Thymosin Peptides: An Overview of the Alpha and Beta Family in Immune and Tissue Function

This content is provided by Superpower Health for educational and informational purposes only. Superpower Health does not prescribe, sell, or facilitate access to the compounds discussed on this page in their research-only or internationally approved forms. None of the thymosin family members discussed here are FDA-approved for human use in the United States, with the exception of thymosin alpha-1, which holds international approvals in over 35 countries (brand name Zadaxin) but is not FDA-approved in the United States. This page is not a substitute for medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider.

In the 1960s, a question was circulating in immunology that seemed almost too simple: if the thymus gland is removed early in life, immune function collapses, so does it secrete something that drives immunity? A researcher named Allan Goldstein thought yes. Over the following decade, he and his colleagues at Albert Einstein College of Medicine and later the University of Texas purified what they called "thymosin fraction 5" from calf thymus tissue: a mix of at least 40 small, mostly acidic polypeptides that appeared to restore T cell function in athymic animals.

The name "thymosins" stuck. The problem is that it describes where these peptides came from, not what they are. Subsequent fractionation work revealed that thymosin alpha-1, thymosin beta-4, and prothymosin alpha are biochemically unrelated. They share no common structure, no common receptor, and no common mechanism. They were grouped together because researchers found them in the same tissue extract, not because they belong to the same molecular family. This page explains what each of the major thymosins actually does, why the naming history matters for anyone reading the clinical literature, and where the evidence stands for each compound.

Key Takeaways

• Regulatory Status: As of April 2026, no thymosin family member is FDA-approved for human use in the United States. Thymosin alpha-1 (Zadaxin) is approved in over 35 countries for hepatitis B, hepatitis C, and immune support in cancer patients. Other thymosins remain in research or developmental stages.
• Research Stage: Thymosin alpha-1 has completed Phase III RCTs in hepatitis B, hepatitis C, and sepsis (mixed results). Thymosin beta-4 has completed Phase II and Phase III trials for ophthalmic indications. Prothymosin alpha is preclinical to early-stage only.
• Availability: None of the thymosin family members are available through Superpower. Thymosin alpha-1 is not currently listed on FDA's 503A Category 1 or Category 2 bulk drug substance lists (it was removed from Category 2 in September 2024 after nominators withdrew their submissions and awaits formal PCAC review). Thymosin beta-4 and TB-500 were both removed from FDA Category 2 effective April 22, 2026 per Federal Register notice and are scheduled for PCAC consultation on July 23, 2026; neither has been added to the Section 503A bulks list, so their current legal status for compounded use is unresolved.
• Prescribing information: View thymosin alpha-1 compound reference data (PubChem CID 16130571)
• How they differ: Thymosin alpha-1 modulates T cell immunity via toll-like receptor signaling. Thymosin beta-4 regulates actin dynamics and drives cell migration in tissue repair. Prothymosin alpha remodels chromatin in the cell nucleus. No shared mechanism exists.
• What the evidence shows: Thymosin alpha-1 has the strongest clinical record: Phase III RCT data in hepatitis B (40.6% vs. 9.4% viral clearance at 26 weeks) and a Phase III sepsis trial (TESTS, 2025) that found no reduction in 28-day mortality, complicating the earlier ETASS trial data. Thymosin beta-4 has Phase III ophthalmic trial data. Both compounds are research-stage in the United States.

The Thymus Gland and Why Researchers Expected Thymic Extracts to Be Immunoactive

Understanding the thymosin family requires understanding why thymic tissue was investigated in the first place. The thymus is a small, bilobed organ located in the upper chest behind the sternum. Its primary function is the education and maturation of T lymphocytes: precursor cells migrate from the bone marrow into the thymus, where they undergo positive and negative selection to emerge as functional, self-tolerant T cells. The thymus is most active in childhood and early adolescence. After puberty, it begins a process called involution: fatty tissue gradually replaces functional thymic epithelium, and the output of naive T cells declines steadily through adulthood.

