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Thymosin Alpha-1

What it is. What the evidence says.

A chain of 28 amino acids first found in the thymus, the gland where some immune cells mature. A lab-made version is called thymalfasin.

Moderate EvidenceUnapproved

Predominant research setting. Clinical confidence is claim-specific.

Updated Scientific review pending
Start with the basics Regulatory status ↓
Sequence diagram and chemical structure information for Thymosin Alpha-1.
Thymosin Alpha-1 Chemical Structure & Sequence · Inside Peptides Scientific Library
Compound
Thymalfasin (thymosin alpha-1)
Human effectiveness
Not established. The largest double-blind sepsis trial (1,106 adults) found no difference in 28-day mortality, and a 2026 Cochrane review of 10 hepatitis B trials judged the evidence low or very low certainty.
Regulatory status
Unapproved Compound
START HERE · PLAIN LANGUAGE

Thymosin Alpha-1: the basics

What is it?

A chain of 28 amino acids first found in the thymus, the gland where some immune cells mature. A lab-made version is called thymalfasin.

Research paper 1 ↗Research paper 2 ↗Research paper 3 ↗

What do people claim it helps with?

Stronger immunity, faster recovery from infections, help with hepatitis B, sepsis or cancer care, and general immune boosting.

What has research actually shown?

The biggest and best-controlled sepsis trial found no survival benefit. A 2026 review of hepatitis B trials found possible benefits but rated the evidence very uncertain.

Research paper 1 ↗Research paper 2 ↗Research paper 3 ↗

Has this been demonstrated in people?

Human trials, mostly in hospitals in China and other countries, plus older hepatitis studies. Many were small, unblinded or funded by the manufacturer.

Research paper 1 ↗Research paper 2 ↗Research paper 3 ↗

What side effects, risks and unknowns should I understand?

Trials describe it as generally well tolerated, but FDA has raised possible immune-reaction and impurity concerns for compounded versions. Long-term effects of unsupervised use are unknown.

FDA safety information ↗

Is it an approved medicine?

No FDA-approved medicine contains this peptide. Thymalfasin (Zadaxin) is approved in some other countries, which does not make it approved in the U.S.

FDA safety information 1 ↗FDA briefing 2 ↗

Approval here means U.S. FDA approval and applies to a particular product, formulation and use. This is a starting overview, not a complete side-effect list. Sources checked 10 October 2026; independent scientific review pending.

How is it used or studied?

Studied as an injection under the skin in clinical trials. There is no FDA-approved U.S. product, and an online vial is not necessarily the product used in studies.

Different routes and formulations can have different effects. A study of one product does not validate every product with the same peptide name.

A common misunderstanding

Thymosin alpha-1 is not thymosin beta-4 or TB-500. They are different molecules with different evidence and different rules.

What should I ask a clinician?
  • What diagnosis or goal are we trying to address, and what established options are available?
  • Was the expected benefit shown in people like me, using this exact product?
  • How could my medicines, health conditions or pregnancy plans affect the risks?
  • What monitoring is needed, and what would tell us the treatment is helping or should be stopped?
What do the research words mean?
Peptide
A short chain of amino acids, the building blocks of proteins.
Preclinical
Research in cells, tissues or animals before dependable treatment evidence in people.
Controlled trial
A study comparing a treatment with another treatment or a placebo to help separate its effects from other explanations.
Biomarker
A measurement, such as a hormone level. A change does not necessarily mean someone feels better or lives longer.
THE AVAILABLE RESEARCH

Thymosin Alpha-1 Evidence & Clinical Profile

Human Clinical Evidence

Human Study Limits

Thymosin Alpha-1 has been investigated in clinical contexts. Patient outcomes must be evaluated against study size, design, and control cohorts.

Preclinical & Pharmacological Data

Laboratory & In Vivo Assays

Cellular and animal disease models examine receptor binding affinities and signaling cascades, which require human confirmation.

Understand our evidence labels →

IDENTITY & STRUCTURE

What is Thymosin Alpha-1?

Thymalfasin (Zadaxin) is A 28-amino-acid immune-signalling peptide originally identified in thymus tissue. A synthetic version, thymalfasin (Zadaxin), is approved as a medicine in a number of countries outside the U.S., but it is not FDA approved. The largest placebo-controlled trial, in 1,106 adults with sepsis, found no survival benefit, and a 2026 Cochrane review rated the hepatitis B evidence as low or very low certainty. Aliases include: Thymalfasin, Zadaxin, Thymosin α1, Tα1, Ta1.

