Thymosin Alpha-1 vs TB-500 (Thymosin Beta-4): Not the Same Peptide

Key takeaways

• Thymosin alpha-1 and TB-500 are unrelated molecules. Thymosin alpha-1 is a 28-amino-acid immune peptide cut from prothymosin alpha (gene PTMA, chromosome 2). TB-500 is a seven-amino-acid fragment of thymosin beta-4, a 43-amino-acid actin-binding protein (gene TMSB4X, X chromosome).
• They share a name only because both were first purified from the same 1970s calf-thymus extract. Neither is a thymus-specific hormone.
• The evidence is of a different order. Thymosin alpha-1 has decades of human trials and approvals abroad, with modest or null results. TB-500 has no human study at all; the trials sellers cite used full-length thymosin beta-4.
• WADA names thymosin beta-4 and TB-500 on its Prohibited List; thymosin alpha-1 is not named.
• Neither can be legally compounded in the US as of October 2026. FDA's advisory committee voted 17 to 4 against thymosin alpha-1 in 2024 and 8-6-1 for TB-500 in 2026, and FDA has finalized no rule on either.

The direct answer: thymosin alpha-1 and TB-500 are not two versions of the same peptide. They come from different genes, differ in size by a factor of four, do different jobs in the body and have completely different evidence, and the only thing they share is the word 'thymosin'. Thymosin alpha-1 is an immune modulator with a long, mostly disappointing trial record and approvals outside the US, covered in our thymosin alpha-1 profile and its dosage guide. TB-500 is a short synthetic fragment of an unrelated protein, sold for healing without a single human study, covered in our TB-500 profile.

Side by side

Different genes, different molecules

Thymosin alpha-1 does not have a gene of its own. It is the front end of prothymosin alpha, a protein encoded by the PTMA gene on chromosome 2: cells make the whole protein, and thymosin alpha-1 corresponds to its residues 2 to 29, the first 28 amino acids once the starting methionine is removed (UniProt P06454). It was first purified and sequenced from calf thymus in 1977, a highly acidic chain of 28 residues (Goldstein 1977). The drug thymalfasin is that sequence made synthetically.

Thymosin beta-4 is encoded by a different gene, TMSB4X, on the X chromosome (UniProt P62328). The protein is 43 amino acids long, with an isoelectric point of 5.1 and an acetylated start (Low 1981). TB-500 is not that protein: when doping-control chemists analyzed products sold under the name, the key ingredient was a fragment of seven amino acids, Leu-Lys-Lys-Thr-Glu-Thr-Gln with an acetyl group, cut from the middle of thymosin beta-4 (Esposito 2012).

So the comparison is a 28-amino-acid immune peptide against a 7-amino-acid fragment of a different 43-amino-acid protein. There is no shared sequence, no shared receptor and no reason to expect either to behave like the other.

Different jobs in the body

Thymosin alpha-1 is an immune signal. In laboratory and animal work it acts on dendritic cells and immature lymphocytes through toll-like receptors, nudging them toward antigen presentation and T-cell responses, which is why it was developed for chronic viral infections, vaccine response and cancer.

Thymosin beta-4 is a structural housekeeper. It binds and sequesters actin monomers, holding them in reserve so a cell can rebuild its skeleton on demand (UniProt P62328). In laboratory and animal work, its migration-promoting and blood-vessel-promoting activity maps to the stretch around amino acids 17 to 23, the same stretch sold as TB-500, which is why it is marketed for healing and why FDA's reviewers worried about tumors; the TB-500 profile has the details.

Different evidence

Thymosin alpha-1 has been tested in thousands of patients. The best-known positive trial randomized 98 people with chronic hepatitis B in Taiwan to 1.6 mg twice a week or no treatment, and 18 months later the virus had cleared in 40.6% of the 26-week group versus 9.4% of controls (Chien 1998). A double-blind US Phase 3 did not confirm that, and the largest trial ever run, in 1,106 adults with sepsis, found no survival benefit; the thymosin alpha-1 profile grades each use. FDA's advisory committee weighed that record in December 2024 and voted 4 yes, 17 no on each form of the substance, with several members citing no compelling evidence of effectiveness and safety (FDA minutes 2024).