As Thomas, Wang, and Su described in a 2020 review published in Immunity and Ageing, this age-related involution has two main consequences: reduced production of naive T cells and disrupted central tolerance, which contributes to the accumulation of autoreactive and senescent immune cells that drive chronic low-grade inflammation. Mittelbrunn and Kroemer, writing in Nature Immunology in 2021, identified thymic involution as one of the primary hallmarks of T-cell aging, linking declining thymic output to organism-wide vulnerability to infection, cancer, and poor vaccine response.

For 1960s-era immunologists, the logic was compelling. An organ that clearly drove immune maturation might secrete hormones that maintained immunity throughout life. If those hormones could be extracted and administered, they might restore immune competence in immunodeficient patients. Goldstein and colleagues set about testing exactly this hypothesis. As Goldstein documented in a first-person historical account published in the Annals of the New York Academy of Sciences in 2007, the research group began by purifying a biologically active fraction from calf thymus that they termed thymosin fraction 5, first reported in the landmark 1966 paper in Proceedings of the National Academy of Sciences by Goldstein, Slater, and White. That fraction appeared to restore T cell function in mice that had been thymectomized, supporting the hormonal hypothesis.

The subsequent challenge was determining what, exactly, was doing the work inside fraction 5.

Why the "Thymosin" Name Does Not Imply a Shared Structure

The name thymosin predates a full biochemical understanding of the individual peptides it encompasses. When Goldstein, Slater, and White purified what they called "thymosin" in 1966, they were describing a heterogeneous mixture, not a defined molecule. As Low, Thurman, McAdoo, and colleagues showed in a 1979 paper in the Journal of Biological Chemistry, separating thymosin fraction 5 into its component peptides revealed chemically distinct molecules with radically different biological activities. Thymosin alpha-1 (28 amino acids) was 10 to 1,000 times as potent as the crude fraction 5 in T cell bioassay systems. Polypeptide beta-1, also isolated from the same fraction, showed no measurable activity in those same assays.

Goldstein and Badamchian formalized this picture in a 2004 review in Expert Opinion on Biological Therapy, describing the thymosins as "a family of biochemically and functionally distinct polypeptides." The term "family" as used here is taxonomic, not structural: these peptides were grouped because they came from the same tissue extract, not because they share a receptor, a signaling pathway, or a scaffold. Huff, Müller, Otto, Netzker, and Hannappel, writing in the International Journal of Biochemistry and Cell Biology in 2001, reinforced this point specifically for the beta-thymosins, noting that beta-thymosins function as actin-sequestering peptides with no structural or functional overlap with thymosin alpha-1 or the prothymosin series.

The clinical implication is direct: research on thymosin alpha-1 says nothing about thymosin beta-4, and vice versa. Anyone reading the thymosin literature needs to identify the specific subfraction being studied before interpreting findings.

Thymosin Alpha-1: The Family Member With International Clinical Approval

Thymosin alpha-1 (abbreviated Tα1) is the best-characterized and most clinically advanced member of the thymosin family. It is a 28-amino-acid synthetic peptide that mirrors the N-terminal fragment of prothymosin alpha and is produced recombinantly for clinical use. It is sold under the brand name Zadaxin and is approved in over 35 countries, primarily for hepatitis B, hepatitis C, and as an immune adjunct in cancer patients. It is not FDA-approved.

Mechanism of action: toll-like receptor signaling and dendritic cell activation

Thymosin alpha-1 does not act directly on T cells in the way that a cytokine does. Its primary mechanism involves dendritic cell activation. Romani, Bistoni, Gaziano, and colleagues from the University of Perugia, writing in Blood in 2004, showed that Tα1 activates dendritic cells through toll-like receptor signaling, specifically via the myeloid differentiation factor 88 (MyD88)-dependent pathway. This activation induces interleukin-12 production and drives differentiation of naive T cells toward a Th1 phenotype, enhancing cellular immunity against pathogens, particularly fungal infections in immunocompromised patients. The dendritic cell activation mechanism explains why Tα1 has been studied across such a wide range of immunocompromised states: it does not replace a specific immune signal but rather restores the upstream decision-making capacity of the innate immune system to direct adaptive responses.

