What Is Thymosin Alpha-1? The Immune Peptide Explained

Thymosin alpha-1 is described as an immunomodulator: it adjusts immune activity rather than simply boosting it.

• T-cell maturation: it promotes the development and function of T cells, especially CD4 helper and CD8 killer T cells.
• Toll-like receptors: it activates toll-like receptors (TLR2 and TLR9) on dendritic cells, which helps the immune system recognize viruses and tumor cells.
• Cytokine balance: it increases antiviral signals such as interferon and IL-2 while helping regulate excessive inflammation.

Hepatitis B. Several randomized trials in the 1990s found higher rates of virus clearance with thymosin alpha-1 than with no treatment, and results comparable to interferon with fewer side effects. Not all trials reached statistical significance, and newer antiviral drugs have largely replaced it.

Sepsis. The largest recent trial, TESTS, published in the BMJ in January 2025, enrolled 1,106 adults with sepsis across 22 hospitals in China. Twenty-eight-day mortality was 23.4% with thymosin alpha-1 versus 24.1% with placebo, which was not a significant difference.

Cancer and vaccines. Thymosin alpha-1 has been studied as an add-on to chemotherapy and to improve vaccine responses in people with weak immune systems, with mixed and mostly small studies. It also holds FDA orphan drug designations for several conditions, which support research but are not approvals.

Thymosin alpha-1 has a long record of clinical use outside the U.S. and is generally considered well tolerated. The most common side effect is local discomfort or redness at the injection site. Because it acts on the immune system, it is used with caution in people with autoimmune disease or organ transplants, where immune activation could cause problems.

No. Thymosin alpha-1 is not FDA-approved in the U.S., although it is approved as Zadaxin in many other countries. It is not on the 503A Bulks List.

In April 2026, the FDA moved this substance out of its “Category 2” table of compounding ingredients that may present significant safety risks and into a separate table of substances whose nominations were withdrawn by the nominators. That move is not a finding that the substance is safe or effective, and it does not place it on the 503A Bulks List.

Similar names, different jobs. Thymosin alpha-1 and thymosin beta-4 were both first found in thymus extracts, which is why they share a name, but they are unrelated molecules with different functions. Tα1 works on immune cells, while Tβ4 is mainly a structural protein involved in healing.

Thymalin vs thymosin alpha-1. Thymalin is a Russian bioregulator preparation containing a mix of thymic peptides. Thymosin alpha-1 is a single, defined molecule with far more clinical trial data.

Products sold as Thymosin Alpha-1 by research suppliers are laboratory chemicals, not medicines. They are not made under drug manufacturing rules, so quality varies a lot between vendors. If you are evaluating a Thymosin Alpha-1 supplier for lab research, these are the checks that matter:

• Batch-specific COA: the certificate should match the lot number on your vial, not a generic sample.
• Independent lab: testing should come from a named third-party lab, ideally with a way to verify the report online.
• Identity, not just purity: HPLC purity alone does not confirm the right molecule; look for mass spectrometry (MS) identity confirmation.
• Endotoxin and sterility results: these matter for any injectable research material.
• Correct fill amount: the measured content should be close to the labeled milligrams.

Our Trusted Sources page lists vendors we track, with current community codes. Our guide to reading a certificate of analysis explains each line of a COA.

• Thymosin alpha-1 is a 28-amino-acid peptide from the thymus that modulates T cells.
• As Zadaxin, it is approved in more than 35 countries, mainly for hepatitis B and C.
• A large 2025 sepsis trial found no mortality benefit.
• It is generally well tolerated in clinical use.
• It is not FDA-approved in the U.S.

What is thymosin alpha-1?

Thymosin alpha-1 is a 28-amino-acid peptide made by the thymus that helps regulate T cells and other parts of the immune system.

Is thymosin alpha-1 FDA-approved?

No. It is approved as Zadaxin in more than 35 other countries, mainly for hepatitis, but not in the U.S.

Does thymosin alpha-1 help with sepsis?

A large 2025 trial of 1,106 adults with sepsis found no significant difference in 28-day mortality compared with placebo.

Is thymosin alpha-1 the same as TB-500?

No. TB-500 is based on thymosin beta-4, a different molecule involved in tissue repair. They only share a name because both were found in thymus extracts.

Is thymosin alpha-1 safe?

It has been generally well tolerated in clinical use abroad. The most common side effect is injection-site discomfort.

• BMJ – Wu et al. (2025), Thymosin alpha 1 in adults with sepsis (TESTS): a multicentre, double-blind, randomised, placebo-controlled trial
• Chien et al. (1998), Hepatology – Efficacy of thymosin α1 in patients with chronic hepatitis B
• Goldstein & Goldstein (2009) – From lab to bedside: emerging clinical applications of thymosin α1
• FDA – Certain bulk drug substances for use in compounding that may present significant safety risks (updated April 22, 2026)