Thymosin α-1.
Thymosin alpha-1 (Tα1, international nonproprietary name thymalfasin; brand name Zadaxin) is a synthetic 28-amino-acid acetylated peptide identical to a naturally occurring fragment derived from prothymosin alpha, a protein produced in the thymus.
Thymosin alpha-1 (Tα1, international nonproprietary name thymalfasin; brand name Zadaxin) is a synthetic 28-amino-acid acetylated peptide identical to a naturally occurring fragment derived from prothymosin alpha, a protein produced in the thymus. Thymosin α-1 is in human trials, and PepCue grades its published evidence C tier (56/100). Also known as Zadaxin. This is a research reference, not medical or dosing advice.
What it is
Thymosin alpha-1 (Tα1, international nonproprietary name thymalfasin; brand name Zadaxin) is a synthetic 28-amino-acid acetylated peptide identical to a naturally occurring fragment derived from prothymosin alpha, a protein produced in the thymus. It belongs to the thymosin family of thymic-derived peptides and is one of the most clinically studied immunomodulatory peptides, marketed as a prescription immune modulator in numerous countries outside the United States.
How it works
Tα1 is an immune modulator rather than a simple immune stimulant: it acts largely through Toll-like receptors, principally TLR2 and TLR9, on dendritic cells, monocytes/macrophages, and other antigen-presenting cells. Downstream MyD88-dependent signaling promotes dendritic cell maturation, biases naive T cells toward Th1 differentiation, augments natural killer cell and cytotoxic T-cell activity, and can modulate regulatory pathways including indoleamine 2,3-dioxygenase. In settings of immune exhaustion or sepsis-associated immunoparalysis, the proposed benefit is restoration of T-lymphocyte numbers and function (e.g., raising depleted CD4+/CD8+ counts and reducing markers of T-cell exhaustion) rather than broad immune activation.
Mechanism pathways
Shifting immune-cell behaviour and cytokine balance rather than simply stimulating immunity.
The immune system is regulated rather than merely switched on or off, and the compounds here act as modulators: they can raise responsiveness in a suppressed state and dampen it in an overactive one, which is a different pharmacological category from a stimulant. The relevant machinery includes pattern-recognition receptors such as the Toll-like receptor family on dendritic cells and macrophages, the transcription factor NF-kB that governs pro-inflammatory gene expression, and the balance between T-helper subsets and regulatory T cells that determines the character of an immune response. One compound in this group is a thymus-derived peptide that acts principally through Toll-like receptors 2 and 9 on antigen-presenting cells. Downstream MyD88-dependent signalling promotes dendritic cell maturation, biases naive T cells toward a Th1 pattern, and augments natural killer and cytotoxic T cell activity, while also engaging regulatory pathways. Its usefulness is described in settings of immune exhaustion rather than in healthy immunity. A second acts largely without classical receptor engagement. It is taken into intestinal epithelial and immune cells by a proton-coupled di- and tripeptide transporter that is normally restricted to the small intestine but becomes induced in the colon during inflammation, meaning uptake is greatest precisely where inflammation is present. Once inside, it inhibits NF-kB and MAP kinase signalling, reducing pro-inflammatory cytokine output. A third is a widely distributed neuropeptide signalling through two class B G-protein-coupled receptors that raise cyclic AMP. Alongside smooth-muscle relaxation and secretory effects, it produces broad anti-inflammatory action by shifting cytokine balance away from pro-inflammatory mediators and supporting regulatory T cell populations. A fourth is included for its anti-inflammatory and anti-apoptotic effects, which accompany its better-known role in cytoskeletal regulation. Evidence quality varies considerably across this group and should not be averaged. One member is an approved medicine in a number of countries for specific infectious and immunological indications, giving it genuine human clinical grounding. Others rest largely on cell and animal work, with human data limited or confined to small studies. Immune modulation also carries a bidirectional risk that is rarely acknowledged: a compound capable of enhancing immune activity is in principle capable of aggravating autoimmune conditions, and that interaction is poorly characterised for most of this group.
The evidence
Human evidence is most developed in chronic hepatitis B and as an adjunct in sepsis, but is mixed and often comes from China-based studies of variable quality. The largest dedicated sepsis trial, ETASS (Wu et al., Critical Care 2013, a multicenter single-blind RCT of 361 patients with severe sepsis), found a reduction in 28-day all-cause mortality that did not reach statistical significance, so its primary endpoint was formally negative; subsequent systematic reviews (e.g., Liu et al., BMC Infect Dis 2016) noted possible mortality benefit but flagged small sample sizes, heterogeneity, and risk of bias. In COVID-19, the widely cited Liu et al. (Clin Infect Dis 2020) report associating Tα1 with lower mortality was a retrospective review of only 76 severe cases, not a randomized trial, and cannot establish efficacy. For hepatitis B, meta-analyses (e.g., entecavir plus Tα1 in HBV-related cirrhosis, BMC Gastroenterol 2020) suggest possible adjunctive benefit on virologic and immune endpoints but again rest on heterogeneous trials; high-quality Western regulatory trials have not confirmed a clear benefit. Much of the broader "immune-restoring" and anti-cancer rationale remains preclinical or mechanistic.
