Skip to content
StackItSmart
BHuman cohort / observational

Thymosin Alpha-1

Zadaxin · Tα1

Thymosin alpha-1 (thymalfasin, Zadaxin) is a synthetic 28-amino-acid peptide identical to a fragment naturally produced by the thymus. It is an immune-response modifier, not an anabolic agent, hormone, or SERM: it acts on T-cell maturation and function rather than on muscle, strength, or body composition. It is approved in several countries (not FDA-approved in the US) as an adjunct for chronic hepatitis B and as a vaccine-response enhancer, and is used off-label in China for sepsis, severe infections, and cancer. Despite decades of use and a generally clean tolerability record (mostly injection-site reactions), rigorous efficacy is weak and inconsistent. The largest, most rigorous trial to date (TESTS, a 1106-patient phase 3 RCT, 2025) found no reduction in sepsis mortality, overturning smaller earlier positive trials. Most positive data come from small, single-region (predominantly Chinese) trials with heterogeneity and bias risk. Because it deliberately stimulates the immune system, the main theoretical dangers are autoimmune activation and unpredictable immune effects in people with autoimmune disease or on immunosuppression/transplant regimens. It should only be used under a physician's supervision for a defined medical indication, not for performance or physique.

Clinical readoutPeptide · immune-peptide
Hepatic strainNone
CardiovascularNone
HPTA suppressionNone
Half-life
2 h
Route
Subcutaneous injection
Evidence
B
Active
Peak serum concentratio…
2 h4 h6 h8 h10 h
Illustrative single-compartment washout · each mark = one half-life · t½ ≈ Approximately 2 hours (serum), per pharmacology review (PMID 11381492).
Pharmacology

Mechanism of action

Thymosin alpha-1 is an immunomodulatory peptide that promotes maturation and differentiation of T lymphocytes, increases circulating CD4+ and CD8+ T-cell numbers and the CD4/CD8 ratio, and enhances thymic output (reflected by increased T-cell receptor excision circles, TRECs). It signals through Toll-like receptors (notably TLR2/TLR9 on dendritic cells and TLR7 on macrophages), augmenting dendritic-cell and NK-cell activity, cytokine production, and antigen presentation, and can up-regulate monocyte HLA-DR expression in immunosuppressed states. In viral and septic contexts it has been reported to reduce T-cell exhaustion markers (PD-1, Tim-3) and restore lymphocyte counts. It is a biological response modifier with no androgenic, anabolic, estrogenic, or direct hormonal (HPTA) activity.
Kinetics

Pharmacokinetics

Half-life

Approximately 2 hours (serum), per pharmacology review (PMID 11381492).

Active duration

Peak serum concentration within ~2 hours; blood levels return to baseline within 24 hours. Pharmacodynamic immunomodulatory effects on T cells outlast plasma presence, which is why chronic regimens dose only twice weekly rather than continuously.

Route

Subcutaneous injection (the only route used clinically). No oral bioavailability for a peptide of this type.

Metabolism & clearance

As a small peptide it is degraded by endogenous peptidases/proteolysis to amino acids; no cytochrome-P450 metabolism and no clinically characterized drug-metabolism interactions. Dedicated renal/hepatic clearance and dose-adjustment data are sparse; this PK profile is relevant for washout and monitoring, not for evading any test.

For monitoring and washout planning, not drug-test evasion.

Reported effects

Physiological & performance effects

  • Increases circulating CD4+ and CD8+ T lymphocytes and the CD4/CD8 ratio in immunosuppressed patients (e.g., sepsis, severe acute pancreatitis)
  • Up-regulates monocyte HLA-DR expression, a marker of restored immune competence, in severe sepsis (ETASS trial)
  • Reduces T-cell exhaustion markers (PD-1, Tim-3) and restored lymphocyte counts in a retrospective severe-COVID-19 cohort
  • As monotherapy for chronic hepatitis B, produces delayed (post-treatment) virological and biochemical responses comparable to or exceeding interferon-alpha, per meta-analysis
  • Added to entecavir in HBV-related cirrhosis, transiently improves early HBV-DNA suppression and HBeAg loss (benefit not sustained at 48-52 weeks)
  • Reduces extrapancreatic infection incidence and APACHE II score in severe acute pancreatitis (meta-analysis)
  • Reported enhancement of vaccine responses in immunocompromised populations (basis of one approved indication)
  • No demonstrated effect on muscle mass, strength, fat loss, or athletic performance — it is not an anabolic or ergogenic agent
Safety

Adverse effects by system

Cardiovascular

No known direct cardiovascular toxicity and no cardiac safety signal reported in the trials reviewed (including the 1106-patient TESTS phase 3 trial, where no safety outcome differed from placebo). Long-term cardiovascular safety has not been formally studied.

