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Thymosin Alpha-1

A 28-amino-acid thymic peptide, approved in dozens of countries outside the US, that pushes immature T-cells toward mature Th1 function and is used for chronic hepatitis, immunosenescence and immune-suppressed states.

Also known as Tα1, Ta1, TA1, thymalfasin, thymosin α1, Zadaxin

Human RCT — Randomised controlled trials in humans, but not an approved product for this use.

Genuinely well studied by peptide standards: approved in more than 30 countries for chronic hepatitis B, hepatitis C and as a vaccine adjuvant, with dozens of randomised trials. Note the honest counterweight - the large 2025 TESTS phase 3 trial of thymosin alpha-1 in 1,106 sepsis patients found no mortality benefit, so it is not a universal immune fix.

How it works

Thymosin alpha-1 is the N-terminally acetylated fragment 1-28 of prothymosin alpha, produced by thymic epithelium. It binds TLR2 and TLR9 on dendritic cells and monocytes, promoting their maturation and driving IL-2, IL-12 and interferon-gamma production while restraining IL-10 and other Th2 signals. Downstream it increases CD4+ and CD8+ T-cell counts, raises IL-2 receptor expression, restores natural killer cell activity and improves antigen presentation via MHC class I upregulation. In immunosuppressed and immunosenescent states it appears to act mainly by restoring a depressed response rather than stimulating a normal one, which is why it behaves as a modulator rather than a blunt stimulant.

Targets: TLR2, TLR9, Dendritic cell maturation, CD4+/CD8+ T-cells, IL-2 / IFN-gamma axis

Dosing

ProtocolDoseFrequencyRoute
Registered hepatitis B/C protocol (Zadaxin)Injections spaced three to four days apart, e.g. Monday and Thursday.1.6 mgtwice weeklysubcutaneous
Common wellness / immune-support protocolNo strong timing signal; most people inject in the evening.450 mcg – 1.6 mgdaily to twice weeklysubcutaneous
Acute infection short courseStarted as early in the illness as possible.1.6 mg – 3.2 mgonce dailysubcutaneous
  • · This is the actual approved regimen in the countries where thymalfasin is licensed, typically run for six to twelve months.
  • · Compounding-clinic protocols usually run 450-900 mcg daily for four to eight weeks, or 1.6 mg twice weekly for longer maintenance. These are extrapolations from the hepatitis dose, not trial-derived.
  • · Mirrors the higher-intensity regimens used in Chinese and Italian sepsis and COVID protocols; short courses of five to ten days.

Cycling

Wellness protocols usually run four to eight weeks and then stop or drop to twice-weekly maintenance. Registered hepatitis use runs six to twelve months continuously, so long duration itself is not the safety problem.

Work out your exact syringe units →

Pharmacology

Half-life
Roughly 2 hours in plasma, but the immune effects clearly outlast the drug because the downstream cellular changes persist for days.
Onset
Cellular immune markers shift within one to two weeks; clinical effects in chronic hepatitis were measured over six to twelve months.
Routes
subcutaneous
Molecule
Synthetic N-acetylated 28-amino-acid thymic peptide
Sequence length
28 amino acids
Molecular weight
3108.3 Da

Handling

Diluent
Bacteriostatic water
Typical mix
2 or 3 mL
Vial sizes
1.6, 5, 10 mg
Lyophilised
Refrigerate at 2-8 C. The lyophilised powder tolerates room temperature in transit but should go back in the fridge on arrival.
Reconstituted
Refrigerated, use within about 14-30 days.
Light sensitive
Yes — keep it out of the light

Mixing

The registered 1.6 mg vial ships with 1 mL of sterile diluent. For a 5 or 10 mg research vial, 2-3 mL of bacteriostatic water gives convenient 100-500 mcg increments on a U-100 syringe.

Side effects

  • commonInjection-site redness or discomfort— The most frequently reported effect in trials; usually settles within a day.
  • uncommonTransient fatigue or flu-like feeling— Consistent with a Th1 shift; typically first few doses only.
  • uncommonTransient ALT elevation in hepatitis patients— Seen during hepatitis B treatment and generally interpreted as an immune-clearance flare rather than toxicity.
  • rareFlare of autoimmune activity— Plausible in anyone with an existing autoimmune condition, since the drug pushes cellular immunity up.

Do not use if

  • Solid organ transplant on immunosuppression - the whole point of the drug works against the transplant regimen.
  • Active, poorly controlled autoimmune disease such as lupus or rheumatoid arthritis, where boosting Th1 activity is the wrong direction.
  • Known hypersensitivity to thymalfasin.

Combining it

  • conflictcyclosporine-a — Directly opposing mechanisms; do not combine outside a physician-run protocol.
  • redundantthymalin — Both target thymic T-cell maturation; stacking adds cost more than effect.
  • synergyInterferon alfa — Combined with interferon in several hepatitis regimens with better response rates than interferon alone.
  • synergyll-37 — Complementary innate and adaptive arms; commonly stacked anecdotally for chronic infection.

What to monitor

  • · CBC with differential to track lymphocyte counts if using it for immune restoration.
  • · Liver function tests if you have any hepatitis history.
  • · Autoimmune symptoms - joint pain, rashes, new fatigue patterns - should prompt stopping.

