Thymosin alpha-1

Thymosin alpha-1 is a small, naturally occurring 28-amino-acid peptide with an acetylated N-terminus and a molecular weight of roughly 3.1 kDa. It is a biologically active fragment cleaved from the larger precursor protein prothymosin alpha and was originally isolated from thymic tissue, the organ central to T-cell maturation. The synthetic version is known generically as thymalfasin and is sold under the brand name Zadaxin in markets outside the United States. It is an immunomodulator: rather than broadly stimulating the immune system, it is described as helping restore balance to immune responses depending on the host's underlying state.

Education only. This page summarises published research and regulatory status. It is not medical advice and gives no dosing, preparation or administration instructions; any decision about use belongs with an independent qualified professional.

Also known as: Thymalfasin, TA1, Tα1, Zadaxin, Thymosin α1, N-acetyl thymosin alpha 1
Class: Thymic immunomodulatory peptide (28-amino-acid, N-acetylated fragment of prothymosin alpha)

How does Thymosin alpha-1 work?

Thymosin alpha-1 acts mainly on the innate-to-adaptive immune handoff. It engages Toll-like receptors (notably TLR9, and TLR2/TLR4 in some contexts) on dendritic cells and monocytes, promoting their maturation and signaling through pathways such as MyD88/IRF7. This supports differentiation and function of T cells, helps shift the Th1/Th2 balance, and drives production of cytokines including interferon-gamma and interleukin-2. It can also help dampen excessive inflammation and restore regulatory/effector T-cell balance, which is why it is framed as a homeostatic immune regulator rather than a one-directional stimulant.

What has Thymosin alpha-1 been studied for?

Chronic hepatitis B and hepatitis C (alone or with interferon/antiviral therapy)
Adjuvant/immune-modulating therapy in certain cancers (hepatocellular carcinoma, melanoma, lung cancer)
Vaccine response enhancement in older or immunocompromised adults (e.g., influenza, COVID-19 boosters)
Severe sepsis and sepsis-associated immunoparalysis
Acute respiratory infections including SARS, COVID-19, and ARDS (investigational)
Immune reconstitution after bone marrow transplant and in immunodeficiency states

What does the evidence show?

Evidence is mixed and indication-dependent. The longest real-world clinical track record is in chronic hepatitis B/C, where thymalfasin has been used for decades in many countries, often combined with interferon. In sepsis, early small/single-blind trials (e.g., the ETASS trial, which showed only a marginally significant mortality signal, P=0.062) and some meta-analyses suggested a possible 28-day mortality benefit, but a 2025 systematic review of 11 RCTs (1,927 patients) found that the apparent benefit weakened or disappeared when restricted to higher-quality and multicenter studies, and trial sequential analysis judged the data inadequate to confirm efficacy. The larger, more rigorous TESTS phase 3 trial (~1,100 patients) did not reduce mortality in the overall sepsis population, with only hypothesis-generating signals in elderly/diabetic subgroups. Key caveats: many positive studies are small, heterogeneous, single-center, single-blind, or used co-interventions; publication bias is plausible; and high-quality randomized evidence in U.S./Western populations is limited. COVID-19 and cancer data remain investigational and not definitive. Nothing here should be read as a promise of benefit or a substitute for individualized clinical judgment.

What is known about the safety of Thymosin alpha-1?

In clinical trials thymosin alpha-1 has generally been well tolerated. The most commonly reported adverse effects are mild and short-lived: injection-site reactions (pain, burning, itching, redness), and occasionally fever, nausea, fatigue, muscle or joint aches, and flu-like symptoms that are typically mild to moderate. Because it modulates immune activity, theoretical and case-level concerns include unpredictable effects in people with autoimmune disease, those on immunosuppressive therapy, and during pregnancy or breastfeeding, where data are insufficient. Long-term safety from rigorous controlled studies is limited, and product quality is a real-world risk for non-pharmaceutical, compounded, or grey-market material. Any use should be evaluated and supervised by an independent qualified professional who can weigh individual contraindications and drug interactions.

What is the legal status of Thymosin alpha-1?

Prescription-only where available, and NOT FDA-approved as a drug in the United States for any indication. Thymalfasin (Zadaxin) is approved in 30+ countries outside the U.S. (notably for hepatitis B/C and as an immune modulator). In the U.S. the FDA granted orphan-drug designation for indications including hepatocellular carcinoma and malignant melanoma, but designation is not approval and the product has never completed the NDA/BLA approval pathway domestically. Its U.S. compounding status has been restrictive: in September 2024 thymosin alpha-1 was removed from Category 2 of the interim 503A bulk-substances list, and at the October and December 2024 Pharmacy Compounding Advisory Committee (PCAC) meetings the committee voted against adding it to the 503A bulks list. As a result, its standing as a substance that pharmacies may legally compound under 503A is, per reporting, not established. Any future compounding pathway would govern only whether a pharmacy may legally compound it with a prescription; it would NOT confer FDA approval, validated indications, or an established benefit-risk profile. It is not a primary WADA-prohibited substance class, but athletes should verify current rules with their governing body.

What is the half-life of Thymosin alpha-1?

Short: the circulating serum half-life of unmodified thymosin alpha-1 is approximately 2 hours (measured around 1.9 hours in animal studies). Peak serum concentrations are reached within about 2 hours of dosing and blood levels generally return toward baseline within roughly 24 hours. Experimental Fc-fusion constructs have been developed to extend the half-life (around 25 hours in mouse models) but are not the standard peptide.

Sources

From the Peppies Peptide Intelligence Library: an educational summary of the published literature, not medical advice. No dosing, preparation or administration instructions are given on these pages. Last updated 2026-09-24. Corrections: support@peppies.eu.