Hepatitis B: the strongest evidence base

The best-supported clinical indication for thymosin alpha-1 is chronic hepatitis B. In a randomized controlled trial published in Hepatology in 1998, Chien, Liaw, Chen, Yeh, and Sheen enrolled 98 patients with chronic hepatitis B and compared 26-week and 52-week courses of thymosin alpha-1 (1.6 mg subcutaneously twice weekly) against a no-treatment control group over 18-month follow-up. The 26-week treatment group achieved a complete virological response rate of 40.6% versus 9.4% in controls (p = 0.004); the 52-week group achieved 26.5% (p = 0.068 vs. control), which did not reach statistical significance. The treatment was well tolerated with no significant adverse effects reported, though the small per-group sample size limits the precision of the treatment effect estimate.

A subsequent meta-analysis by Yang, Zhao, Zhong, and colleagues, published in Antiviral Research in 2008, pooled four randomized controlled trials (n = 199) comparing thymosin alpha-1 against interferon-alpha in chronic hepatitis B and reported that Tα1 showed comparable end-of-treatment virological response to interferon-alpha, with a trend toward continued accumulation of response during post-treatment follow-up rather than the relapse pattern seen with interferon. The review noted that trial heterogeneity, small individual sample sizes, and the evolving hepatitis B treatment landscape complicate direct comparisons against modern nucleoside/nucleotide analog standards of care. The hepatitis B indication remains the most consistent clinical signal in the Tα1 evidence base, though current guidelines do not include Tα1 as a first-line option.

For hepatitis C, Andreone, Gramenzi, Cursaro, and colleagues published a randomized controlled pilot trial in the Journal of Viral Hepatitis in 2004 examining interferon-alpha 2b plus thymosin alpha-1 versus interferon-alpha 2b alone in 41 treatment-naive chronic HCV patients (22 combination, 19 monotherapy) over a 6-month treatment period with 6-month follow-up. The combination demonstrated significantly higher end-of-treatment virological response rates than monotherapy (p = 0.03), though sustained response at 6-month follow-up was equivalent between groups. The small sample size limits the strength of this pilot finding.

Sepsis: a mixed and evolving evidence base

Thymosin alpha-1 has been studied extensively in severe sepsis based on the observation that sepsis causes profound immune suppression, characterized by depletion and exhaustion of lymphocytes, reduced monocyte HLA-DR expression, and failure to mount effective adaptive immune responses. The hypothesis was that Tα1, by restoring dendritic cell activation and Th1 signaling, might reverse this immune paralysis and improve survival.

The ETASS trial, a multicenter, single-blind, randomized controlled trial published in Critical Care in 2013 by Wu and colleagues, enrolled 361 patients across six Chinese hospitals and compared thymosin alpha-1 combined with standard treatment against standard treatment alone. The Tα1 group showed a 28-day mortality rate of 26.0% versus 35.0% in controls. However, statistical significance was marginal (P = 0.062 in the primary non-stratified analysis and P = 0.049 by log-rank), and the immune recovery findings (improved monocyte HLA-DR expression) were more consistently significant than the mortality data.

