seven amino acids (Ac-LKKTETQ) — not a steroid hormone. Steroids are lipid molecules built on a four-ring carbon skeleton and act largely through nuclear hormone receptors. TB-500 carries the actin-binding motif of thymosin beta-4 and works on the cytoskeleton [1], a completely different chemistry and a completely different mechanism.
The confusion is usually a product-category artifact: TB-500 appears in the same research-peptide and anti-doping conversations as anabolic agents, so it inherits the label by association. It does not fit it. TB-500 is, however, prohibited in sport under the World Anti-Doping Agency's peptide and growth-factor categories — a regulatory fact, not a statement that it is a steroid. The WADA status of TB-500 is on the legal-status page.
What the readouts say, in one screen
Four numbers describe TB-500 honestly. It is 7 amino acids long. It maps to residues 17-23 of thymosin beta-4 — the actin-binding motif. The parent protein binds monomeric (G-)actin in a 1:1 complex, capping both ends of the monomer [1]. And validated human pharmacokinetics for the fragment: NONE. There is no completed controlled clinical trial of the TB-500 heptapeptide for any indication [12]. Human data exist only for full-length Tβ4 — a Phase 1 intravenous safety study and topical ophthalmic trials [6].
That last readout is the one the rest of the site keeps returning to. The structural story is genuinely established. The human story, for the fragment, is a gap — and the honest move is to label the gap rather than paper over it with the parent protein's data.