What are thymic peptides?
Thymic peptides, also called thymus peptides, are peptides first isolated from the thymus gland or prepared from thymus extracts. The best known are the thymosins, isolated from a calf thymus preparation called thymosin fraction 5, and thymalin, a Russian calf thymus extract.
The label describes where these peptides were first found, not a shared structure: a 2003 review concluded that none of the peptides isolated from fraction 5 is really a thymic hormone, though they have diverse functions inside and outside cells (Hannappel and Huff, 2003). Disguised Alpha supplies thymosin alpha-1, TB-500 (thymosin beta-4) and thymalin as lyophilized powders for laboratory research use only. Nothing on this page is guidance for use in people.
From thymosin fraction 5 to single peptides
The work began in the early 1960s in Abraham White's laboratory at the Albert Einstein College of Medicine in New York, and a 1966 paper from Allan Goldstein and colleagues named the thymus factors they studied "thymosins" (Goldstein, 2007). By 1972 the team had moved to the University of Texas Medical Branch in Galveston, where it prepared a standardized, partially purified calf thymus extract, thymosin fraction 5: a family of at least 40 mostly small, acidic polypeptides of 1,000 to 15,000 Da (Goldstein, 2007).
Several peptides were then isolated and studied, among them thymosin alpha-1, prothymosin alpha, parathymosin and thymosin beta-4 (Hannappel and Huff, 2003). Thymosin alpha-1 was sequenced in 1977 as a heat-stable, highly acidic peptide of 28 residues (Goldstein et al., 1977), and thymosin beta-4 in 1981 as a 43-residue peptide with an acetylated N-terminus (Low et al., 1981).
Why "thymosin" names unrelated molecules
The thymosin peptides were named for where they were found, not for what they are. The 1977 paper also proposed a nomenclature for the fraction 5 polypeptides (Goldstein et al., 1977), and the label stayed with each peptide after later work showed that the two best known are unrelated and that neither is confined to the thymus.
Thymosin alpha-1 and prothymosin alpha
Antibodies against thymosin alpha-1 mainly recognized a larger protein in rat tissues, prothymosin alpha, of about 112 residues. It was most concentrated in the thymus but also present in brain, liver, kidney, lung and spleen, and the authors believed thymosin alpha-1 forms from it by proteolysis during preparation of fraction 5 (Haritos et al., 1984). Our thymosin alpha-1 guide covers later work that found the 28-residue peptide itself in serum.
Thymosin beta-4: an actin-sequestering peptide found far beyond the thymus
The 1981 sequencing paper called thymosin beta-4 a thymic hormone (Low et al., 1981). The next year it was found in spleen, brain, lung, liver and heart muscle of rats and mice, and at high concentrations in tissues of athymic nude mice, so it does not come only from the thymus (Hannappel et al., 1982).
In 1991 it proved identical to Fx, the peptide holding the bulk of unpolymerized actin in resting human platelets: it binds actin monomers one to one, blocks their polymerization and is widely distributed at high concentration inside cells (Safer et al., 1991).
TB-500: a research name, not one molecule
TB-500 is a research name tied to thymosin beta-4, and the name does not point to one molecule. Analysis of material sold as TB-500 identified a seven-residue fragment, residues 17 to 23 of thymosin beta-4 with an acetylated N-terminus, Ac-LKKTETQ (Esposito et al., 2012), while most biological data concern the full-length protein. Our product page lists full-length thymosin beta-4's formula and weight under a "fragment" class, so our TB-500 guide does not assume which molecule a vial holds.
Thymalin: a thymus extract, not a thymosin
Thymalin comes from a separate program: a polypeptide preparation made from calf thymus by mild acid extraction in V. G. Morozov and V. Kh. Khavinson's group, from which the dipeptide Glu-Trp, later the drug thymogen, was isolated (Morozov and Khavinson, 1997). The St Petersburg group describes it as a thymus polypeptide complex (Khavinson et al., 2020), so it has no single sequence or molecular weight. Our thymalin guide explains why a PubChem record under that name describes a different peptide.
