Thymosin Peptides

What Defines a Thymosin Peptide?

"Thymosin" is a historical family name, not a single structure or receptor. In 1977, researchers sequenced thymosin alpha-1, a 28-residue peptide from a calf-thymus extract called thymosin fraction 5, and proposed a naming system for the related polypeptides [R1]. Thymosin beta-4 was sequenced from the same tissue in 1981 as a 43-residue peptide with an acetylated N-terminus [R4].

The two families are studied for different things. Thymosin beta-4 binds and sequesters monomeric (G-) actin, so it is investigated in models of cytoskeleton organization and cell migration [R5]. Thymosin alpha-1 is investigated in immunology models, for example dendritic-cell signaling in mice [R6].

TB-500 is not full-length thymosin beta-4

FDA defines TB-500 as N-acetyl-Leu-Lys-Lys-Thr-Glu-Thr-Gln (Ac-LKKTETQ), residues 17–23 of thymosin beta-4, with a molecular weight of 889.01 g/mol. FDA states that "thymosin beta-4 and TB-500 are not the same substance" [R3]. Doping-control laboratories identified the same acetylated heptapeptide as the key ingredient of products sold as TB-500 [R7][R8]. Some vendors and papers still use "TB-500" for full-length thymosin beta-4, so check the mass on the COA. See TB-500 vs thymosin beta-4.

Thymosin Peptides Compared

Sources: [R3][R2][R9][R5]. Thymosin β4 formula and weight calculated from the UniProt sequence with N-terminal acetylation.

Regulatory Status

Neither compound is an FDA-approved drug in the United States. Thymosin alpha-1 (thymalfasin) is approved as a medicine in a number of other countries; FDA has approved no product containing it [R10]. Both appear on FDA's list of substances nominated but withdrawn from Category 2 of the 503A interim compounding policy [R11]. In July 2026, an FDA advisory committee voted 8–6 (one abstention) to recommend TB-500 for the 503A bulks list; the vote is non-binding [R12]. In December 2024, the same committee voted 4–17 against listing thymosin alpha-1 [R13].

Thymosin Peptides FAQ

What are thymosin peptides?

They are peptides named after the thymosins, polypeptides first isolated from calf thymus. The family includes thymosin alpha-1 and the beta-thymosins such as thymosin beta-4, which differ in structure and in what they are studied for [R1][R4].

Is TB-500 the same as thymosin beta-4?

No, by FDA's definition. TB-500 is the acetylated seven-residue fragment 17–23 of the 43-residue thymosin beta-4 protein [R3].

Is thymosin alpha-1 related to thymosin beta-4?

Only by name and tissue of origin. Their sequences differ, and they are studied in different research areas [R2][R5].

How can I confirm which compound is in a vial?

Check the mass-spectrometry result on the batch COA. TB-500 has a molecular weight near 889 g/mol, thymosin alpha-1 near 3108 g/mol, and full-length thymosin beta-4 near 4963 g/mol. See how to read a peptide COA.

Can these peptides be used in people?

No. Sequora supplies them as research chemicals for laboratory use only. See RUO peptides explained.

References

1. Goldstein AL, Low TLK, McAdoo MH, et al. Thymosin α1: isolation and sequence analysis of an immunologically active thymic polypeptide. Proc Natl Acad Sci U S A. 1977;74(2):725–729. doi:10.1073/pnas.74.2.725. PMID 265536.
2. KEGG DRUG. Thymalfasin, D06116. kegg.jp/entry/D06116
3. U.S. Food and Drug Administration. FDA briefing document: evaluation of TB-500-related bulk drug substances for inclusion on the 503A bulk drug substances list. Pharmacy Compounding Advisory Committee, July 23–24, 2026. fda.gov/media/193349/download
4. Low TLK, Hu SK, Goldstein AL. Complete amino acid sequence of bovine thymosin β4: a thymic hormone that induces terminal deoxynucleotidyl transferase activity in thymocyte populations. Proc Natl Acad Sci U S A. 1981;78(2):1162–1166. doi:10.1073/pnas.78.2.1162. PMID 6940133.
5. UniProt. Thymosin beta-4 (TMSB4X), human, P62328. uniprot.org/uniprotkb/P62328
6. Romani L, Bistoni F, Gaziano R, et al. Thymosin α1 activates dendritic cells for antifungal Th1 resistance through Toll-like receptor signaling. Blood. 2004;103(11):4232–4239. doi:10.1182/blood-2003-11-4036. PMID 14982877.
7. Ho ENM, Kwok WH, Lau MY, et al. Doping control analysis of TB-500, a synthetic version of an active region of thymosin β4, in equine urine and plasma by liquid chromatography–mass spectrometry. J Chromatogr A. 2012;1265:57–69. doi:10.1016/j.chroma.2012.09.043. PMID 23084823.
8. Esposito S, Deventer K, Goeman J, Van der Eycken J, Van Eenoo P. Synthesis and characterization of the N-terminal acetylated 17–23 fragment of thymosin beta 4 identified in TB-500, a product suspected to possess doping potential. Drug Test Anal. 2012;4(9):733–738. doi:10.1002/dta.1402. PMID 22962027.
9. FDA Global Substance Registration System. Timbetasin (thymosin β4), UNII 2D5MRE3SSY. precision.fda.gov
10. U.S. Food and Drug Administration. FDA briefing document: thymosin alpha-1 (free base) and thymosin alpha-1 acetate. Pharmacy Compounding Advisory Committee, December 4, 2024. fda.gov/media/183820/download
11. U.S. Food and Drug Administration. Certain bulk drug substances for use in compounding that may present significant safety risks. Content current as of April 22, 2026. fda.gov
12. McDermott Will & Schulte. Bulk list bound? PCAC backs majority of peptides in two-day public meeting. July 27, 2026. mcdermottlaw.com
13. U.S. Food and Drug Administration. Final summary minutes of the Pharmacy Compounding Advisory Committee meeting, December 4, 2024. fda.gov/media/185642/download

For laboratory research use only. Not for human or veterinary use.