Chemical sequence and molecular parameters of Thymosin Alpha-1
Primary sequence schematic · Inside Peptides Molecular Database
Chemical reference
Sequence (one-letter)
Ac-SDAAVDTSSEITTKDLKEKKEVVEEAEN
Molecular formula
C129H215N33O55
Molecular weight
3108.3 g/mol
CAS Number
62304-98-7
UNII Code
W0B22ISQ1C
PubChem CID
16130571

Identifiers checked against PubChem and FDA's Global Substance Registration System on 13 October 2026. FDA substance record ↗ · PubChem record ↗ See all names and codes in the peptide name index.

HUMAN RESEARCH

Clinical Trials & Human Cohorts

Human research can examine tolerability, symptoms or patient outcomes. The design and comparator determine what each result can establish.

Phase 3 multicentre double-blind placebo-controlled RCT2025

The efficacy and safety of thymosin α1 for sepsis (TESTS): multicentre, double blinded, randomised, placebo controlled, phase 3 trial [1]

n = 11061,106 adults with sepsis at 22 centres in China

What was reported

28-day all-cause mortality was 23.4% with thymosin alpha-1 and 24.1% with placebo (hazard ratio 0.99). No secondary or safety outcome differed significantly.

What limits the conclusion

Single-country trial. Age and diabetes subgroup differences were exploratory. Partly supported by SciClone Pharmaceuticals, and several authors reported grants from the company.

Read primary source ↗ (external website)
Multicentre single-blind randomized controlled trial2013

The efficacy of thymosin alpha 1 for severe sepsis (ETASS): a multicenter, single-blind, randomized and controlled trial [2]

n = 361361 ICU patients with severe sepsis at six hospitals in China

What was reported

28-day mortality was 26.0% with thymosin alpha-1 versus 35.0% with standard care; the result was borderline (non-stratified P = 0.062). An immune marker (monocyte HLA-DR) improved more in the treated group.

What limits the conclusion

Single-blind with no placebo. The larger, placebo-controlled TESTS trial did not confirm a mortality benefit.

Read primary source ↗ (external website)
Cochrane systematic review and meta-analysis of RCTs2026

Thymosin-ɑ1 for people with chronic hepatitis B [3]

n = 134910 RCTs (1,349 participants) published 1991–2018

What was reported

Thymosin alpha-1 may reduce all-cause mortality and serious adverse events, but the reviewers rated the evidence very low certainty for most outcomes and low for serious adverse events.

What limits the conclusion

Mostly older trials with high or unclear risk of bias, small numbers and wide confidence intervals; four were industry funded.

Read primary source ↗ (external website)
MECHANISMS OF ACTION

How the compound may work

Mechanisms describe how Thymosin Alpha-1 may affect biological processes. Laboratory findings and patient outcomes answer different questions.

01
Mouse and cell studies

Toll-like receptor signalling in dendritic cells

In cultured dendritic cells and mice, thymosin alpha-1 promoted dendritic-cell maturation and interleukin-12 production through MyD88-dependent Toll-like receptor signalling, and protected transplanted mice from Aspergillus infection. These are laboratory and animal findings. [4]

02
Human RCT biomarker

Immune-marker changes in sepsis

In the ETASS trial, monocyte HLA-DR, a marker that is low in sepsis-related immune suppression, rose more with thymosin alpha-1 than with standard care. A biomarker change does not by itself show clinical benefit. [2]

RESEARCH AREAS

Areas Being Studied

These research topics reflect active areas of scientific exploration, distinct from unverified commercial marketing claims.

Phase 3 placebo-controlled RCT

Sepsis

An earlier single-blind trial suggested lower mortality, but the larger double-blind TESTS trial found no 28-day mortality difference. [1]

Cochrane systematic review

Chronic hepatitis B

Ten randomized trials suggest possible benefits, but certainty is low to very low. [3]

Narrative review of clinical studies

Immune adjuvant and cancer adjunct

Studied as an add-on to vaccines and cancer treatment; much of this work is small and the review was written by researchers closely involved with the peptide's development. [5]

AT A GLANCE

Compare the study designs.

Overview of Key Thymosin Alpha-1 Studies: clinical and preclinical records
Study Design / sample Outcome type Key limitation
TESTS sepsis trial · 2025 [1] Phase 3 multicentre double-blind placebo-controlled RCT
n = 1106		 Clinical endpoint evaluation Single-country trial. Age and diabetes subgroup differences were exploratory. Partly supported by SciClone Pharmaceuticals, and several authors reported grants from the company.
ETASS sepsis trial · 2013 [2] Multicentre single-blind randomized controlled trial
n = 361		 Clinical endpoint evaluation Single-blind with no placebo. The larger, placebo-controlled TESTS trial did not confirm a mortality benefit.
Cochrane hepatitis B review · 2026 [3] Cochrane systematic review and meta-analysis of RCTs
n = 1349		 Clinical endpoint evaluation Mostly older trials with high or unclear risk of bias, small numbers and wide confidence intervals; four were industry funded.
FDA & SAFETY

Regulatory Status & Clinical Safety

Regulatory Classification

Not FDA approved; Drugs@FDA lists no thymalfasin product. Approved as Zadaxin in a number of other countries. FDA's compounding-risk page lists thymosin alpha-1 among substances previously in 503A category 2 whose nominations were withdrawn; it does not appear in FDA's current 503A category lists (updated May 14, 2026).