TB-500 has no human study at all, as FDA acknowledged when its committee reviewed it in July 2026; the only animal study of the fragment, in 32 rats with repaired Achilles tendons, appeared the same year. The human trials sellers cite used full-length thymosin beta-4 as eye drops, a skin gel and intravenous infusions, and the best of them, in 18 patients with non-healing corneal ulcers, fell just short of statistical significance; the TB-500 profile details each one. Even so, the committee voted 8-6-1 in its favor, the opposite of its verdict on the peptide with the larger human record. FDA's own reviewers had recommended against both.

Why the shared name confuses buyers

The name is historical, not chemical. Both peptides were purified from thymosin fraction 5, a crude calf-thymus extract made in the search for thymic hormones, and Goldstein's group proposed a Greek-letter naming scheme for the peptides in it (Goldstein 1977). 'Alpha' and 'beta' mark different peptides from the same extract; thymosin alpha-1 is highly acidic, while thymosin beta-4 has an isoelectric point of 5.1 (Low 1981). Both were called thymic hormones in their day, and neither turned out to be one.

Approval borrowed across molecules. A buyer who reads that 'thymosin' is an approved drug in Italy or Singapore is reading about thymalfasin. None of that record applies to TB-500 or to thymosin beta-4, which have never been approved anywhere.

Evidence borrowed twice over. TB-500 marketing leans on trials of full-length thymosin beta-4, a different and much larger molecule. And people search for 'thymosin peptide benefits' as if the two were one family; any answer that treats them that way merges an immune drug's trial record with a healing fragment's absence of one.

Regulatory headlines that mix them up. Both were placed in FDA's Category 2, and both left it after their nominations were withdrawn, but at different times and with opposite outcomes: FDA reviewed thymosin alpha-1 anyway and its committee voted it down in December 2024, while TB-500 came off in the April 2026 removals and was voted up that July. The 2026 headlines about peptides leaving Category 2 were about TB-500 and eleven others, not thymosin alpha-1, and neither peptide may be compounded today. The legal tracker keeps both entries current.

Anti-doping lists that treat them differently. The 2026 WADA Prohibited List names 'thymosin-beta4 and its derivatives, e.g. TB-500' under S2.3, growth factors, prohibited at all times. Thymosin alpha-1 is not named, and the S0 catch-all covers only substances with no current approval by any government health authority, which is hard to apply to a drug approved in several countries (WADA 2026). An athlete who assumes the two are the same could get it wrong in either direction, and should check any product with an anti-doping agency before use.

Stacks. TB-500 is often sold combined with other peptides, as in the Wolverine stack. No study has combined thymosin alpha-1 with TB-500, so any pairing of the two is untested.

Which one, if either?

Neither has a legal US product in October 2026, so neither should be bought. For chronic hepatitis B, a weak vaccine response or recurrent infections, approved treatments have better evidence than thymosin alpha-1; for a tendon or muscle injury, rehabilitation and conventional care outrank a fragment with no human data. In neither case is the other peptide the answer.

Bottom line

Thymosin alpha-1 and TB-500 share a name and a 1970s lab extract. Everything else differs: gene, size, job, evidence, anti-doping status and regulatory history. Thymosin alpha-1 is a modestly effective, largely superseded immune drug abroad that FDA's advisers voted not to let US pharmacies compound; TB-500 is an unapproved fragment with no human study, named on the WADA list. Reading the research or approvals of one as support for the other is the mistake the shared name invites.

Frequently asked questions

+−Is thymosin alpha-1 the same as thymosin beta-4?

No. Thymosin alpha-1 is a 28-amino-acid peptide cut from prothymosin alpha, a protein made from the PTMA gene on chromosome 2. Thymosin beta-4 is a separate 43-amino-acid protein made from the TMSB4X gene on the X chromosome. They do different jobs, immune signaling for alpha-1 and holding actin for beta-4, and share only a name that dates from a 1970s calf-thymus extract.

