Thymosin α1 – Learning Health

What it is

Thymosin α1 is a 28-residue acetylated peptide, a fragment of the larger precursor prothymosin α, originally identified in calf thymus extract during the search for the thymic hormones that direct T-cell maturation. Unlike most peptides in this atlas it became a real pharmaceutical: synthetic thymosin α1 is marketed as thymalfasin under the brand name Zadaxin.

It is also the clearest example here of a compound whose regulatory standing and evidence base point in different directions depending on the indication.

What it does in the body

Thymosin α1 is an immune modulator rather than an immune stimulant, and the distinction matters for how it has been tested. It signals through Toll-like receptor pathways on dendritic cells and monocytes, influences T-cell maturation and function, and shifts the balance of T-helper responses. In the clinical literature its main proposed role is restoring an immune response that has become dysregulated — either suppressed, as in chronic viral infection and sepsis-associated immunoparalysis, or exhausted.

This is why it has been trialled where it has: chronic hepatitis B, sepsis, cancer as an adjunct, and during the COVID-19 pandemic.

What the human evidence shows

Sepsis — the big, clean, negative result. The TESTS trial is the most rigorous test any peptide in this atlas has faced outside of elamipretide. It was a multicentre, double-blinded, placebo-controlled phase 3 trial across 22 centres in China, enrolling 1,106 adults aged 18–85 with sepsis by sepsis-3 criteria between September 2016 and December 2020, randomised 1:1 to subcutaneous thymosin α1 or placebo every 12 hours for seven days. The primary outcome was 28-day all-cause mortality.

The result: 23.4% versus 24.1% (hazard ratio 0.99, 95% CI 0.77–1.27, p=0.93). No secondary or safety outcome differed statistically significantly. Pre-specified subgroup analyses showed interactions by age (under 60: HR 1.67, 1.04–2.67; 60 and over: 0.81, 0.61–1.09; interaction p=0.01) and by diabetes (diabetes: 0.58, 0.35–0.99; no diabetes: 1.16, 0.87–1.53; interaction p=0.04). Those subgroups are exploratory and hypothesis-generating — and one of them points the wrong way, suggesting possible harm in younger patients. The authors concluded there was no clear evidence that thymosin α1 decreases 28-day mortality in adults with sepsis. The trial was supported in part by the manufacturer.

It is worth noting what this replaced. Earlier reviews of smaller studies had reported that single or combined treatment reduced sepsis mortality, improved monocyte HLA-DR expression and reduced secondary infection, and described it as a promising adjuvant therapy. A properly powered trial reversed that.

Chronic hepatitis B — the approved indication, with a nuance. A meta-analysis of four randomised trials in 199 patients compared thymosin α1 with interferon-α. At the end of six months of treatment the odds ratios favoured interferon (complete response 0.54, 95% CI 0.30–0.97). But at six months after treatment ended, they favoured thymosin α1 — virological response 3.71 (2.05–6.71), biochemical 3.12 (1.74–5.62), complete 2.69 (1.47–4.91). The pattern the authors described is a benefit that accumulates gradually after therapy rather than appearing during it, which fits an immune-modulating rather than directly antiviral mechanism. Three of the four trials studied HBeAg-negative patients, so that is where the result mainly applies.

COVID-19 — retrospective only. A review of 76 severe COVID-19 cases in two Wuhan hospitals reported lower mortality in treated patients (11.1% versus 30.0%, p=0.044), along with restored T-cell counts and reduced PD-1 and Tim-3 expression on CD8+ T cells. This was retrospective, not randomised, in 76 patients during the chaotic first months of a pandemic. It generated a great deal of interest and does not constitute controlled evidence.

Cancer adjunct. An active area with scoping reviews and phase 2 work published through 2026, including trials combining thymalfasin with immune checkpoint inhibitors. The FDA has granted orphan drug designation for malignant melanoma and hepatocellular carcinoma — a designation is an incentive to develop a drug for a rare disease, not a finding that it works.

