Thymosin Alpha-1 — questions, answered plainly.

Thymosin Alpha-1 (Tα1, thymalfasin) is a synthetic 28-residue immunomodulatory peptide based on a fragment of the endogenous thymic protein prothymosin α. It is marketed as Zadaxin in many jurisdictions; it is not FDA-approved in the United States at the time of writing.

Zadaxin (thymalfasin) is approved in over 30 countries — most prominently Italy and China — primarily for chronic hepatitis B, chronic hepatitis C, and as an immunomodulator. Approval status varies widely; clinical use requires verification of local regulatory status.

Wu et al. 2025 (BMJ, PMID 39814420; n=1106) was a multicentre double-blind RCT testing thymosin α1 in adults with sepsis. The primary endpoint — 28-day all-cause mortality — did not reach statistical significance overall, though subgroup analyses suggested potential differential effects by age and diabetes status.

Shehadeh et al. 2023 (J Infect Dis, PMID 36056913; n=49) was an open-label randomised pilot in hospitalised COVID-19 patients with hypoxaemia and lymphocytopenia. Thymosin α1 was associated with accelerated CD4+ T-cell recovery versus standard care in the pilot cohort.

The Wu 2015 review (Expert Opinion on Biological Therapy, PMID 25640173) covers the chronic-hepatitis-B literature in which thymosin α1 alone or with antivirals has been associated with HBV DNA suppression and HBeAg seroconversion in subsets of treated patients. This is one of the longer-running clinical use cases for the compound.

Mechanistic research describes effects on T-cell maturation, macrophage polarisation, and Toll-like receptor signalling. The compound is characterised as an immunomodulator rather than a direct antimicrobial or antiviral agent.

Approval status varies widely by jurisdiction; the compound is not FDA-approved in the United States. The most recent Phase 3 sepsis trial (Wu 2025) did not show a significant primary outcome benefit overall, and subgroup signals require confirmation. Older hepatitis-B literature is heterogeneous in design.