Hepatic

No evidence of hepatotoxicity; it is used therapeutically in hepatitis B and HBV cirrhosis, and combination meta-analysis reported fewer adverse events than entecavir alone. No liver-injury signal.

Endocrine / HPTA

No hormonal or hypothalamic-pituitary-gonadal (HPTA) activity. It is not a steroid, androgen, or SERM and has no reported effect on testosterone, LH, FSH, estradiol, or the gonadal axis. No adequate data on endocrine effects beyond the immune system.

Reproductive

No adequate human data on fertility, pregnancy, or lactation; not studied in these populations and should be avoided in pregnancy/breastfeeding absent a compelling supervised indication.

Neuropsychiatric

No characterized psychiatric adverse effects; no neuropsychiatric signal reported in trials. Data are limited and not specifically collected.

Renal

No established nephrotoxicity and no renal safety signal reported; dedicated renal-impairment and dose-adjustment data are sparse.

Hematologic

Intended pharmacologic effect is to raise lymphocyte subsets; no adverse hematologic toxicity (cytopenias, thrombosis) reported. As an immune activator, theoretical concern for autoimmune cytopenias exists but is not documented in the reviewed literature.

Dermatologic

Most commonly reported adverse effect is local injection-site reaction/irritation (erythema, discomfort). Rare hypersensitivity is theoretically possible with any injected peptide.

Recovery

HPTA suppression & recovery

Suppression: None expected — no HPTA/gonadal axis activity

Thymosin alpha-1 is an immune peptide with no androgenic, anabolic, or hormonal mechanism, so it does not suppress the hypothalamic-pituitary-gonadal axis and there is no rationale for post-cycle therapy or any SERM. If a user is combining it with agents that do suppress the axis (e.g., anabolic steroids), any recovery decision — including whether a single SERM is appropriate — should be individualized and directed by an endocrinologist, not self-managed; dual-SERM protocols are not endorsed.

Bloodwork & vitals

Monitoring

Recommended labs & checks
CBC with lymphocyte subsets (CD4/CD8) when used for immune indicationsLiver function tests and HBV DNA / HBeAg when treating hepatitis B (per hepatology protocol)Baseline autoimmune screen (e.g., ANA) if there is personal or family history of autoimmune diseaseInflammatory markers (CRP) and clinical severity scores in acute-care use, per treating team

Cadence: Directed by the treating clinician and indication: acute/critical-care use is monitored continuously in hospital; chronic hepatitis B regimens follow standard hepatology intervals (e.g., every 4-12 weeks). There is no validated self-monitoring schedule for non-medical use.

Warning signs — seek care
  • Signs of new or flaring autoimmune disease (unexplained joint pain, rash, fatigue, thyroid symptoms)
  • Allergic/hypersensitivity reaction (widespread rash, swelling, difficulty breathing) — stop and seek emergency care
  • Persistent or worsening injection-site reaction, infection, or fever
  • Any unexpected symptom in a transplant recipient — contact the transplant team immediately
Do not use if

Contraindications

  • Known hypersensitivity to thymosin alpha-1 or excipients
  • Organ transplant recipients on immunosuppression — deliberate immune stimulation risks graft rejection; use only under transplant-specialist direction
  • Active autoimmune disease (e.g., lupus, rheumatoid arthritis, autoimmune thyroiditis) — an immune activator may theoretically worsen autoimmunity; avoid unless a specialist judges benefit exceeds risk
  • Pregnancy and breastfeeding — no adequate safety data
  • Use for non-medical goals (physique, performance, longevity self-experimentation) is not an evidence-supported indication and is discouraged
Combinations

Interaction profile

  • ContraindicatedWith DNP: Additive cardiovascular strain

Check a specific combination in the interaction checker.

