TB-500 Is Not Thymosin Beta-4: The Fragment Problem

The Confusion at the Heart of TB-500

TB-500 is one of the most popular grey-market peptides, cited for wound healing, cardiac repair, hair growth, and athletic recovery. But there’s a fundamental problem that most discussions ignore: the research that drives TB-500’s reputation was conducted on a completely different molecule.

The grey market sells a 7-amino-acid fragment (Ac-LKKTETQ) with a molecular weight of approximately 847–889 Da. The research literature — including every landmark study — used full-length Thymosin Beta-4 (Tβ4), a 43-amino-acid protein with a molecular weight of 4,921 Da.

These are not the same thing.

Why the Distinction Matters

Proteins derive their function from their three-dimensional structure, which is determined by their full amino acid sequence. A 7-amino-acid fragment of a 43-amino-acid protein retains only a small portion of the original structure. While TB-500 contains the active site region of Tβ4, it lacks:

• The full tertiary structure that determines receptor binding affinity
• Flanking sequences that may affect stability, distribution, and half-life
• Post-translational modifications present in the native protein
• The pharmacokinetic profile established in clinical studies

Claiming TB-500 has the same effects as Tβ4 is like claiming that a single chapter has the same impact as the complete book. The active site fragment may retain some biological activity, but the assumption that it’s equivalent is scientifically unsupported.

The Research: What Was Actually Studied

The Landmark Cardiac Study

Bock-Marquette I et al., Nature, 2004;432(7016):466–472 — This is the study that put thymosin on the map for regenerative medicine. Published in Nature, it demonstrated that full-length Tβ4 promoted cardiac repair after myocardial infarction in mice. This study used the full 43-amino-acid protein, not the TB-500 fragment.

Human Safety Data

Ruff D et al., Ann N Y Acad Sci, 2010;1194:56–59 — The only Phase I safety trial data in humans used full-length Tβ4 administered to healthy volunteers. TB-500 (the fragment) has no published human safety data.

The Eye Drop Success Story

RGN-259, a topical ophthalmic formulation of full-length Tβ4, completed Phase III clinical trials showing positive results for dry eye disease. This represents the most advanced clinical development of any thymosin product — but again, it’s the full protein, not the fragment, and it’s topical (eye drops), not injectable.

Wound Healing

Multiple animal studies demonstrate wound healing acceleration with full-length Tβ4, including Malinda KM et al., J Invest Dermatol, 1999;113(3):364–368. These studies established Tβ4 as a potent promoter of cell migration and angiogenesis.

Hair Growth

Philp D et al., FASEB J, 2004;18(2):385–387 — Demonstrated hair growth stimulation in mice using full-length Tβ4.

What’s Known About TB-500 Specifically

The honest answer: very little from published peer-reviewed research.

The 7-amino-acid fragment (Ac-LKKTETQ) was identified as the active site of Tβ4 responsible for actin binding and cell migration. In vitro studies confirm this fragment retains some ability to promote cell migration. However:

• No published human trial has used the TB-500 fragment
• No published pharmacokinetic study compares the fragment to full-length Tβ4
• No published dose-response study establishes therapeutic doses for the fragment
• The half-life, biodistribution, and clearance of the fragment may differ significantly from the full protein

The Regulatory Landscape

FDA: TB-500 falls under Category 2 — prohibited for compounding. Full-length Thymosin Beta-4 is similarly restricted.

WADA: Thymosin Beta-4 has been on the prohibited list under S2.3 (Peptide Hormones, Growth Factors, Related Substances and Mimetics) for years. TB-500, as a synthetic fragment, falls under the same prohibition.

The horse racing connection: TB-500 gained early notoriety through its use in horse racing, where it was detected in post-race drug testing. This veterinary/racing use preceded its adoption in the human biohacking community. Tailor Made Compounding’s federal conviction involved TB-500 alongside connections to equine doping operations.

What Users Should Understand

If you’re considering or currently using TB-500:

The research you’ve read about almost certainly describes full-length Thymosin Beta-4, not the fragment you’re buying. The cardiac repair, wound healing, and hair growth studies used a different molecule.

The safety profile established in human Phase I trials applies to full-length Tβ4, not TB-500. You cannot assume the fragment has the same safety characteristics.

The dosing protocols circulated in biohacking communities are not derived from published research on the fragment. They’re extrapolated from Tβ4 studies and anecdotal user reports.

The quality question: Grey-market TB-500 faces the same quality challenges as all unregulated peptides — identity, purity, sterility, and endotoxin contamination are not guaranteed. The additional complication with TB-500 is that even a “pure” product is a fragment with an unestablished therapeutic profile.

The Bottom Line

The distinction between TB-500 and Thymosin Beta-4 isn’t pedantic — it’s the difference between a molecule with published human safety data and one without any. Vendors who cite Tβ4 research to market TB-500 are, at best, making an optimistic extrapolation. At worst, they’re misleading consumers about the evidence base for the product they’re selling.

This doesn’t mean TB-500 has no biological activity. The actin-binding active site fragment likely retains some functional properties. But “likely retains some activity” is a very different claim from “clinically proven for cardiac repair and wound healing” — and the gap between those statements is filled with missing research, not evidence.

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This article will be updated as new research becomes available. Last reviewed: February 20, 2026.