Thymic peptides at a glance
| Peptide | Origin | Length | Main research line | Our guide |
|---|---|---|---|---|
| Thymosin alpha-1 (thymalfasin) | Calf thymus fraction 5; linked to prothymosin alpha | 28 residues | Immune-cell signaling; human trials, including a null phase 3 sepsis trial | Thymosin alpha-1 guide |
| Thymosin beta-4 (timbetasin) | Calf thymus fraction 5; found in many tissues | 43 residues | Actin sequestering; cell migration and tissue-repair models | TB-500 guide |
| TB-500 | Research name; analyzed material held the acetylated 17 to 23 fragment of thymosin beta-4 | 7 residues as analyzed; also used for the 43-residue protein | Doping-control detection, metabolism and actin-binding motif assays | TB-500 guide |
| Thymalin | Calf thymus extract from a St Petersburg group | No single length: a peptide mixture | Russian immune-cell, rodent and explant studies | Thymalin guide |
At the bench, molecular weights differ widely: 3108.31 g/mol for thymosin alpha-1 on its product page (PubChem CID 16130571), 4963 g/mol for full-length thymosin beta-4 as our TB-500 page lists it (CID 16132341, titled timbetasin), and 889.0 g/mol for the Ac-LKKTETQ fragment (CID 62707662). Thymalin, a mixture, has no single value. All three product pages list -20°C storage for the lyophilized powder, protected from light, and 2 to 8°C after reconstitution. Use each lot's certificate for identity and measured content.
What each peptide has been studied for
The literatures differ in size: PubMed returns about 960 records for thymosin alpha-1, about 1,200 for thymosin beta-4 and about 300 for thymalin (searched October 2026).
Thymosin alpha-1: immune-cell signaling
In dendritic cells exposed to the fungus Aspergillus fumigatus, thymosin alpha-1 induced maturation and interleukin-12 production through MyD88-dependent Toll-like receptor signaling, and in mice it activated T helper 1 responses to the fungus (Romani et al., 2004). No receptor for it has been established. About 100 of its PubMed records are typed as clinical trials. A 2025 placebo-controlled phase 3 trial at 22 centers in 1106 adults with sepsis found 28-day mortality of 23.4 percent against 24.1 percent with placebo, and no secondary outcome differed between the groups (Wu et al., 2025).
Thymosin beta-4: actin, cell migration and tissue-repair models
In a rat full-thickness wound model, thymosin beta-4 increased re-epithelialization, wound contraction, collagen deposition and new vessel growth, and in a chamber assay it stimulated keratinocyte migration (Malinda et al., 1999). Our TB-500 guide covers the heart, endothelial and actin-binding motif studies and the small human studies of the full-length protein; we found no published human study of the acetylated fragment.
Thymalin: the Russian literature and its limits
The founding group reported that the extract and the group's synthetic dipeptides affected T-cell differentiation and cyclic nucleotide levels in blood lymphocytes (Morozov and Khavinson, 1997). Our thymalin guide covers the rodent, explant and cell work that followed. The limits are plain: of its roughly 300 PubMed records, about 100 list V. Kh. Khavinson or V. G. Morozov as an author, about 268 are in Russian or Ukrainian, and more than three quarters date from 1980 to 1999 (searched October 2026). The human reports behind its registration come almost entirely from the institute that created it.
Regulatory and anti-doping status
Thymalfasin, the drug name for thymosin alpha-1, is approved as a medicine in a number of countries outside the United States, while remaining unapproved in the United States and Europe (Tuthill, 2007). Thymalin is registered as a medicine in the Russian Federation (registration number LS-000267, 26 February 2010), as stated by its founding group (Khavinson et al., 2020).