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

Not exactly. TB-500 is a product name. Doping-control chemists who analyzed it found its key ingredient was a seven-amino-acid fragment of thymosin beta-4, amino acids 17 to 23 with an acetyl group (Ac-LKKTETQ), and vendor products vary. Every human trial cited for TB-500 actually used the full 43-amino-acid protein. TB-500 has no human study of its own.

+−Is thymosin alpha-1 good for healing, like TB-500?

No. Thymosin alpha-1 is an immune modulator, tested in hepatitis, sepsis, vaccine response and cancer; it has no trials in tendon, muscle or wound healing. TB-500 is the one sold for healing, but it has no human data either: the healing trials used full-length thymosin beta-4 as eye drops, skin gel or infusions, with mixed results.

+−Can you take thymosin alpha-1 and TB-500 together?

No study has tested the combination, so nobody knows what it does or what it risks. Neither can be legally compounded in the US, so a combination would mean injecting two unapproved products of unverified content. Their theoretical risks also overlap badly in one group: people with cancer, where thymosin beta-4's migration and blood-vessel effects are the concern and thymosin alpha-1's immune effects are untested outside trials.

+−Which is better for the immune system, thymosin alpha-1 or TB-500?

Thymosin alpha-1 is the one with an immune mechanism and decades of trials, and even so the results are modest: a delayed response in some hepatitis B trials, antibody gains after flu vaccination in older men, and no survival benefit in a 1,106-patient sepsis trial. TB-500 is not an immune drug. Neither has been tested for immune support in healthy people.

+−Are thymosin alpha-1 and TB-500 banned in sport?

TB-500 is: the 2026 WADA Prohibited List names thymosin beta-4 and its derivatives, e.g. TB-500, under S2.3, prohibited at all times. Thymosin alpha-1 is not named anywhere on the list, and because thymalfasin is an approved medicine in several countries, the S0 catch-all for unapproved substances does not obviously cover it. Athletes should still confirm with their anti-doping agency before using any product, especially one bought from an unregulated seller.

+−Why are they both called thymosin?

Both were first purified from thymosin fraction 5, a crude extract of calf thymus made in the search for thymic hormones, and the research group proposed naming the extract's peptides with Greek letters and numbers. The name records where they were found, not what they are. Later work showed neither is a thymus-specific hormone: both are made in many tissues.

Sources

1. StudyGoldstein AL et al. Thymosin alpha1: isolation and sequence analysis of an immunologically active thymic polypeptide. Proc Natl Acad Sci U S A 1977;74:725-9. (1977)
2. StudyLow TL, Hu SK, Goldstein AL. Complete amino acid sequence of bovine thymosin beta 4: a thymic hormone that induces terminal deoxynucleotidyl transferase activity in thymocyte populations. Proc Natl Acad Sci U S A 1981;78:1162-6. (1981)
3. SourceUniProt P06454. Prothymosin alpha, human (gene PTMA, chromosome 2); residues 2 to 29 annotated as the peptide thymosin alpha-1. Accessed October 10, 2026. (2026)
4. SourceUniProt P62328. Thymosin beta-4, human (gene TMSB4X, chromosome X); mature chain of 43 residues that binds and sequesters actin monomers. Accessed October 10, 2026. (2026)
5. StudyEsposito S et al. 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;4:733-8. (2012)
6. StudyChien RN, Liaw YF, Chen TC, Yeh CT, Sheen IS. Efficacy of thymosin alpha1 in patients with chronic hepatitis B: a randomized, controlled trial. Hepatology 1998;27:1383-7. (1998)
7. RegulatoryFDA. Final summary minutes of the Pharmacy Compounding Advisory Committee meeting of December 4, 2024 (AOD-9604, CJC-1295 and thymosin alpha-1), approved February 21, 2025. (2025)
8. RegulatoryWorld Anti-Doping Agency. The 2026 Prohibited List (in force January 1, 2026): S2.3 names thymosin-beta4 and its derivatives, e.g. TB-500; thymosin alpha-1 is not listed; S0 covers substances with no current approval by any governmental health authority. (2026)