Claims and what backs them

Claim as usually stated	Verdict	What the published evidence actually shows
Reduces mortality in sepsis	Not supported	TESTS, 1,106 patients, 22 centres: 28-day mortality 23.4% versus 24.1%, HR 0.99, p=0.93. No secondary outcome differed either.
Helps clear chronic hepatitis B	Partly supported	Approved for this in around 35 countries. Meta-analysis of 4 small trials found it underperformed interferon during treatment and outperformed it six months after — a delayed, accumulating effect.
Modulates T-cell function	Supported	Consistent across mechanistic and clinical work: effects on T-cell counts, HLA-DR expression on monocytes and exhaustion markers are repeatedly measured.
Improved COVID-19 survival	Unproven	The supporting study was retrospective, in 76 patients, without randomisation. No adequately powered randomised trial established this.
Boosts immunity in healthy people	Unproven	Every trial has been in people with a specific disease and a disturbed immune system. There is no evidence base for use in healthy adults, and an immune modulator is not self-evidently desirable in someone whose immune system is working.
FDA-approved	Not supported	It is not. Thymalfasin has approvals in roughly 35 other countries and FDA orphan designations — which are not approvals — for two cancers.

Risks and unknowns

Safety looks reassuring, from the best possible source. In TESTS no safety outcome differed significantly between drug and placebo across 1,089 analysed patients. That is a far stronger safety dataset than anything else in this atlas apart from elamipretide, and it is genuinely informative.

The younger-patient subgroup. In TESTS, patients under 60 had a hazard ratio of 1.67 (1.04–2.67) — nominally worse survival on the drug. Subgroup findings in a null trial should not be over-read in either direction, but this one should not be ignored while the diabetes subgroup is quoted.

Immune modulation is directional. Shifting T-helper balance or TLR signalling in a person with no immune disorder has unpredictable consequences, and that population has never been studied.

Supply. Thymalfasin is a licensed pharmaceutical in dozens of countries. Material sold online as “thymosin alpha-1” for research use is not that product and carries the identity and purity problems regulators raised about this whole category.

Where regulators stand

United StatesNot approved. The FDA has granted orphan drug designation for malignant melanoma and hepatocellular carcinoma, which is a development incentive rather than a marketing authorisation. Thymosin α1 appears in the FDA’s table of bulk drug substances previously in Category 2 whose nominations were withdrawn by the nominators.

The July 2026 voteOn 23–24 July 2026 the FDA’s Pharmacy Compounding Advisory Committee reviewed seven peptides for the 503A bulks list and voted six of them through, against the unanimous recommendation of the agency’s own scientific staff. Thymosin α1 was not among the seven peptides reviewed at that meeting, so the committee took no position on it.

Other countriesApproved as thymalfasin (Zadaxin) in roughly 35 countries, mainly across Asia and Latin America, chiefly for chronic hepatitis B. First approval was in China in 1995.

SportNot named on the WADA 2026 Prohibited List. Because it holds regulatory approval for human therapeutic use in many countries, the S0 catch-all for non-approved substances does not apply.

The bottom line

Thymosin α1 is the entry in this atlas that shows most clearly why trial size matters. For years, small studies and reviews suggested it reduced sepsis mortality. Then someone ran a 1,106-patient randomised trial and the effect was not there — 23.4% against 24.1%, with a confidence interval tight enough to rule out a meaningful benefit.

It remains a legitimate licensed medicine in around 35 countries for chronic hepatitis B, where the evidence is modest but real and has an interesting delayed-benefit pattern. It is not FDA-approved, it has no evidence base in healthy people, and the main lesson it offers is about how often promising small studies evaporate.

References

Wu J, Pei F, Zhou L, et al. (TESTS study collaborator group). The efficacy and safety of thymosin α1 for sepsis (TESTS): multicentre, double blinded, randomised, placebo controlled, phase 3 trial. BMJ. 2025;388:e082583 PMID 39814420
Yang YF, Zhao W, Zhong YD, et al. Comparison of the efficacy of thymosin alpha-1 and interferon alpha in the treatment of chronic hepatitis B: a meta-analysis. Antiviral Res. 2008;77(2):136–141 PMID 18078676
Pei F, Guan X, Wu J. Thymosin alpha 1 treatment for patients with sepsis. Expert Opin Biol Ther. 2018;18(sup1):71–76 PMID 30063866 — pre-TESTS review; superseded by the trial
Liu Y, Pan Y, Hu Z, et al. Thymosin alpha 1 reduces the mortality of severe coronavirus disease 2019 by restoration of lymphocytopenia and reversion of exhausted T cells. Clin Infect Dis. 2020;71(16):2150–2157 PMID 32442287 — retrospective, 76 patients
King R, Tuthill C. Immune modulation with thymosin alpha 1 treatment. Vitam Horm. 2016;102:151–178 PMID 27450734
Kim SD, Kim KR, Kim DH, et al. Clinical applications and evidence landscape of thymosin alpha 1 as an immunomodulatory adjunct in cancer care: a scoping review. Pharmaceuticals. 2026;19:1366 PMID 42797537