Thymosin Alpha-1 (Thymalfasin): Mechanism, Trial Evidence and Research Profile

Thymosin alpha-1 (Tα1, synthetic form thymalfasin) is a 28-amino-acid peptide originally isolated from calf thymus as the fraction that restored immune function in thymectomised mice. It is one of the few research peptides with a genuine pharmaceutical track record: sold as Zadaxin in China, Italy and a number of other markets for hepatitis B and as a vaccine adjuvant, tested in a 1,100-patient phase 3 sepsis trial published in the BMJ in 2025, and now under a 2026 Cochrane review. This profile summarises the mechanism, what the large trials actually found, and what the 59 suppliers on Peptides.SO charge for the 10 mg research vial.

> Research use only. Thymosin alpha-1 sold through the suppliers listed here is a laboratory reagent for in-vitro and preclinical investigation. It is not the licensed drug Zadaxin, and nothing on this page is a protocol for human or animal administration.

What thymosin alpha-1 is

Tα1 is an N-terminally acetylated peptide of 28 residues, cleaved from the precursor prothymosin alpha. Allan Goldstein's group described it in 1972 during fractionation of thymosin fraction 5. It is highly acidic, unstructured in solution, and circulates at low nanomolar concentrations in healthy humans. The synthetic version, thymalfasin, is chemically identical.

The 2016 Vitamins and Hormones chapter Immune Modulation with Thymosin Alpha 1 Treatment (PMID 27450734) is the most useful single overview: it traces the peptide from its isolation "as the compound responsible for restoring immune function to thymectomized mice" to its pleiotropic effects on the immune cell subsets involved in immune suppression.

Mechanism of action

Toll-like receptor signalling in dendritic cells

The central mechanism is activation of Toll-like receptors on dendritic cells. Luigina Romani's group at Perugia established this in a series of papers in the mid-2000s. Thymosin alpha1: an endogenous regulator of inflammation, immunity, and tolerance (PMID 17495242) summarises the model: Tα1 primes dendritic cells through TLR signalling, and because dendritic cells "sense infection and tissue stress and translate collectively this information into an appropriate immune response", an action on them predicts a role in inducing both immunity and tolerance depending on context.

The specific receptor pathway was mapped in Thymosin alpha1 activates the TLR9/MyD88/IRF7-dependent murine cytomegalovirus sensing for induction of anti-viral responses in vivo (PMID 17804687). In TLR-deficient mice infected with murine cytomegalovirus, Tα1 protection depended on intact TLR9, the adaptor MyD88 and the transcription factor IRF7, which links the peptide to the innate antiviral sensing machinery rather than to a dedicated receptor of its own. The Vitamins and Hormones review adds that Tα1 acts "through Toll-like receptors in both myeloid and plasmacytoid dendritic cells", which is why both Th1 priming and type I interferon production show up downstream.

Modulation rather than stimulation

Tα1 also has tolerogenic effects. The same review reports that Tα1 also induces indoleamine 2,3-dioxygenase in dendritic cells, "affecting tolerization toward self as well as microbial non-self-antigens", with transplantation tolerance and protection from inflammatory allergy as the in-vivo readouts, which is why the peptide has been studied in inflammatory conditions and in transplantation as well as in infection. Readers who want the contrast with a pure innate-immune effector should compare the LL-37 vs thymosin alpha-1 comparison.

A neuronal receptor

In 2026 a group reported that Tα1 binds a receptor outside the immune system entirely. Thymosin Alpha-1 Provides Direct Neuroprotection by Engaging the Orexin Receptor HCRTR1 to Suppress Neuronal Necroptosis (PMID 42693587) describes Tα1 as a non-canonical ligand for the hypocretin/orexin receptor 1 on neurons, where it suppresses RIPK3-driven necroptosis. This is a single recent paper and the finding has not yet been replicated, but it is the first molecularly defined non-immune target for the peptide.

Clinical evidence

Chronic hepatitis B

Hepatitis B is the indication on which Zadaxin was originally licensed, and it is where the evidence is most contested. The 2026 Cochrane review Thymosin-α1 for people with chronic hepatitis B (PMID 42713852) searched to June 2026, included randomised trials at any dose or route, and rated the certainty of evidence "very low for all outcomes except for serious adverse events (low)". Its conclusion is that the reviewers "are not sure whether thymosin-α1 monotherapy versus placebo or no intervention, or with the same co-interventions, reduces all-cause mortality, serious adverse events, HBV-related mortality, and non-serious adverse events". Earlier meta-analyses that reported virological benefit were built on the same small trials; the Cochrane assessment is the current standard.