Harm reduction

Reducing harm & when to stop

  • Use only under a physician's supervision for a defined medical indication; there is no evidence supporting non-medical use for physique, performance, or longevity, and self-sourced peptides carry contamination and dosing-accuracy risks.
  • Recognize that the strongest evidence is negative or modest: the largest rigorous trial found no sepsis mortality benefit, and most positive data are from small single-region trials — do not assume efficacy.
  • Because it stimulates the immune system, avoid it if you have active autoimmune disease or are on immunosuppression/post-transplant regimens unless a specialist directs otherwise.
  • Stop and seek medical care for signs of allergic reaction (facial/throat swelling, difficulty breathing, widespread rash), new autoimmune symptoms, or persistent injection-site infection/fever.
  • Do not use in pregnancy or breastfeeding given the absence of safety data.
  • It has no hormonal activity and is not a substitute for, nor a remedy against, any steroid-related endocrine suppression; hormonal recovery questions belong with an endocrinologist.
  • Maintain appropriate lab monitoring for the specific indication (e.g., LFTs and HBV markers for hepatitis, CBC/lymphocyte subsets for immune indications) with your clinician.
Evidence

Citations (8)

Every clinical claim above is tied to a primary source. Overall evidence grade B — graded to the best available evidence for its core claims.

  1. 01

    Thymosin alpha-1 is a synthetic 28-amino-acid thymic peptide and immune-response modifier approved for chronic hepatitis B and as a vaccine-response enhancer; it acts on T-cell maturation/differentiation via Toll-like receptor signaling and has an exceptional tolerability record; used off-label for cancer, sepsis, and severe infection.

    ReviewPMID 36871535 ↗

  2. 02

    Serum half-life is approximately 2 hours; peak serum concentration within ~2 hours; blood levels return to baseline within 24 hours; well tolerated with mostly local injection-site irritation; typical hepatitis dosing 1.6 mg subcutaneously twice weekly.

    ReviewThymosin alpha-1.PMID 11381492 ↗

  3. 03

    In a double-blind, placebo-controlled phase 3 RCT of 1106 adults with sepsis (TESTS), thymosin alpha-1 (subcutaneous every 12 hours for 7 days) did NOT reduce 28-day all-cause mortality (23.4% vs 24.1%; HR 0.99) and no secondary or safety outcome differed from placebo.

    RCTThe efficacy and safety of thymosin alpha1 for sepsis (TESTS): multicentre, double blinded, randomised, placebo controlled, phase 3 trial.DOI 10.1136/bmj-2024-082583 ↗

  4. 04

    An earlier multicenter single-blind RCT (ETASS, n=361) in severe sepsis found a marginal 28-day mortality reduction (26.0% vs 35.0%; RR 0.74, log-rank P=0.049) and greater improvement in monocyte HLA-DR, with no serious drug-related adverse events.

    RCTPMID 23327199 ↗

  5. 05

    In a retrospective cohort of 76 severe COVID-19 patients, thymosin alpha-1 was associated with lower mortality (11.1% vs 30.0%, P=.044), increased CD4+/CD8+ T-cell counts, reduced PD-1/Tim-3 exhaustion markers, and increased thymic output (TRECs).

    CohortDOI 10.1093/cid/ciaa630 ↗

  6. 06

    Meta-analysis of 4 RCTs (199 patients) found that compared with interferon-alpha, thymosin alpha-1 monotherapy for chronic hepatitis B produced delayed but greater virological, biochemical, and complete responses at post-treatment follow-up.

    Meta-analysisDOI 10.1016/j.antiviral.2007.10.014 ↗

  7. 07

    Meta-analysis of 7 RCTs (1144 subjects) in HBV-related cirrhosis found entecavir plus thymosin alpha-1 improved early complete response and HBV-DNA/HBeAg outcomes (benefit not sustained at 48-52 weeks) and had significantly fewer adverse events than entecavir alone (RR 0.48).

    Meta-analysisDOI 10.1186/s12876-020-01477-8 ↗

  8. 08

    Meta-analysis of 5 RCTs (706 patients) in severe acute pancreatitis found thymosin alpha-1 increased CD4+ cells and CD4/CD8 ratio, reduced extrapancreatic infection incidence (RR 0.56) and APACHE II score, without significant reduction in hospital stay.

    Meta-analysisDOI 10.3389/fimmu.2025.1571456 ↗

Last reviewed 2026-07-06 · Verified against PubMed · Educational, not medical advice