What it is
"Thymosin" is a name, not a molecule. In 1966 Allan Goldstein, Frank Slater and Abraham White published the preparation of a "thymic lymphocytopoietic factor" from thymus and called it thymosin (PNAS, PMID 5230175). Their group went on to make a partly purified calf-thymus preparation, thymosin fraction 5, which, in Goldstein's own later account, "consists of a family of at least 40 mostly small acidic polypeptides, with molecular weights ranging from 1000 to 15,000 Da" (Ann N Y Acad Sci 2007, PMID 17600284). Individual peptides isolated from that mixture were named thymosins in turn.
So the name records where a peptide was first found, not what it is or does. Two of the best-known members are unrelated molecules from different genes.
The thymosins, side by side
| Name | Length | What it is | US status | Entry |
|---|---|---|---|---|
| Thymosin alpha-1 (thymalfasin, Zadaxin) | 28 | fragment of prothymosin alpha (UniProt P06454) | not approved; licensed in other countries | thymosin alpha-1 |
| Thymosin beta-4 (Tβ4) | 43 | actin-holding protein found in most cells (UniProt P62328) | not approved anywhere | thymosin beta-4 |
| TB-500 | 7 | synthetic Ac-LKKTETQ, residues 17–23 of beta-4 | not approved; sold for research use | TB-500 |
| Thymosin fraction 5 | mixture | the original calf-thymus extract | historical; first IND 1974 | this page |
Two Russian thymus preparations are often searched beside them: thymalin, a polypeptide mixture extracted from calf thymus, and thymogen, a synthetic two-amino-acid peptide (Glu-Trp). Neither carries the name thymosin, and both are licensed in Russia rather than the US.
Alpha and beta are not relatives
Lined up, the two sequences have almost nothing in common:
- Thymosin alpha-1: SDAAVDTSSEITTKDLKEKKEVVEEAEN — residues 2–29 of the 111-residue prothymosin alpha.
- Thymosin beta-4: SDKPDMAEIEKFDKSKLKKTETQEKNPLPSKETIEQEKQAGES — residues 2–44 of the TMSB4X gene product.
The longest run of identical amino acids they share is two — the Ser-Asp each starts with (our comparison). The Greek letters do not mean "first and second forms of one hormone"; both peptides simply came out of the same extract.
Thymosin beta-4 is not a thymus hormone. UniProt describes its job as binding and sequestering actin monomers — the building blocks of the cell's internal scaffolding — and so inhibiting their assembly, in many cell types. It was named for the extract it was isolated from.
The extract that started it
Goldstein's 2007 history records that his group received the first IND for a thymic hormone preparation from the FDA in 1974, to begin a phase I study of thymosin fraction 5 in children with primary immunodeficiency, and that the first clinical report followed in 1975. Those were small, early studies of a mixture, and fraction 5 never became an approved product. The member that did reach a licence, outside the US, is thymosin alpha-1.
Where it stands
- No thymosin is an FDA-approved drug. Thymosin alpha-1 is licensed in a number of other countries; its entry records the Drugs@FDA search that found no US application.
- Research record: PubMed indexes 3,677 records for "thymosin", 89 tagged as randomised trials; ClinicalTrials.gov lists 89 studies with a thymosin as an intervention (2026-10-26). Most of that record belongs to alpha-1 and beta-4 separately — see their entries for the counts that matter.
Related reading
What the use-case claims for these peptides rest on is set out in the library's pages on what the immune-support claims rest on and peptides for healing. Where a protein ends and a peptide begins — beta-4's 43 residues sit just over the line — is explained in peptide vs protein.