In hepatitis-B-related acute-on-chronic liver failure the picture is different and more recent. Thymosin α1 improves the outcomes of patients with hepatitis B virus-related acute-on-chronic liver failure by restoring immune balance (PMID 41887933) reports an open-label randomised trial (NCT03082885) of 73 patients in which the peptide was added to standard medical therapy, with flow-cytometry and cytokine data linking 90-day transplant-free survival to restoration of immune cell balance. It is a mechanistic trial with a survival signal, not a definitive outcome study.

Sepsis: from ETASS to TESTS

In sepsis, a promising single-blind trial was later overturned by a larger double-blind one.

The efficacy of thymosin alpha 1 for severe sepsis (ETASS): a multicenter, single-blind, randomized and controlled trial (PMID 23327199) ran at six Chinese teaching hospitals from 2008 to 2010. The relative risk of death with Tα1 was 0.74 (95% CI 0.54 to 1.02), with the log-rank test just reaching significance (p=0.049), and monocyte HLA-DR expression improved more in the treated group at days 3 and 7. The authors concluded the peptide "may be effective in improving clinical outcomes in a targeted population of severe sepsis".

That hypothesis was tested properly a decade later. The efficacy and safety of thymosin α1 for sepsis (TESTS): multicentre, double blinded, randomised, placebo controlled, phase 3 trial (PMID 39814420), published in the BMJ in 2025, enrolled 1,106 adults with sepsis at 22 Chinese centres between 2016 and 2020. Twenty-eight-day all-cause mortality was 23.4% with Tα1 and 24.1% with placebo (hazard ratio 0.99, 95% CI 0.77 to 1.27). No secondary or safety outcome differed. Prespecified subgroups suggested possible differential effects by age and by diabetes status, but the trial "found no clear evidence" of a mortality benefit in sepsis overall. ETASS should not be cited without TESTS.

COVID-19

Tα1 was used widely in Chinese COVID-19 wards in 2020, and the observational data were encouraging. The controlled evidence is thinner. Thymosin alpha1 use in adult COVID-19 patients: A systematic review and meta-analysis on clinical outcomes (PMID 36527881) pooled nine studies covering 5,352 patients and found no statistically significant effect on mortality overall (RR 1.03, 95% CI 0.60 to 1.75, with 90% heterogeneity). Subgroups of older and more severely ill patients showed lower mortality, but the authors concluded that the results "do not support the use of Ta1 in hospitalized adult COVID-19 patients".

The one prospective randomised trial from outside China, A Pilot Trial of Thymalfasin (Thymosin-α-1) to Treat Hospitalized Patients With Hypoxemia and Lymphocytopenia Due to Coronavirus Disease 2019 Infection (PMID 36056913), randomised 49 patients. Clinical recovery was not significantly different, but among patients on low-flow oxygen the treated group had 3.84 times more CD4+ T cells on day 5 relative to day 1 (p=0.01). The immunological effect is reproducible; the clinical effect has not been shown.

Vaccine adjuvant

The adjuvant use is the best supported of the peptide's smaller indications. Thymosin-alpha 1 (Zadaxin) enhances the immunogenicity of an adjuvated pandemic H1N1v influenza vaccine (Focetria) in hemodialyzed patients: a pilot study (PMID 22178096) found that a larger proportion of haemodialysis patients seroconverted by day 21 when the peptide was given with the vaccine, that the European CHMP immunogenicity criteria were fully met only in the Tα1 groups, and that no adverse event was attributed to the peptide. It was a pilot in a specific immunosuppressed population; the authors called for larger studies that have not been published.

Oncology

Thymic peptides have been added to chemotherapy in trials for decades. The Cochrane review Thymic peptides for treatment of cancer patients (PMID 21328265) assessed purified thymus extracts and synthetic thymic peptides including Tα1 alongside chemotherapy or radiotherapy in adults. It found no evidence overall that adding thymic peptides reduced the risk of death or disease progression, though for Tα1 specifically there was "a trend for a reduced risk of dying and of improved DFS", and preliminary evidence that purified extracts lowered the risk of severe infectious complications during chemotherapy (RR 0.54). Most trials carried at least moderate risk of bias. Hepatocellular carcinoma remains the most-studied tumour, in combination with transarterial chemoembolisation and more recently with immune checkpoint inhibitors, but the trials are small and mostly single-centre.