Searched through openFDA in October 2026, Drugs@FDA lists no product containing thymalfasin, thymosin beta-4 or thymalin. The World Anti-Doping Agency's 2026 Prohibited List names "Thymosin-β4 and its derivatives e.g. TB-500" under S2.3, growth factors and growth factor modulators, in class S2, which is prohibited at all times. Our materials are research chemicals, not these medicines.
Key sources
- Goldstein AL. "History of the discovery of the thymosins." Ann N Y Acad Sci. 2007. PMID 17600284. DOI 10.1196/annals.1415.045.
- Goldstein AL, et al. "Thymosin alpha1: isolation and sequence analysis of an immunologically active thymic polypeptide." Proc Natl Acad Sci U S A. 1977. PMID 265536. DOI 10.1073/pnas.74.2.725.
- Hannappel E, Huff T. "The thymosins. Prothymosin alpha, parathymosin, and beta-thymosins: structure and function." Vitam Horm. 2003. PMID 12852257. DOI 10.1016/s0083-6729(03)01007-0.
- Safer D, et al. "Thymosin beta 4 and Fx, an actin-sequestering peptide, are indistinguishable." J Biol Chem. 1991. PMID 1999398.
Frequently asked questions
What are thymic peptides?
Peptides first isolated from the thymus or prepared from thymus extracts, such as thymosin alpha-1, thymosin beta-4 and thymalin. None of the fraction 5 peptides proved to be a true thymic hormone (Hannappel and Huff, 2003).
Are all thymosins made in the thymus?
No. Thymosin beta-4 occurs in many tissues, including those of mice with no thymus (Hannappel et al., 1982), and prothymosin alpha, linked to thymosin alpha-1, was found in brain, liver, kidney, lung and spleen as well as thymus (Haritos et al., 1984).
Thymosin alpha 1 vs thymosin beta 4: what is the difference?
They are unrelated molecules that share a name because both came from the same calf thymus fraction. Thymosin alpha-1 is a highly acidic 28-residue peptide (Goldstein et al., 1977) studied mainly in immune-cell signaling; thymosin beta-4 is a 43-residue actin-sequestering peptide (Safer et al., 1991) studied in cell migration and tissue-repair models.
Is TB-500 the same as thymosin beta-4?
Not by the published analyses, which found Ac-LKKTETQ, the acetylated 17 to 23 fragment, in material sold as TB-500 (Esposito et al., 2012). Confirm identity on each lot's certificate. The BPC-157 vs TB-500 comparison sets it beside BPC-157.
Is thymalin a thymosin?
No. It is a calf thymus polypeptide preparation from a St Petersburg group, not one defined peptide (Morozov and Khavinson, 1997). Our peptide bioregulators explainer covers the program it came from.
Are thymic peptides approved drugs?
Thymalfasin is approved in a number of countries outside the United States, but not in the United States or Europe (Tuthill, 2007), and thymalin is registered in Russia (Khavinson et al., 2020). Drugs@FDA lists none of them (openFDA search, October 2026).
Is thymosin beta-4 prohibited in sport?
Yes. The WADA 2026 Prohibited List names thymosin-β4 and its derivatives, such as TB-500, under S2.3 of class S2, prohibited at all times.
Where can researchers buy thymic peptides?
Disguised Alpha lists Thymosin Alpha-1, TB-500 (Thymosin Beta-4) and Thymalin as lyophilized research powders, with lot certificates on each product page and in the COA portal.
Related guides
- Thymosin alpha-1 research guide
- TB-500 (thymosin beta-4) research guide
- Thymalin research guide
- BPC-157 vs TB-500 comparison
- Peptide bioregulators explained
- LL-37 research guide: an antimicrobial peptide, not a thymic one
- What is a COA?
- COA portal: published certificates, by lot
- Research hub
Disguised Alpha products are for research use only. They have not been evaluated for safety or effectiveness in humans. Not for human consumption. All products are intended for laboratory research purposes only.