The more definitive test came from the TESTS trial, a multicentre, double-blind, placebo-controlled Phase 3 study published in BMJ in January 2025 by Wu, Pei, Guan, and colleagues, which randomized 1,089 sepsis patients across 22 Chinese medical centers to subcutaneous thymosin alpha-1 (n = 542) or placebo (n = 547) every 12 hours for 7 days. The trial found no meaningful difference in survival: 28-day all-cause mortality occurred in 127/542 (23.4%) of the thymosin alpha-1 group versus 132/547 (24.1%) in the placebo group (HR 0.99, 95% CI 0.77 to 1.27, p = 0.93 by log-rank), with no secondary or safety outcome differing statistically between arms. The authors concluded that the trial "found no clear evidence to suggest that thymosin alpha-1 decreases 28-day all-cause mortality in adults with sepsis." Subgroup analyses suggested potential differential effects in patients over 60 and in those with specific comorbidities, but these findings require prospective evaluation. As of April 2026, the sepsis indication for thymosin alpha-1 should be understood as clinically unproven at the current level of evidence.

The limitations of the pre-TESTS evidence base had already been flagged by Liu, Wang, and colleagues in a 2016 systematic review in BMC Infectious Diseases, which rated the quality of available sepsis trial evidence as low due to small sample sizes, methodological variability, and risk of bias across the included studies. That critique helps explain why the large, double-blind TESTS trial was necessary and why its negative primary outcome should now weigh more heavily than the earlier positive signals in clinical interpretation.

Cancer immunology: an adjunct research application

Costantini, Bellet, Pariano, Renga, and colleagues published a reappraisal of thymosin alpha-1 in cancer therapy in Frontiers in Oncology in 2019, reviewing its application across melanoma, hepatocellular carcinoma, and lung cancer as both a standalone immune adjunct to chemotherapy and as a potential combination partner for checkpoint inhibitor immunotherapy. The rationale is that Tα1's dendritic cell activation and Th1-polarizing effects may convert immunologically "cold" tumors, which are poorly infiltrated by effector T cells, into "hot" tumors responsive to PD-1 or CTLA-4 blockade. The authors frame this as a reappraisal of a historical therapeutic approach in light of modern immunology. The evidence base for oncology applications remains largely exploratory, and no regulatory approval for a cancer indication has been granted anywhere.

COVID-19: no routine benefit supported by meta-analysis

The hypothesized relevance of thymosin alpha-1 to COVID-19 followed from the same immune-paralysis logic as sepsis: severe SARS-CoV-2 infection features lymphopenia, monocyte dysfunction, and Th1 suppression. Shang, Zhang, Ren, and colleagues published a systematic review and meta-analysis of nine studies involving 5,352 patients (1,152 receiving Tα1, 4,200 controls) in International Immunopharmacology in 2023. Across the full pooled population, thymosin alpha-1 had no statistically significant effect on mortality (RR 1.03, 95% CI 0.60 to 1.75, p = 0.92), with high between-study heterogeneity (I² = 90%). Pre-specified subgroups showed reduced mortality in patients with severe or critical disease (RR 0.66, 95% CI 0.57 to 0.76, p < 0.0001, I² = 0%) and in cohorts with mean age over 60 (RR 0.68, 95% CI 0.58 to 0.78, p < 0.0001), but the authors concluded the results do not support routine use in hospitalized adult COVID-19 patients, and subgroup findings require prospective confirmation. As of April 2026, thymosin alpha-1 is not recommended as a routine intervention in COVID-19 on the basis of current synthesized evidence.

Regulatory status and availability

As documented by Goldstein and Goldstein in a 2009 review in Expert Opinion on Biological Therapy tracing the compound's clinical development from laboratory discovery through international approvals, thymosin alpha-1 (Zadaxin) is approved for hepatitis B and hepatitis C in over 35 countries. As of April 2026, thymosin alpha-1 is not FDA-approved for any indication in the United States. Its availability in the United States is limited to compassionate use or clinical trial contexts. Superpower Health does not prescribe, sell, or facilitate access to thymosin alpha-1.

Thymosin Beta-4: The Actin-Sequestering Peptide

Thymosin beta-4 (Tβ4) is an endogenous 43-amino acid peptide encoded by the TMSB4X gene. It is not an immune hormone. Its primary function is the regulation of actin cytoskeleton dynamics: Tβ4 binds G-actin (globular monomeric actin) with high affinity, buffering the intracellular pool of actin subunits available for polymerization into filamentous F-actin. When a cell receives a migration or repair signal, the sequestered actin is released and assembled into the lamellipodia and stress fibers that drive cell movement.