Safety

The narrative review Comprehensive Review of the Safety and Efficacy of Thymosin Alpha 1 in Human Clinical Trials (PMID 38308608) covers more than 30 trials and over 11,000 subjects and describes the peptide as "well-tolerated". Its efficacy conclusions are more enthusiastic than the randomised evidence above supports, and the review is an advocacy piece for US compounding access, so read it for the safety summary rather than the efficacy claims. Both TESTS and the Cochrane hepatitis B review reached the same conclusion on safety from randomised data: no signal of serious harm. The peptide's problem in trials has been efficacy, not tolerability.

Research specifications

Because the peptide is acidic and unstructured it dissolves readily in water or bacteriostatic water and does not need acetic acid. See the reconstitution guide for handling.

Thymosin alpha-1 on Peptides.SO: supplier pricing

The platform tracks 66 listings of the 10 mg Tα1 vial from 59 suppliers, price-checked through 25 September 2026. The median list price is $72.75; the interquartile range is $60 to $99.99, and the full range runs from $35 to $338. Thirteen of the 66 listings show a discounted price. Across all Tα1-related rows (including 5 mg vials, 5/10 mg kits and the Tα1/thymalin blend) there are 84 listings from 70 suppliers with a combined median of $70.70; the top of that range is a multi-vial bulk pack, not a single vial.

Set against the rest of the catalogue, a 10 mg Tα1 vial at the median costs slightly more than a 5 mg BPC-157 vial ($65 median across roughly 100 suppliers) and about the same as a 5 mg CJC-1295 no-DAC vial ($64.95 across 56 suppliers). Given that it is a 28-mer synthesised at 10 mg, that pricing is consistent with the peptide being straightforward to manufacture; the $300-plus listings are outliers, not a quality tier.

• •Current offers and per-supplier comparison: thymosin alpha-1 10 mg listings
• •Checking a supplier before ordering: supplier checklist
• •Reading the certificate: COA interpretation guide

Comparison with related peptides

Thymosin beta-4 (TB-500). Despite the shared name, Tβ4 is a 43-residue actin-sequestering peptide studied for tissue repair, with no TLR mechanism. The two are compared in detail in thymosin alpha-1 vs thymosin beta-4.

Thymulin. The other classical thymic hormone, a zinc-dependent nonapeptide that acts on T-cell differentiation through a different pathway; see the thymulin research profile.

LL-37. A human cathelicidin that kills microbes directly and signals through formyl-peptide receptors; Tα1 does neither. The LL-37 vs Tα1 comparison covers the distinction.

Frequently asked questions

Is thymosin alpha-1 an approved drug?

Thymalfasin (Zadaxin) is licensed in China, Italy and a number of other countries, mainly for chronic hepatitis B and as a vaccine adjuvant. It has never been approved by the US FDA. Research-grade Tα1 on this site is not the licensed product.

Did it reduce mortality in sepsis?

No. The 2025 BMJ phase 3 trial of 1,106 patients (PMID 39814420) found 28-day mortality of 23.4% versus 24.1% on placebo. The earlier single-blind ETASS trial (PMID 23327199) had suggested a benefit.

What does the Cochrane review say about hepatitis B?

That the evidence is of very low certainty and it is unclear whether Tα1 reduces mortality, serious adverse events or HBV-related outcomes (PMID 42713852).

How does it act on the immune system?

Through Toll-like receptors on dendritic cells, with TLR9/MyD88/IRF7 signalling demonstrated in mice (PMID 17804687). Downstream it promotes Th1 priming and, in other contexts, tolerance.

Is it safe?

Randomised trials and a review of over 11,000 subjects (PMID 38308608) report no serious safety signal. Efficacy, not safety, is where the trial record is weak.

What is a fair research price?

The 10 mg vial has a $72.75 median across 59 suppliers on this platform; listings above $150 are not explained by any purity tier the certificates support.

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This article is for educational and research purposes only. Thymosin alpha-1 sold through the suppliers listed on Peptides.SO is a laboratory reagent and is not intended for human or animal use. Trial regimens belong to the cited studies and are not guidance. Nothing on this page is medical advice.