Thymosin alpha-1 is one of the few peptides in this catalogue that is an actual licensed medicine somewhere. Its synthetic version, thymalfasin, is the active ingredient in the brand Zadaxin and is approved in more than 35 countries for chronic hepatitis B and as an immune enhancer, though not by the US FDA. CAS number 62304-98-7. You'll also see it written as Tα1, thymosin alpha 1, or alpha1-thymosin.
The peptide was first isolated from thymus tissue in 1977, out of a crude extract called thymosin fraction 5. It is a chain of 28 amino acids and, as the name suggests, the synthetic drug is made to be identical to the natural human peptide.
What it is and how it's thought to work
Thymosin alpha-1 behaves as an immunomodulator rather than a hormone in the usual sense. Per DrugBank and a comprehensive review in International Immunopharmacology, it acts largely through Toll-like receptors, particularly TLR-2 and TLR-9, on dendritic and myeloid cells. Downstream, that nudges naive T cells toward maturation into CD4+ and CD8+ subsets and shifts cytokine output, including interferon-gamma and interleukin-2. The short version: it pushes the adaptive immune response, which is why most of its studied uses sit in infection, immune deficiency, and as a vaccine adjuvant.
That mechanism is reasonably well characterised compared with most research peptides. It is not a muscle, tendon, or growth peptide, and it should not be confused with thymosin beta-4 or its fragment TB-500, which are different molecules with a different biology.
State of the evidence
The research base here is genuinely substantial, decades deep, but the headline results are mixed.
- Hepatitis B. This is the indication behind most of the approvals. Trials have reported virological response, and Tα1 is used clinically for chronic HBV in China and elsewhere, sometimes alongside interferon.
- Vaccine response and immune support. It is approved in China to enhance vaccine response, and has been studied in immunocompromised and elderly patients.
- Sepsis. This is where the most important recent data landed. The TESTS trial, a multicentre, double-blind, placebo-controlled phase 3 study published in BMJ in 2025, randomised more than 1,000 adults with sepsis. Twenty-eight-day all-cause mortality was 23.4% with thymosin alpha-1 versus 24.1% with placebo (hazard ratio 0.99, 95% CI 0.77–1.27; p=0.93). The authors found no clear evidence it reduces mortality, though they flagged possible subgroup signals worth further study. A large, rigorous trial returning a null result is useful information, and it tempers earlier, smaller positive studies.
- COVID-19. Studied during the pandemic in several trials; the data were limited and not practice-changing.
Safety, across this long history, has generally been reported as good, with injection-site reactions the most common complaint. That track record is part of why it stayed in use abroad despite never clearing the FDA.
For buyers: regulatory status and quality
This is the part that matters if you're looking at a vial. Thymalfasin is a prescription drug where it's licensed, and that licensed product is not the same thing as research-grade peptide sold online. In the United States it is not FDA-approved, and in 2023 the FDA placed thymosin alpha-1 in the category of bulk drug substances that should not be used in compounding. Material sold by research-chemical vendors is for research use only, not for human consumption.
peptideone does not sell peptides, run lab tests, or give medical or dosing advice. Nothing here is a recommendation to use this compound. Because research-grade vials are unregulated, identity and purity vary between sellers, so the questions worth asking are the obvious ones: is there a recent, batch-specific certificate of analysis, and does it cover identity (often by mass spec) and purity (typically HPLC)? We aggregate third-party vendor reputation and testing signals where they exist; treat any vendor claim without a matching COA as unverified.
On sport: thymosin alpha-1 is an immunomodulating peptide and is not the same as the WADA-prohibited thymosin beta-4 / TB-500. Athletes should not assume status from the name alone and should check the current WADA Prohibited List directly.