Huff and colleagues, in their 2001 review in the International Journal of Biochemistry and Cell Biology, described the beta-thymosins as "the main intracellular G-actin sequestering peptides." The relevance to tissue repair is that cell migration is prerequisite to wound healing: fibroblasts, endothelial cells, and keratinocytes must all reach the injury site and traverse the wound bed, and the speed and efficiency of this migration depends partly on cytoskeletal dynamics regulated by Tβ4. Goldstein, Hannappel, and Kleinman expanded this picture in a 2005 review in Trends in Molecular Medicine, framing Tβ4 as an actin-sequestering peptide that moonlights as a multifunctional tissue-repair factor with activities that extend well beyond the cytoskeleton. In addition to its actin-buffering role, Tβ4 has anti-inflammatory, pro-angiogenic, and survival-signaling activities that appear to involve multiple distinct structural domains of the peptide.

Cardiac regeneration: the landmark preclinical work

The most striking preclinical finding in Tβ4 research came from a 2004 study published in Nature by Bock-Marquette, Saxena, White, Dimaio, and Srivastava. The study showed that thymosin beta-4 treatment following coronary artery ligation in mice promoted cardiomyocyte migration and survival, activated integrin-linked kinase and downstream Akt survival signaling, and improved cardiac function after experimental myocardial infarction. This was a landmark result because cardiomyocytes are largely post-mitotic, and the possibility that an endogenous peptide could mobilize repair in infarcted myocardium attracted significant investment in cardiac regeneration research. Subsequent work showed that Tβ4 also mobilizes adult epicardial progenitor cells. The cardiac findings, compelling as they are, remain preclinical in the context of infarction; the human cardiac trial data available as of April 2026 is limited and preliminary.

Corneal wound healing: the pathway to human trials

Thymosin beta-4 has the most advanced human trial record in ophthalmic indications. Sosne, Szliter, Barrett, Kernacki, Kleinman, and Hazlett published an early bench-level demonstration in Experimental Eye Research in 2002, showing that topical Tβ4 accelerated corneal re-epithelialization and reduced inflammatory mediator expression after alkali injury in a mouse model. This work provided the mechanistic foundation for subsequent clinical development. As Sosne reviewed in Expert Opinion on Biological Therapy in 2018, the translational arc from that preclinical discovery through Phase 2 and Phase 3 clinical trials in dry eye and neurotrophic keratopathy represented one of the most developed clinical development pathways for any thymosin. RegeneRx's RGN-259 (0.1% Tβ4 ophthalmic solution) has completed Phase III trials for neurotrophic keratopathy; no FDA approval has been granted as of April 2026.

TB-500: the synthetic fragment and its distinct identity

TB-500 is a synthetic 7-amino-acid acetylated fragment of thymosin beta-4 (Ac-LKKTETQ), covering the actin-binding domain (residues 17 through 23) of the parent protein but not the full sequence. A 2004 paper in Chemistry and Biodiversity by Leeanansaksiri, DeSimone, Huff, Hannappel, and colleagues characterized the activity of full-length Tβ4 and its N-terminal tetrapeptide Ac-SDKP on mast cells, reporting that both molecules inhibited mast cell proliferation (maximal effect at 10⁻¹⁴ M) and triggered degranulation at 10⁻⁸ M (57% for Tβ4, 89% for Ac-SDKP), while other fragments of the peptide were inactive. This provided partial mechanistic evidence that distinct sub-regions of Tβ4 retain independent bioactivity. TB-500 is not thymosin beta-4: it lacks the C-terminal AGES domain that appears to drive many of the cardiac repair findings and the full-length protein's anti-inflammatory activities. As of April 22, 2026, TB-500 was removed from the FDA 503A Category 2 list per Federal Register notice (effective April 22, 2026) and referred to the July 23, 2026 PCAC meeting for formal review; it has not been added to the Section 503A Category 1 bulks list, and removal from Category 2 does not, by itself, authorize its use in compounding. Superpower does not prescribe, sell, or facilitate access to TB-500. For a detailed treatment of TB-500's regulatory and evidence status, see the TB-500 overview.

