Comparison
Same family name, completely different molecules: an approved immune drug versus an actin-binding repair protein with a doping history.
Thymosin alpha-1 is a 28-amino-acid immune peptide approved in dozens of countries with a defined schedule; thymosin beta-4 is a 43-amino-acid repair-associated protein whose market fragments (TB-500) are prohibited in sport.
Both were originally isolated from thymus tissue, but they are distinct molecules with distinct jobs: alpha-1 is an immune signal acting on dendritic cells and T-cell maturation; beta-4 is an actin-sequestering protein tied to cell migration and wound biology.
Alpha-1 has the stronger record by far: approved drug status outside the US (Zadaxin) with label-defined schedules for hepatitis and immune-adjuvant use. Beta-4 has a genuine research literature and registered trials, but nothing approved, and its human repair claims remain unproven.
Beta-4 and its TB-500 fragments are prohibited in sport with LC-MS detection methods in place, and TB-500 is on FDA's 2026 compounding advisory agenda. Alpha-1's regulated status is pharmaceutical approval, not a gray-market category.
| Dimension | Thymosin alpha-1 | Thymosin beta-4 |
|---|---|---|
| Overall evidence grade | C | D |
| Evidence label | Limited human evidence | Limited human evidence |
| Regulatory status | Varies by product and jurisdiction | Investigational / not approved |
| Editorial confidence | Moderate confidence | Low confidence |
| Efficacy | 2/5 limited | 1/5 limited |
| Safety | 2/5 limited | 1/5 limited |
| Regulatory risk | 4/5 high | 4/5 high |
| Product quality risk | 4/5 high | 5/5 very high |
| Primary claim categories | Immune modulation, Chronic hepatitis B evidence, Sepsis evidence, COVID and vaccine-response claims, FDA compounding review | Wound-healing research, Ocular surface research, Cardiovascular research, Recovery and soft-tissue claims |
Thymosin alpha-1 has to be judged by clinical setting: chronic hepatitis B has the clearest historical positive signal, sepsis has a modern non-confirmatory phase 3 result, COVID and vaccine-response evidence is mixed or selected, and clinic wellness claims need direct evidence.
Thymosin beta-4 has its own evidence. Eye-drop, wound, IV, and registry sources do not prove consumer recovery benefit, and TB-500/Ac-LKKTETQ fragment sources are not interchangeable full-length thymosin beta-4 evidence.
ClinicalTrials.gov. ClinicalTrials.gov record NCT00382174 2006.
NCT00382174 Accessed 2026-06-08.
Full-length thymosin beta-4 trial record; adjacent to TB-500 fragment discussion, not interchangeable evidence.
PMID:22962027 Accessed 2026-06-08.
Identity and anti-doping source for TB-500/Ac-LKKTETQ; not human efficacy evidence.
FDA. July 23-24, 2026: Meeting of the Pharmacy Compounding Advisory Committee 2026.
Accessed 2026-07-24.
FDA meeting page for the July 23-24, 2026 PCAC review of BPC-157, KPV, TB-500, MOTs-C, emideltide/DSIP, Semax, and Epitalon bulk substances, including the uses evaluated for each, briefing documents, and docket FDA-2025-N-6895.
FDA. Certain Bulk Drug Substances for Use in Compounding that May Present Significant Safety Risks 2026.
Accessed 2026-06-08.
FDA summarizes potential significant safety risks and missing safety information for several nominated peptide bulk substances.