The Molecule

What Is TB-500?

Note: This page explains chemistry and cell biology. It does not describe any way of using TB-500.

A 2012 paper from the racing laboratory of the Hong Kong Jockey Club defines TB-500 clearly: its key ingredient is the peptide LKKTETQ with an acetyl group attached to the start of the chain. That seven-letter sequence is the section of thymosin beta-4 between positions 17 and 23.

Fragment Versus Full Protein

	TB-500 / Thymosin beta-4
Length	7 amino acids / 43 amino acids
Weight	About 889 / about 4,900
Occurs naturally?	No, synthetic / Yes, in most human cells
Clinical trials	None published / Several, mainly for wounds and the eye

The name is sometimes used loosely online for thymosin beta-4 itself, so it is worth checking which molecule a paper, article or label actually means.

The Parent Protein and Actin

Actin exists in cells either as single units or as long filaments. Cells constantly build and dismantle these filaments when they crawl, divide or change shape. In 1991 researchers in Philadelphia showed that thymosin beta-4 is the main protein holding single actin units in reserve in blood platelets. It binds them one to one and stops them joining filaments until the cell needs them.

The LKKTETQ region is central to this binding. Researchers have also linked it to cell migration and wound closure, which is why a short copy of it attracted interest.

How the Body Handles TB-500

A 2024 study by the doping control centre of the Korea Institute of Science and Technology followed TB-500 in human serum, in liver enzyme systems and in rats:

It is quickly cut into shorter pieces. The main breakdown product in rat urine during the first six hours was Ac-LK, only two amino acids long.
A slightly longer piece, Ac-LKK, could still be found up to 72 hours later, which makes it a useful marker for drug testing.
In cell tests, neither TB-500 nor its fragments harmed cells. Only one fragment, Ac-LKKTE, sped up closure of a scratch wound, not the original peptide.

The authors suggested that some of the effects attributed to TB-500 might come from one of its breakdown products. This is a single study, but it shows how little is known about the fragment itself.

In Brief

TB-500 is an acetylated seven-amino-acid peptide, not the full thymosin beta-4 protein.
It copies the actin-binding site of the parent protein.
It breaks down within hours into shorter fragments.
Most research on healing used the full protein, not TB-500.