Prothymosin Alpha: The Nuclear Member

Prothymosin alpha (ProTα) is the largest and least clinically discussed member of the thymosin family. It is a 111-amino acid protein encoded by the PTMA gene that is found primarily in the cell nucleus, where it interacts with histone H1. Gómez-Márquez and Rodríguez, writing in the Biochemical Journal in 1998, established that prothymosin alpha is a chromatin-remodeling protein: overexpression of ProTα increases micrococcal nuclease accessibility to chromatin and produces nucleosomes depleted of histone H1. The authors proposed that ProTα functions biologically in the remodeling of chromatin fibers through its interaction with histone H1, regulating transcriptional availability of DNA in actively dividing cells.

ProTα has dual biological roles depending on its subcellular location. Intracellularly, it functions as a cell survival and proliferation factor, with its expression closely linked to cell cycle progression. It has also been characterized in extracellular contexts, where it appears to exhibit immunostimulatory activity separate from its nuclear functions, though this extracellular biology is less well characterized than either thymosin alpha-1 or thymosin beta-4. Samara, Ioannou, and Tsitsilonis reviewed this dual biology in Vitamins and Hormones in 2016, describing prothymosin alpha as both an intracellular survival and proliferation mediator and an extracellular immune-activating molecule that traces its identification back to the original thymosin fraction 5. That review bridges the historical fraction 5 work to ProTα's standalone immune function and helps explain why a nuclear chromatin-remodeling protein has been repeatedly re-examined in immunology. ProTα has no clinical development program of comparable scale to either Tα1 or Tβ4. It is included in this overview to complete the taxonomy of the thymosin family and to underscore that the shared name encompasses three radically different biological entities.

Which Biomarkers Are Relevant When Exploring Thymosin Biology?

Because the thymosin family spans immune function and tissue repair, the most informative biomarkers vary by which subfraction and which indication is being discussed. For immune function, which is the domain of thymosin alpha-1 and, in different ways, prothymosin alpha, the relevant markers center on the adaptive immune compartment and systemic inflammation. For tissue repair and cellular dynamics, which is thymosin beta-4's domain, the relevant markers are broader and less specific to Tβ4 directly.

• Absolute lymphocyte count: The circulating pool of lymphocytes reflects the output of both the thymus (naive T cells) and peripheral lymphoid organs. Thymic involution, the same biological context that motivated original thymosin research, manifests partly as reduced naive T cell output and altered lymphocyte subset ratios. Baseline and tracking relevant regardless of compound use.
• Neutrophil-to-lymphocyte ratio (NLR): An elevated NLR reflects both relative neutrophilia and lymphopenia and is one of the simplest blood markers of systemic immune imbalance. In the sepsis research context where Tα1 has been studied, immune paralysis is characterized by lymphopenia and disrupted NLR. An NLR outside the reference range in otherwise healthy individuals may indicate chronic inflammatory or immune dysregulation worth characterizing.
• Lymphocyte subsets (CD4+, CD8+ T cells, NK cells): Thymosin alpha-1's mechanism of action specifically involves T cell differentiation and Th1 polarization. In clinical practice, detailed lymphocyte subset panels provide more mechanistic information about immune compartment composition than total lymphocyte counts alone. These are available through specialty immune panels and are particularly relevant for individuals with suspected immune dysfunction.
• hs-CRP and inflammatory markers: Both thymosin alpha-1, through Th1 polarization, and thymosin beta-4, through direct anti-inflammatory activity on wound-site monocytes and macrophages, are proposed to modulate inflammatory signaling. Baseline inflammatory markers provide context for immune and tissue-repair status. Superpower's baseline panel includes hs-CRP as a standard marker of systemic low-grade inflammation.
• Liver function panel (ALT, AST, GGT): Given thymosin alpha-1's primary clinical applications in hepatitis B and C, liver function markers are directly relevant to anyone exploring Tα1 in the context of liver disease. ALT and AST are the primary markers of hepatocellular inflammation. Reference ranges vary by lab and individual; your provider will interpret your specific results in context.
• Complete blood count with differential: The CBC with differential provides a full picture of circulating immune cell populations, including neutrophils, lymphocytes, monocytes, eosinophils, and basophils. It is the foundational immune-status measurement and anchors any clinical decision-making related to immune function. Superpower's immune system biomarker guide covers the CBC components most relevant to immune strength assessment.

