Thymosin alpha 1 vs beta 4 comes down to two different molecules with different jobs. Alpha-1 is studied mainly for immune signaling. Beta-4 binds actin, the protein framework cells use to change shape and move. Neither is a substitute for the other.
Both are peptides, short chains of amino acids, the building blocks of proteins. They share a name because researchers isolated them from the calf-thymus mixture called thymosin fraction 5. That shared discovery source does not make their molecules or treatment evidence interchangeable.
Thymosin alpha 1 vs beta 4 at a glance
The numbers in their names identify molecules. They do not describe doses, strength or a ranking.
| Question | Thymosin alpha-1 | Full-length thymosin beta-4 |
|---|---|---|
| How long is it? | 28 amino acids | 43 amino acids |
| What is its main research focus? | Signals involving immune cells | Actin binding and cell migration, meaning cell movement |
| What has been studied in people? | Hepatitis B, vaccine response and sepsis | Leg-wound healing, dry eye and other eye-surface conditions |
| What name appears in drug studies? | Thymalfasin; Zadaxin is a brand | Tβ4; RGN-259 is an eye-drop candidate |
| What is its overseas position? | Zadaxin is registered in Singapore for specified hepatitis uses | RGN-259 remains in clinical development |
| What is its U.S. position? | Not FDA-approved | Not FDA-approved |
| What does WADA's 2027 list say? | Not named; absence alone does not establish permission | Named under S2.3, prohibited at all times; TB-500 is included |
Goldstein's team reported alpha-1's sequence in PNAS in 1977; the alpha-1 discovery story follows that history. Low, Hu and Goldstein reported beta-4's sequence in PNAS in 1981; the TB-500 discovery story explains the later fragment name.
The same extract family yielded separate peptides in separate studies. These are not two strengths of one medicine, and a result for one cannot establish what the other does.
Alpha-1: immune signaling and condition-specific trials
Thymalfasin is the medicine name for synthetic thymosin alpha-1, made by assembling the peptide rather than extracting it. Its research centers on how immune cells develop and communicate. “Immune modulation” means changing immune activity; it does not necessarily mean turning it up or preventing infections.
The human studies ask particular questions. A 1989 vaccine study assigned 90 older men by chance to alpha-1 or placebo, a comparison preparation without the active peptide. It measured antibody response, the production of proteins that recognize infection, after flu vaccination. That is different from measuring whether generally healthy adults catch fewer infections.
The larger TESTS study examined sepsis, a life-threatening response to infection that damages organs. It found no clear reduction in deaths. The TESTS sepsis-trial explainer covers its results and correction.
As of October 1, 2026, the day this article was written, a September 9 Cochrane review brings together 10 hepatitis B trials involving 1,349 adults. Some combined results favored alpha-1. However, the authors rated most evidence very uncertain because of study limitations and imprecise findings. Hepatitis B is a viral liver infection; those results do not establish routine immune-support benefits.
Beta-4: cell movement, wound studies and the TB-500 distinction
Beta-4's best-established molecular role concerns actin. Cells continually build and dismantle this framework as they move. Beta-4 binds individual actin units and helps regulate their availability, as shown in cell experiments published in PNAS in 1992.
That helps explain the interest in wound repair. It does not, by itself, show that a medicine heals an injury.
A 2010 trial in Annals of the New York Academy of Sciences enrolled 73 people with venous ulcers, leg wounds caused by poor vein circulation. It tested full-length beta-4 applied to the wound. The abstract described a possible healing signal at one concentration and a safety profile comparable with placebo. It was an exploratory study, not evidence for every injury.
A 2015 Clinical Ophthalmology trial tested full-length beta-4 eye drops in 72 people with dry eye for 28 days. Neither main outcome showed a clear difference from placebo, although some other measures improved. An eye-drop finding answers an eye-drop question.
TB-500 adds another distinction: the fragment FDA reviewed contains seven amino acids, compared with the parent's 43. As of October 1, 2026, the day this article was written, FDA's review dated May 15, 2026 identified no studies administering that fragment to people. The thymosin beta-4 guide and TB-500 myths ledger explain why parent-molecule studies cannot establish fragment-specific results.
Hair-growth claims are one example: TB-500 for hair growth traces them to studies of full-length thymosin beta-4 in animals and cells.
A newer finding broadens alpha-1’s research
The immune-versus-repair shorthand also has limits. As of October 1, 2026, the day this article was written, a September 3 Advanced Science study reports alpha-1 binding to HCRTR1, a signal-receiving protein on nerve cells. Treatment experiments involved mice and cells; the human portion compared blood samples. It does not establish a stroke-treatment benefit in people.
Overseas registration and U.S. status are different questions
As of October 1, 2026, the day this article was written, Singapore's Zadaxin formulary entry reads “Last updated 16 September 2026.” It lists a prescription medicine for chronic hepatitis B and combination treatment for hepatitis C, another viral liver infection. That date documents a registry update, not a new authorization.
Full-length beta-4's RGN-259 program remains clinical development of a drug candidate. It does not have the same overseas product history as thymalfasin.
As of October 1, 2026, the day this article was written, thymosin alpha-1 and thymosin beta-4 are not FDA-approved. FDA's records for alpha-1 in hepatitis B and beta-4 in an eye condition show orphan designations without marketing approval. An orphan designation provides rare-disease development incentives; it is not permission to market a medicine.
Promise's thymosin alpha-1 is dispensed as a compounded medication, prepared by a pharmacy for an individual prescription. It is different from an FDA-approved product: the formulation offered here is not FDA-approved. A licensed provider may still prescribe — that decision is between you and your doctor.
Sport rules do not follow the shared name
As of October 1, 2026, the day this article was written, WADA, the World Anti-Doping Agency, has published its 2027 list on September 21. It takes effect January 1, 2027. The list names thymosin beta-4 and its derivatives, including TB-500, under S2.3, its section for growth signals and substances that alter them. They are prohibited in and out of competition. Derivatives means related versions of the molecule.
Alpha-1 is not named. That alone does not clear every preparation: the list includes broader classes, including substances without any governmental authorization for human treatment. A tested athlete's anti-doping organization determines how those rules apply to the exact medicine. The 2027 WADA explainer covers the broader changes. A prescription alone is not an exemption.
What makes the comparison useful in a visit
A useful discussion starts with the intended goal, then asks whether the evidence concerns the same molecule, condition and preparation. For alpha-1, transplant history, autoimmune disease, where immunity attacks the body's own tissues, and medicines affecting immunity matter to that review.
At Promise, a licensed provider reviews every request, and not everyone qualifies. The provider sets any individual plan and follow-up. Shared names can start a conversation; matching the evidence to the actual prescription is what makes it meaningful.
