Thymosin Alpha-1 (Thymalfasin)

Thymosin alpha-1 (Tα1, marketed as thymalfasin or Zadaxin) is a naturally occurring immune-modulating peptide first isolated from the thymus gland in the 1970s. A 28-amino-acid peptide, thymosin alpha-1 does not attack pathogens directly; instead it fine-tunes the immune response. It is approved in many countries to treat chronic hepatitis B and has been studied widely in infections and immune suppression.

What is Thymosin alpha-1?

Thymosin alpha-1 is an acetylated 28-amino-acid peptide (sequence Ac-Ser-Asp-Ala-Ala-Val-Asp-…-Glu-Asn; formula C129H215N33O55, about 3,108 g/mol; CAS 62304-98-7). It is a fragment of a larger precursor protein, prothymosin alpha, and is produced naturally by the thymus — the gland that trains the immune system’s T cells. As a manufactured drug it carries the international name thymalfasin and the brand name Zadaxin.

How Thymosin alpha-1 is studied to work

Thymosin alpha-1 acts as an immunomodulator: rather than stimulating the immune system across the board, it helps restore balance, which is especially relevant when immunity is suppressed or exhausted. Studied mechanisms include:

• Signals through Toll-like receptors (TLR2 and TLR9) on dendritic cells and monocytes
• Promotes maturation of dendritic cells, improving antigen presentation
• Supports T-cell development and function, and can help restore depleted T-cell counts
• Shifts immunity toward a Th1 (antiviral) profile and modulates cytokine signaling
• Reported to reverse T-cell “exhaustion” in severely ill patients

Reported effects and benefits in the research literature

Effects reported for thymosin alpha-1 in clinical and laboratory studies include:

• Improved immune markers (T-cell counts and function) in immunosuppressed patients
• Benefit as an add-on to interferon in chronic hepatitis B — its main approved use in many countries
• Investigated in severe sepsis, where it may improve immune function
• Studied as an adjunct in some cancers and to improve vaccine responses
• Examined in COVID-19, with mixed results

What this does NOT mean: approval for hepatitis B in some countries does not establish benefit for other conditions. Much of the non-hepatitis evidence is preliminary or mixed, and thymosin alpha-1 is not approved in the United States.

What the human evidence shows

Chronic hepatitis B is where the evidence is strongest: thymalfasin is approved and marketed in many countries (used alone or with interferon). In severe sepsis, the ETASS randomized controlled trial (China, 2013; 1,450 patients) found a trend toward lower 28-day mortality (24.1% versus 26.8%) that did not reach statistical significance, prompting calls for larger trials. In COVID-19, retrospective cohort studies were mixed — some reported lower mortality in critically ill patients, others found no benefit. In the United States thymosin alpha-1 is not FDA-approved and remains investigational.

Handling, storage and reconstitution (research context)

Thymosin alpha-1 is supplied as a lyophilized powder and reconstituted before use. General handling principles:

• Lyophilized powder: refrigerate (2–8 °C), protected from light
• Reconstitute with the appropriate diluent; once in solution, refrigerate and use promptly
• Use the reconstitution calculator to turn vial strength and water volume into a concentration (mg/mL)
• Review why units are not a dose before reading a syringe

Cautions and considerations

• Thymosin alpha-1 is approved for hepatitis B in many countries but is not FDA-approved in the US
• Most non-hepatitis evidence is preliminary or mixed — interpret claims cautiously
• In trials it has generally been well tolerated; injection-site reactions are the most common complaint
• Confirm identity and purity with a Certificate of Analysis (COA)
• This page is informational only and is not medical advice

Frequently asked questions

Is thymosin alpha-1 FDA-approved?

No — not in the United States. It is approved and marketed as thymalfasin in many other countries, mainly for chronic hepatitis B.

What does thymosin alpha-1 actually do?

It is an immunomodulator: it helps the immune system’s own cells (dendritic cells and T cells) work more effectively, particularly when immunity is suppressed.

Is it the same as TB-500 (thymosin beta-4)?

No. Despite the shared “thymosin” name, thymosin alpha-1 and thymosin beta-4 are different peptides with different structures and functions.

Does thymosin alpha-1 have side effects?

In clinical trials it has generally been well tolerated, with injection-site reactions reported most often. As with any compound, individual responses vary.

Related compounds and further reading

• TB-500 (Thymosin β4) — a different thymic peptide studied for tissue repair
• How to reconstitute peptides
• Sterile technique
• Peptide Library · Guides
• IU vs mL: why units are not a dose

For informational use only. Not medical advice; consult a qualified healthcare professional. 21+.