Testing gives you a picture of where your immune biology actually stands, separate from any compound. That picture is informative regardless of whether any thymosin-related compound is ever part of the clinical conversation. Superpower's approach to preventive health is grounded in the principle that objective measurement should come before any clinical decision, as described in the Superpower manifesto.

Legal and Regulatory Status

As of April 2026, the regulatory status of the thymosin family in the United States is as follows.

Thymosin alpha-1 (Zadaxin): Not FDA-approved for any indication. Approved in over 35 countries for hepatitis B, hepatitis C, and immune support in cancer patients. Available in the United States only through compassionate use requests or clinical trial enrollment. Not available through Superpower.

Thymosin beta-4: Not FDA-approved for any indication. RGN-259 (RegeneRx's ophthalmic formulation) completed Phase III trials for neurotrophic keratopathy without receiving FDA approval as of April 2026. Thymosin beta-4 was removed from the FDA Category 2 list effective April 22, 2026 per Federal Register notice and is scheduled for PCAC review on July 23, 2026; it has not been placed on the Section 503A Category 1 bulks list, and removal from Category 2 is not equivalent to approval for use in compounding. Not available through Superpower. For the dedicated article, see the thymosin beta-4 overview.

TB-500 (thymosin beta-4 fragment): TB-500 was removed from the FDA Category 2 list effective April 22, 2026 per Federal Register notice and was referred to the July 23, 2026 PCAC meeting for formal review. It has not been added to the Section 503A bulks list, and removal from Category 2 does not, by itself, authorize compounded use. Not available through Superpower.

Prothymosin alpha: No approved indication anywhere. No active clinical development program as of April 2026. Research compound only.

IMPORTANT SAFETY INFORMATION

This page discusses multiple compounds in the thymosin family with varying regulatory statuses. No member of the thymosin family discussed here is FDA-approved for human use in the United States as of April 2026. Thymosin alpha-1 (Zadaxin) holds international approvals in over 35 countries for hepatitis B, hepatitis C, and immune-adjunct oncology use, but is not FDA-approved in the United States; it is not listed on the FDA's 503A Category 1 or Category 2 bulk drug substance lists (removed from Category 2 in September 2024 following nominator withdrawals and awaiting formal PCAC review). Thymosin beta-4 has not received FDA approval for any indication; RGN-259 (RegeneRx's ophthalmic formulation) completed Phase 3 trials for neurotrophic keratopathy without approval. Thymosin beta-4 and TB-500 were both removed from the FDA 503A Category 2 list effective April 22, 2026 per Federal Register notice and are scheduled for formal PCAC review on July 23, 2026; neither has been added to the Section 503A Category 1 bulks list, and removal from Category 2 is not, by itself, authorization for use in compounding. The legal status of compounded thymosin beta-4 and TB-500 remains unresolved pending FDA action. Prothymosin alpha has no approved indication and no active clinical development program. Superpower is a technology platform; Superpower does not prescribe, sell, compound, or facilitate access to thymosin alpha-1, thymosin beta-4, TB-500, or prothymosin alpha.