The evidence, in brief
A synthetic immunomodulatory peptide (Zadaxin / thymalfasin) approved in several countries for chronic hepatitis B/C and as an immune adjuvant, though not FDA-approved in the US. It has a substantial real clinical base, including randomized controlled trials in chronic hepatitis B; results vary by indication.
- Thymosin α1 treatment of chronic hepatitis B: a phase III randomized, double-blind, placebo-controlled studyJ Viral Hepat, 1999 (PMID 10607256)
Evidence maturity
An evidence-only reading: approval status, human vs preclinical data, mechanism and safety. Popularity never raises it. Research file #065
Some human signal atop preclinical work; gaps remain.
Studied in human clinical trials; evidence is meaningful.
Claim receipts
Popular claims about Thymosin α-1, checked against the state of the evidence. The verdict describes evidence maturity, never an invented study result.
Thymalfasin is a prescription product in roughly thirty countries, but it is not approved by the FDA or centrally by the EMA.
The largest dedicated multicenter randomized trial did not meet its primary mortality endpoint, so the claim overstates a formally negative result.
The most cited supporting report was a small retrospective review of severe cases rather than a randomized trial.
Meta-analyses suggest possible adjunctive benefit on virologic and immune endpoints, drawn from heterogeneous trials of variable quality.
Its action is immunomodulatory, raising unstudied questions in autoimmune disease and transplant settings, and compounded or research-grade material has no assured identity or purity.
Safety profile
Across decades of clinical use as thymalfasin, Tα1 has generally been reported as well tolerated, with injection-site reactions among the more commonly noted effects; it is a peptide given by injection in studied settings. Because its action is immunomodulatory, theoretical and context-dependent concerns include effects in autoimmune disease, transplant recipients, or other immune-sensitive populations, and immunogenicity is one reason regulators have scrutinized compounded versions. Importantly, much safety data comes from regulated pharmaceutical product used under medical supervision; the purity, identity, and safety of research-grade or compounded "gray market" material are not assured, and long-term safety outside approved indications is not well characterized.
Compound notes
- Thymosin alpha-1 (Tα1) is an immune-modulating peptide that influences T-cell function.
- It is approved in a number of countries (brand Zadaxin) for hepatitis B/C and as an immune adjuvant, but it is not FDA-approved in the US.
- Regulatory status varies by country; US use is research/compounded.
- “Boosts immunity” in healthy people is not an established benefit.
Regulatory status
Thymalfasin (Zadaxin) is approved and marketed as a prescription drug in roughly 30-plus countries (including China, Italy, and parts of Asia, Latin America, and the Middle East), chiefly for chronic hepatitis B and as an immune adjunct, but it is NOT approved by the US FDA or centrally by the EMA; in the US it has held orphan-drug designations and been studied investigationally. In US compounding, FDA placed thymosin alpha-1 in Category 2 of the interim 503A bulk substances list in 2023 (citing safety/data concerns), and in 2024 it was removed from Category 2 after its nomination was withdrawn, leaving it without a clear compounding pathway.
Investigational, currently in registered human clinical trials.
By the numbers
- 01Synthetic 28-amino-acid peptide corresponding to a fragment of prothymosin alpha; INN thymalfasin, brand Zadaxin
- 02Acts mainly via TLR2/TLR9 on dendritic cells and macrophages as an immune modulator, not a generic stimulant
- 03Approved as a prescription drug in numerous countries for chronic hepatitis B and as an immune/cancer adjunct, but not FDA- or EMA-approved
- 04The largest sepsis RCT (ETASS, 2013) missed statistical significance on its primary 28-day mortality endpoint
- 05Cited COVID-19 mortality benefit came from a small retrospective study (n=76), not a randomized trial
- 06Placed on FDA's 503A Category 2 compounding list in 2023, then removed in 2024 after nomination withdrawal
Thymosin α-1: research formats
Choose the format you are researching to see route-specific notes.
Used clinically (Zadaxin); 1.6 mg/vial SC injection is the standard format.
Thymosin α-1 (Zadaxin) is used clinically in several countries for immune modulation and chronic hepatitis B/C treatment. It is provided as a 1.6 mg lyophilized vial reconstituted with 1 mL sterile water (since clinical use is single-dose, sterile water rather than BAC water is supplied). Research contexts often mirror this format.
Sources
Every factual claim above resolves to a real, published source.
- The efficacy of thymosin alpha 1 for severe sepsis (ETASS): a multicenter, single-blind, randomized and controlled trialCritical Care, 2013; PMID 23327199
- The efficacy of thymosin alpha1 as immunomodulatory treatment for sepsis: a systematic review of randomized controlled trialsBMC Infectious Diseases, 2016; PMID 27633969
- Thymosin Alpha 1 Reduces the Mortality of Severe COVID-19 by Restoration of Lymphocytopenia and Reversion of Exhausted T CellsClinical Infectious Diseases, 2020 (retrospective, n=76); PMID 32442287
- Bulk Drug Substances Used in Compounding Under Section 503A of the FD&C Act (interim list categories)U.S. Food and Drug Administration, regulatory page
Cite this page
PepCue. “Thymosin α-1: the evidence.” PepCue, reviewed June 1, 2026. https://www.pepcue.app/p/thymosin-alpha-1.
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