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Peptide reference

Thymosin Alpha-1

Thymosin Alpha-1

Animal data

How it works

A 28-amino-acid peptide originally isolated from calf thymic tissue. It modulates both innate and adaptive immunity — upregulating dendritic cell maturation, increasing T-cell activity, and enhancing natural killer cell function. Approved in over 35 countries under the brand name Zadaxin for viral hepatitis and as an adjuvant in immunocompromised patients. Not FDA-approved. It occupies an unusual regulatory position: extensive international clinical history, no approved US indication, compoundable as a 503A bulk substance.

FDA status
Not FDA-approved in the US. Approved in 35+ countries for viral hepatitis (Zadaxin). Compoundable by 503A pharmacies in the US as it has no active FDA NDA.
WADA status
Permitted in sport
Route
subcutaneous injection

Key studies

Frequently asked

  • What is Thymosin Alpha-1?

    Thymosin alpha-1 is a naturally occurring 28-amino-acid peptide secreted by the thymus gland, involved in maturing T-cells and shaping immune response. A synthetic version, called thymalfasin, has been used as an immune-modulating drug outside the US for decades. It's structurally and functionally distinct from thymosin beta-4, the peptide behind TB-500; the shared "thymosin" name reflects thymus origin, not a chemical relationship.

  • What does the clinical evidence for Thymosin Alpha-1 actually show?

    The largest controlled human trial of Thymosin alpha-1 to date tested it in over 1,100 sepsis patients and found no difference in 28-day mortality versus placebo, a null result. A subgroup analysis suggested a possible harm signal in patients under 60 that couldn't be ruled out. That 2025 trial substantially weakens the international case for using Thymosin alpha-1 as a sepsis adjuvant, even where it's already approved for that use.

  • Is Thymosin Alpha-1 approved by the FDA or on the PCAC docket?

    The synthetic version, thymalfasin, is approved in dozens of countries outside the US for hepatitis B and C adjuvant therapy, but has never received FDA marketing approval, only orphan-drug designations. It's not on either of the FDA's 2026 or 2027 PCAC review waves either: PCAC already reviewed it in December 2024 and voted against adding it to the Section 503A compounding list.

  • Is Thymosin Alpha-1 banned for athletes?

    Yes. Thymosin alpha-1 falls under WADA's S2 category for peptide hormones and immunomodulatory substances without recognised therapeutic approval in the testing jurisdiction, prohibited at all times for athletes subject to testing.

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Research on Thymosin Alpha-1

Thymosin Alpha-1 Corrects a Measurable Immune Deficit. Yours Might Not Have One Yet.

WADA

Thymosin Alpha-1 is a real, internationally approved drug for a documented immune deficit, not a peptide-store rumour. Whether that same signal holds for a healthy 50-year-old with no diagnosis is the part nobody has actually tested.

thymosin-alpha-1

The thymus: the vanishing organ behind your ageing immune system

WADA

The thymus trained every immune cell you own, then started shrinking in your teens. Its slow fade is a big reason immunity gets weaker with age — and the reason a family of thymus-signal peptides is worth understanding. Here is the biology, in plain language.