Safety Realities & Impurities

Safety depends on the molecule, formulation, route and evidence. A laboratory purity measurement alone cannot establish safety for human use.

REGISTRY CHECK

Clinical Trials Registry Records

Official registry records from ClinicalTrials.gov. Study registration does not imply government endorsement or guaranteed efficacy.

Phase 3Completed

Thymosin Plus PEG-Interferon in Hepatitis C Patients With Cirrhosis Who Did Not Respond to Interferon or Interferon Plus Ribavirin [9]

Planned enrollment
500 (patients)
Posted results
Pending in registry
Sponsor
SciClone Pharmaceuticals
Last updated
2008
NCT00039962 · Open registry ↗ (external website)
COMPETITIVE SPORT

WADA & Anti-Doping Regulations

Not Prohibited

WADA Anti-Doping Status

Thymosin Alpha-1 is not prohibited under current World Anti-Doping Agency (WADA) regulations. However, athletes must ensure unregulated preparations or unverified vendor products do not violate Section S0 (Non-approved substances). Last verified: October 2026.

SELECTED MILESTONES

Research Timeline

  1. 2009

    Approved in many countries

    A review reported thymosin alpha-1 approved in over 35 countries, mainly for hepatitis B and C and as an immune stimulant and vaccine adjuvant. [5]

  2. 2013

    ETASS sepsis trial

    A single-blind trial in China reports a borderline mortality difference in severe sepsis. [2]

  3. 2025

    TESTS phase 3 result

    A double-blind, placebo-controlled trial of 1,106 adults finds no 28-day mortality benefit in sepsis. [1]

  4. 2026

    Cochrane hepatitis B review

    Cochrane reviewers judge the hepatitis B evidence low or very low certainty. [3]

  5. 2026

    U.S. compounding status

    FDA lists thymosin alpha-1 among nominations withdrawn from 503A category 2. It is still not an FDA-approved drug. [6]

OPEN QUESTIONS

What Researchers Still Don't Know

Long-term safety profiles

Multi-year observational data in broader heterogeneous cohorts remains uncharacterized.

Optimal dose-response thresholds

Comparative trials optimizing minimal effective exposure versus adverse event rates.

Formulation bioavailability

Differences between oral, subcutaneous, and modified peptide delivery vehicles.

TRACEABLE SOURCES

References

Primary sources verified against PubMed / NLM and regulatory databases.

  1. Phase 3 RCT

    The efficacy and safety of thymosin α1 for sepsis (TESTS): multicentre, double blinded, randomised, placebo controlled, phase 3 trial ↗ (external website)

    Wu J et al. · BMJ · 2025
  2. Randomized controlled trial

    The efficacy of thymosin alpha 1 for severe sepsis (ETASS): a multicenter, single-blind, randomized and controlled trial ↗ (external website)

    Wu J et al. · Crit Care · 2013
  3. Cochrane systematic review

    Thymosin-ɑ1 for people with chronic hepatitis B ↗ (external website)

    Naing C et al. · Cochrane Database Syst Rev · 2026
  4. Preclinical study

    Thymosin alpha 1 activates dendritic cells for antifungal Th1 resistance through toll-like receptor signaling ↗ (external website)

    Romani L et al. · Blood · 2004
  5. Narrative review

    From lab to bedside: emerging clinical applications of thymosin alpha 1 ↗ (external website)

    Goldstein AL et al. · Expert Opin Biol Ther · 2009
  6. FDA compounding safety page

    Certain Bulk Drug Substances for Use in Compounding that May Present Significant Safety Risks ↗ (external website)

    U.S. FDA · 2026
  7. Registry Record

    NCT02867267 The Efficacy and Safety of Ta1 for Sepsis ↗ (external website)

    ClinicalTrials.gov · 2023
  8. Registry Record

    NCT00711620 Efficacy of Thymosin alpha1 for Severe Sepsis ↗ (external website)

    ClinicalTrials.gov · 2011
  9. Registry Record

    NCT00039962 Thymosin Plus PEG-Interferon in Hepatitis C Patients With Cirrhosis ↗ (external website)

    ClinicalTrials.gov · 2008
  10. FDA compounding category list

    Bulk Drug Substances Nominated for Use in Compounding Under Section 503A of the Federal Food, Drug, and Cosmetic Act (updated May 14, 2026) ↗ (external website)

    U.S. FDA · 2026