Do not use any thymosin family compound if you: have an active malignancy (immunomodulatory peptides may theoretically affect tumor biology); are pregnant, may become pregnant, or are breastfeeding (safety not established); are taking immunosuppressive therapy following organ transplant (thymosin alpha-1 activation of adaptive immunity could theoretically provoke rejection); have an autoimmune condition (immune activation is a theoretical concern); have a history of severe hypersensitivity to peptide formulations or to any excipient; or are a minor (pediatric safety not established for any thymosin family compound outside limited international pediatric hepatitis B programs for thymosin alpha-1).

Warnings (vary by compound): Thymosin alpha-1 international labeling lists injection site reactions, transient lymphocyte changes, and rare allergic reactions; its mechanism as a toll-like-receptor-mediated immunomodulator carries theoretical concerns in autoimmunity. Thymosin beta-4 and TB-500 have limited published human safety data; products sold online as TB-500 for research use are not subject to FDA oversight for identity, purity, sterility, or potency. Neither thymosin beta-4 nor TB-500 is currently on the FDA's Section 503A Category 1 bulks list; pending PCAC review in July 2026, their legal status for use in compounded preparations is unresolved, and products marketed as compounded thymosin beta-4 or TB-500 are being dispensed without clear FDA authorization. Prothymosin alpha has no clinical safety database in humans.

Common side effects: Compound-specific. For thymosin alpha-1 (per international Zadaxin labeling): injection site reactions, transient discomfort, rare flu-like symptoms. For thymosin beta-4 and TB-500: not systematically characterized in published human trials. Reported anecdotal effects from unregulated use cannot substitute for controlled safety data.

WADA status: TB-500 and thymosin beta-4 are prohibited at all times under the World Anti-Doping Agency 2026 Prohibited List. Thymosin alpha-1 is not currently specifically listed but may fall under S0 (non-approved substances). Athletes subject to anti-doping controls should confirm status with their governing body.

Long-term data limitations: No U.S. Phase 3 controlled efficacy trial has been completed for any thymosin family compound for indications outside thymosin alpha-1's international hepatitis B use. For TB-500 and prothymosin alpha, no completed U.S. Phase 2 efficacy trial exists. Long-term cancer risk, immune consequences, and safety in special populations have not been characterized.

Compound references: PubChem CID 16132341 (thymosin alpha-1); PubChem CID 16129704 (thymosin beta-4). No FDA-approved prescribing information exists for any thymosin family compound in the United States.

Additional Questions

What is the thymus gland and why does it matter for immunity?

The thymus is a small organ in the upper chest that educates and matures T lymphocytes before they enter circulation. It is most active during childhood and involutes progressively after puberty, producing fewer naive T cells with age. Thomas, Wang, and Su documented in a 2020 review in Immunity and Ageing that this involution is a primary driver of the age-related immune decline known as immunosenescence, characterized by reduced adaptive immune responses and increased chronic inflammation. The original hypothesis behind thymosin research was that thymic involution could be partially compensated by thymic hormones, which motivated the extraction and study of thymosin fraction 5 in the 1960s.

Can you get thymosin alpha-1 in the United States?

As of April 2026, thymosin alpha-1 is not FDA-approved and is not available through standard medical channels in the United States. Access options are limited to participation in clinical trials or compassionate use requests submitted to the FDA. It cannot be compounded by standard 503A pharmacies in the United States because it is not on the FDA's approved bulk drug substance list for compounding. Superpower Health does not prescribe, sell, or facilitate access to thymosin alpha-1.

Where can I learn more about individual thymosin compounds?

Dedicated articles are available for the two most clinically discussed thymosins. The thymosin beta-4 overview covers the actin biology, the cardiac and corneal trial data, and current availability through compounding. The TB-500 article covers the synthetic fragment in detail, including the regulatory reclassification and the distinction between the fragment and the full-length protein.