Thymosin alpha-1 is a 28-amino-acid, N-terminally acetylated peptide corresponding to a fragment of prothymosin alpha. It was originally isolated from thymic tissue in the 1970s and is now made synthetically. Under the international non-proprietary name thymalfasin it is a registered medicine in a number of countries, marketed as Zadaxin, with a labelled subcutaneous regimen for chronic hepatitis B and use as a vaccine adjuvant.
Among the compounds profiled on this site, thymosin alpha-1 has one of the more substantial evidence bases: a defined chemical identity, an approved label in multiple jurisdictions, a well-worked-out mechanism at Toll-like receptors on dendritic cells, and — unusually — a completed multicentre Phase 3 trial with over a thousand patients. It is also a compound whose most rigorous trial was negative, which is the single most important thing to know about it.
It is an immune modulator rather than an immune stimulant, and the distinction is meaningful. The mechanistic work describes it restoring or coordinating immune responses in states of impaired immunity — fungal infection after transplantation, chronic viral hepatitis, sepsis-associated immunoparalysis — rather than amplifying immune function in people whose immune systems are working normally. There is no trial of thymosin alpha-1 in healthy people, and no evidence that a healthy immune system benefits from modulation.
It is not approved by the FDA for any indication, though it holds orphan drug designations in the United States for several conditions. Orphan designation is a development incentive granted before efficacy is established, not a finding about the drug.
A 28-amino acid peptide fragment of prothymosin alpha, marketed as thymalfasin and used as an immune modulator. Supplied as a lyophilised powder that is reconstituted with bacteriostatic water before measurement.
| Weeks | Dosage | Syringe units (U-100) |
|---|---|---|
| Weeks 1–26 | 1.6 mg | 32 units0.32 mL |
FrequencyTwice weekly, subcutaneous.
The single row is the labelled regimen for thymalfasin in chronic hepatitis B — 1.6 mg twice weekly for six months — and there is no titration in it, so the amount is the same in week 26 as in week 1. Thymosin alpha-1 is approved under that labelling in a number of countries but not in the United States, and the amount is established for an infectious-disease indication in patients rather than for immune support, recovery or any other use it is discussed for. Athena lists one strength: a 10 mg vial made up with 2 mL comes to 5 mg/mL, which puts the 1.6 mg labelled amount at 32 units per injection and leaves six of them in the vial. The registered product is supplied as a 1.6 mg single-dose vial instead, which is the presentation the labelled regimen was written around. Sepsis and COVID-19 trials have used different amounts and schedules from the one charted here, and none of those has produced a labelled regimen.
What this evidence establishes. Thymosin alpha-1 is approved as thymalfasin in a number of countries — though not the United States — for chronic hepatitis B and as a vaccine adjuvant, and the amount shown above is its labelled dose for that indication. It is a dose established for an infectious-disease indication in patients, which is not evidence for any other use.
Sources: Zadaxin (thymalfasin) product labelling
For educational and laboratory research purposes only. It does not provide medical advice, dosing recommendations, or instructions for human or veterinary use. Syringe units assume a U-100 syringe, on which 1 mL is 100 units and a 50-unit syringe holds 0.5 mL.
Thymosin alpha-1's mechanism is better characterised than that of most compounds in this catalogue, and it is a genuinely interesting one: it works upstream, at the innate immune system's pattern-recognition machinery, rather than acting directly on the effector cells it ultimately influences.
Romani and colleagues showed that thymosin alpha-1 induces functional maturation of fungus-pulsed dendritic cells by signalling through distinct Toll-like receptors via the MyD88-dependent pathway.
Dendritic cell maturation and interleukin-12 production proceed through the p38 mitogen-activated protein kinase and NF-κB-dependent pathway.
IL-12 from matured dendritic cells drives T-helper 1 differentiation. In mice receiving haematopoietic transplants, this translated into antifungal resistance and protection against aspergillosis.
The clinical rationale in every studied indication is correcting an impaired immune state — not boosting a normal one. In severe COVID-19, retrospective data described restoration of lymphocyte counts in patients with severe lymphocytopenia.
The same retrospective series reported reduced PD-1 and Tim-3 expression on CD8+ T cells, with changes tracking alongside a rise in T-cell receptor excision circles — a marker of thymic output.
Every studied regimen is subcutaneous and time-limited: twice weekly for six months on the hepatitis B label, or twice daily for seven days in the sepsis trials.
