Thymosin β4

Thymosin β4 is a naturally occurring 43-amino-acid peptide that binds monomeric actin and is abundant in many cell types. It has been studied in animal models of tissue repair and in human trials of ophthalmic and topical formulations, and is not approved.

Last reviewed 2026-10-25 by the PeptideWiki editorial team. Editorial policy.

Overview

Thymosin β4 is one of the most abundant intracellular peptides in many mammalian cells, particularly platelets and white blood cells. It was first isolated from thymus extracts, which gave the thymosin family its name, but it is not a thymic hormone in the classical sense and is expressed widely. Its best-characterized role is sequestering monomeric G-actin, which helps regulate the pool of actin available for building filaments.

Beyond this intracellular role, thymosin β4 has been studied as an extracellular signaling molecule. Animal and cell studies have examined its effects on cell migration, blood vessel formation, inflammation and repair in skin, eye, heart and other tissues. Formulations of the full-length peptide have progressed into human clinical trials, chiefly as eye drops and topical gels. It should not be confused with the short synthetic fragment often sold as TB-500.

Compare: TB-500 vs thymosin β4, Thymosin α1 vs thymosin β4.

Structure

Thymosin β4 is a 43-residue peptide encoded in humans by the TMSB4X gene, with an acetylated N-terminal serine. Its sequence begins with Ac-Ser-Asp-Lys-Pro, the tetrapeptide also known as Ac-SDKP, which can be released by enzymatic processing and has been studied separately. The central region contains the LKKTETQ motif (around residues 17–23) that is important for actin binding. The peptide is largely unstructured in solution and adopts more ordered conformation when bound to actin. It contains a single methionine near the N-terminus, which is susceptible to oxidation. Research material is typically synthetic or recombinant and supplied as an acetate or TFA salt.

Mechanism

Inside cells, thymosin β4 binds G-actin in a 1:1 complex and inhibits its polymerization, acting as a major actin-sequestering protein. Proposed extracellular and signaling roles, derived mainly from animal and in-vitro studies, include promotion of cell migration, angiogenesis, modulation of inflammatory signaling, and effects on cardiac progenitor cells and the epicardium in mouse models. Some of these effects have been attributed in part to its Ac-SDKP fragment. Several reported cardiac findings have been debated in follow-up studies, so mechanisms beyond actin sequestration should be regarded as proposed rather than settled.

Research

Eye and cornea

The most advanced clinical work uses ophthalmic formulations of thymosin β4. Preclinical studies in corneal injury models reported effects on epithelial healing, and eye-drop formulations were subsequently tested in human trials in conditions including dry eye disease and neurotrophic keratopathy. Reported outcomes have been mixed, with some endpoints met and others not, and no thymosin β4 product has received FDA approval. This remains the area with the most human data.

Skin and wound models

Thymosin β4 has been studied in rodent models of dermal wounds, where authors reported effects on re-epithelialization, angiogenesis and collagen deposition. Topical gel formulations were evaluated in early human trials in chronic wounds such as pressure ulcers, venous stasis ulcers and epidermolysis bullosa. These studies were relatively small, and their results have not led to an approved product. Human wound evidence remains limited.

Cardiac models

Mouse studies reported that thymosin β4 could promote survival of cardiomyocytes and activate epicardial progenitor cells after experimental myocardial infarction, which generated considerable interest. Later work questioned how robust some of these effects are and to what extent they depend on experimental conditions. An injectable formulation was tested in early human safety studies, but cardiac efficacy in people has not been demonstrated.

Evidence quality

Thymosin β4 has a deeper evidence base than many repair-signaling peptides, with substantial cell and animal biology and a series of human trials of specific full-length formulations. Even so, efficacy results in humans have been inconsistent and no product is approved. Importantly, these data concern the full-length 43-residue peptide and do not transfer to short synthetic fragments such as Ac-LKKTETQ.

Regulatory status

Thymosin β4 is not approved by the FDA for any use, and no thymosin β4 product has been approved in the US. Its compounding status should be checked against current FDA lists; the July 2026 PCAC vote concerned TB-500, and FDA noted that human thymosin β4 trials are not evidence for the TB-500 fragment. WADA prohibits thymosin β4 and its fragments under class S2.

Status changes often. See the regulatory tracker for dated updates.

Quality and identity testing

A good certificate of analysis should confirm identity by mass spectrometry, with the observed mass matching the expected mass shown on this page for the N-acetylated 43-residue sequence, and report HPLC purity with the chromatogram. Confirm that the sample is full-length thymosin β4 and not a fragment sold under a similar name, since the masses differ greatly. Check for loss of the N-terminal acetyl group, for truncated sequences typical of long syntheses, and for a +16 Da species indicating methionine oxidation. The salt form and net peptide content should be stated. For recombinant material, host-cell protein and endotoxin results are also relevant.

Open Thymosin β4 in the sequence analyzer to see the expected [M+H]⁺ and multiply charged ions for an MS identity check (set the N-acetyl, amide and disulfide options to match).

For a walkthrough of each section of a certificate, see how to read a COA.

Storage and stability

Store lyophilized thymosin β4 sealed at -20 °C or colder, protected from light and moisture, and equilibrate to room temperature before opening. Its methionine residue is oxidation-prone, so minimize air exposure, consider purging vials with inert gas for long-term storage, and avoid oxidizing buffers. In solution, keep refrigerated for short-term lab use, aliquot for frozen storage at -20 °C or below, and avoid repeated freeze-thaw cycles. Long, flexible peptides can adsorb to plastic surfaces at low concentrations, so low-binding tubes may help.

Download the Thymosin β4 data sheet (PDF): sequence, masses, expected MS ions and handling on one printable page.

Latest research

Frequently asked questions

Related compounds

• TB-500

  TB-500 is a research name most often applied to a short synthetic N-acetylated fragment of thymosin β4, Ac-LKKTETQ, but it is also used loosely for full-length thymosin β4. There are no direct human studies of the fragment.

• BPC-157

  BPC-157 is a synthetic 15-amino-acid peptide whose sequence was derived from a protein fraction of gastric juice. It is studied mainly in rodent models of gastrointestinal, tendon and soft-tissue injury and is not approved for any use.

• Thymosin α1

  Thymosin α1 is a 28-amino-acid N-acetylated peptide derived from prothymosin α. Its synthetic form, thymalfasin (Zadaxin), is approved in several countries outside the US and has been studied as an immune modulator in clinical trials.

• GHK-Cu

  GHK-Cu is the copper(II) complex of GHK (glycyl-histidyl-lysine), a tripeptide found in human plasma. It is widely used as a cosmetic ingredient and studied in cell, animal and small human skin studies.

References

1. PubMed: all indexed literature on Thymosin β4
2. ClinicalTrials.gov: registered studies mentioning Thymosin β4