A well-described mechanism is a reason to run a trial, not a substitute for one. Thymosin alpha-1's dendritic cell and Th1 biology is solid, was published in a leading haematology journal, and correctly predicted a benefit in a mouse aspergillosis model. It did not predict the outcome of the largest randomised trial ever run on the compound. That gap between mechanism and clinical result is the recurring lesson of immunomodulator development, and it applies here more clearly than almost anywhere else on this site.
The TESTS result deserves to be read carefully rather than dismissed or overextended. It shows that thymosin alpha-1, at the studied dose and duration, does not reduce 28-day mortality in a broad sepsis population. It does not show that the compound is inert — the mechanistic biology is real — and it does not directly speak to the labelled hepatitis B indication, which is a different disease, a different regimen and a different endpoint. What it does show is that a marginal signal in a smaller single-blind trial did not survive a properly powered double-blind one. That sequence is common enough in critical care that it should shape how the earlier positive results are weighted.
Geographic concentration is a limitation across the whole clinical literature: the major randomised evidence comes from trials conducted in China, and the labelled hepatitis B indication reflects registration decisions made largely outside North America and Western Europe. The COVID-19 data are retrospective and small. The oncology literature is largely mechanistic and review-based.
Most relevant to how this compound is actually used: there is no trial of thymosin alpha-1 in healthy adults, for general immune support, longevity, recovery or resilience. Every studied population had a specific disease and a specific measurable immune deficit. Extending a restoration mechanism to a system that is not impaired is not supported by anything in this literature.
Tolerability across the trials has been good. In TESTS, no safety outcome differed significantly between thymosin alpha-1 and placebo across more than a thousand patients — the most substantial safety dataset for any compound on this site. The considerations below are drawn from product labelling and from the studied contexts.
| Condition / Factor | Risk Level | Rationale |
|---|---|---|
| Hypersensitivity to thymalfasin | CONTRAINDICATED | Standard labelled contraindication for the registered product. |
| Immunosuppressive therapy / organ transplant recipients | CONSULT SPECIALIST | The compound's action is to promote Th1 immune responses. In someone whose immunosuppression is deliberate and necessary, that is a potential antagonism of therapy and a decision for the transplant team. |
| Autoimmune disease | CAUTION | A Th1-polarising immunomodulator in a condition driven by inappropriate immune activation is a theoretical concern. The trials excluded such populations, so there is no evidence either way. |
| Pregnancy and lactation | INSUFFICIENT DATA | Not studied in pregnancy. Labelling advises use only where clearly needed. |
| Injection-site reactions | COMMON, MILD | Local discomfort and erythema are the most frequently reported effects. Systemic adverse events did not differ from placebo in the Phase 3 trial. |
| Use for sepsis on the basis of ETASS | SUPERSEDED | The marginal 2013 result has been superseded by a definitive Phase 3 trial in over a thousand patients showing no mortality benefit. |
| Use in healthy people for immune support | UNSTUDIED | No trial has enrolled healthy participants. The mechanism is restoration of impaired immunity, which is not a rationale that transfers to an unimpaired immune system. |
| Research-grade material | MODERATE | A registered pharmaceutical product exists in several countries. Research-grade peptide sold under the same name is not that product and carries no assurance of identity, purity or sterility. |
FDA: thymosin alpha-1 is not approved for any indication in the United States. It holds orphan drug designations for several conditions, which is a development incentive granted on the basis of rarity of the target disease, not a determination of safety or efficacy.
Outside the United States: thymalfasin is a registered medicine in a number of countries, with a labelled subcutaneous regimen of 1.6 mg twice weekly for chronic hepatitis B and use as an adjuvant to influenza and hepatitis B vaccination. That regimen — the one shown in the dosing section of this page — is a dose established for an infectious disease indication in patients, not for immune support, recovery or any other use it is informally discussed for.
Sport: thymosin alpha-1 is not a growth factor and is not the same molecule as thymosin beta-4 (TB-500), which is explicitly prohibited under WADA section S2. Athletes subject to anti-doping rules should verify current status directly with their governing body rather than relying on the distinction between the two thymosins.
How to weigh this compound: thymosin alpha-1 sits in an unusual position. It has more regulatory recognition, better mechanistic characterisation and a far larger randomised safety database than almost anything else in this catalogue — and its definitive efficacy trial in the indication it was most promising for returned a null result. Both halves of that sentence are the evidence.
Test your understanding of the mechanism, the labelled indication, and what the Phase 3 sepsis trial found.
This profile is for educational and research purposes only. It is not medical advice, and nothing on it is a protocol, a recommendation, or an instruction for use in a person or an animal.
Athena Peptides Education does not prescribe, sell, or recommend any compound. Compounds discussed here are for laboratory research only and are not for human consumption. Always consult a qualified physician before making any decision about your health.