Legal status

Not FDA-approved in the US (orphan designation only) but approved in over 30 countries including China and Italy. In the US it appeared on the FDA's category 2 bulk-substance list, which pushed it out of legitimate compounding; it is now sold mainly as a research chemical.

References

  • TESTS: multicentre, double-blind, randomised, placebo-controlled phase 3 trial of thymosin alpha-1 in sepsis, BMJ 2025 (trial)
  • Zadaxin (thymalfasin) product information, SciClone Pharmaceuticals (label)
  • Garaci and colleagues, thymosin alpha-1 mechanism and clinical applications review (review)
  • Systematic review and meta-analysis of thymosin alpha-1 in sepsis, Frontiers in Cellular and Infection Microbiology 2025 (review)

Mechanism in depth

The Core entry says TLR2 and TLR9. The mechanistically interesting part is what happens downstream of TLR9, because that is where the clinical picture comes from. Bozza and colleagues showed in 2007 that thymosin alpha-1 signals through TLR9 into the MyD88/IRF7 axis, which is the plasmacytoid dendritic cell pathway that produces type I interferon rather than the classical NF-kB inflammatory pathway. That is why the drug produces an antiviral, interferon-flavoured immune shift without the cytokine storm you would expect from a blunt TLR agonist. The second arm, from Romani's Blood paper in 2006, is that thymosin alpha-1 simultaneously induces indoleamine 2,3-dioxygenase in dendritic cells, which catabolises tryptophan and generates regulatory T-cells. So the drug pushes the Th1 antiviral response and the regulatory brake at the same time. That dual action is the honest explanation for why it behaves as a modulator rather than a stimulant, and it is also the best explanation for why it failed in sepsis - in a patient whose problem is a dysregulated hyperinflammatory-then-immunoparalytic cascade, pushing both accelerator and brake produces nothing measurable. The T-cell maturation effect that gets quoted in wellness marketing is real but downstream and slow: CD4 and CD8 counts, IL-2 receptor density and NK cytotoxicity shift over one to three weeks, not over hours.

What usually goes wrong

Three failure modes, in order of how often they actually happen. First and most common: nothing happens. If your immune function was not depressed to begin with, a drug whose signature is restoring a suppressed response has nothing to restore, and eight weeks and several hundred dollars later you have no measurable change. Second: an autoimmune flare. This is the mechanistically predictable harm - you are pushing Th1 and interferon signalling in someone whose immune system is already pointed at their own tissue. It usually presents as new joint pain, a rash, or unexplained fatigue two to six weeks in, and it is the reason to know your ANA before starting. Third, and specific to hepatitis B: a transient ALT rise that people panic about. In that population it is usually immune clearance doing its job, but distinguishing that from real hepatotoxicity requires bilirubin and INR, not guesswork. The commercial failure mode worth naming separately is product identity - since the FDA category 2 bulk-substance listing pushed thymalfasin out of legitimate US compounding, what is sold as thymosin alpha-1 is research-chemical supply with no released certificate of analysis in most cases, and a 28-residue acetylated peptide is not trivial to synthesise correctly.

Titration ladder

  1. 450 mcgWeek 1 — Convention, not evidence. There is no trial-derived ramp for this drug - the registered hepatitis regimen starts at the full 1.6 mg. The reason to ramp is the transient flu-like feeling some people get on the first doses, which reflects the Th1 shift rather than a toxicity.
  2. 900 mcgWeek 2 — If the first week produced no fatigue or joint ache, this is where most clinic protocols sit for wellness use.
  3. 1.6 mgWeek 3 onward — The actual registered dose. Beyond this you are extrapolating from sepsis and COVID protocols, which used 1.6 mg once or twice daily for five to ten days - short courses, not maintenance.

Bloodwork worth running

MarkerWhenWhy it matters
CD4+ / CD8+ T-cell counts and the CD4:CD8 ratio (lymphocyte subset panel)Baseline, then at four and twelve weeks. There is no point testing at week one - the cellular changes take longer than that.This is the single marker that actually tracks what the drug does. It is also the marker that moved in the COVID cohort work, where lymphocyte recovery was the mechanism proposed for the mortality signal.Act if: If absolute CD4 count has not moved at all by twelve weeks in someone who started low, the drug is not doing anything for you and continuing is spending money on hope.
CBC with differential (absolute lymphocyte count)Baseline and every four to eight weeks on a long course.Cheap proxy for the subset panel and available anywhere. Absolute lymphocyte count below about 1.0 x 10^9/L is the state in which this drug has the best chance of doing something measurable.Act if: A falling lymphocyte count on treatment is not a thymosin effect - go looking for another cause.
ALT and ASTBaseline, then monthly if you have any hepatitis history. Otherwise baseline and at eight weeks.In chronic hepatitis B, a transient ALT rise on treatment is the expected immune-clearance flare and is a good sign, not a toxic one. Outside hepatitis, a rise is not expected and means something else.Act if: ALT above five times the upper limit of normal, or any rise accompanied by rising bilirubin, means stop and get assessed rather than assume it is a clearance flare.
hs-CRPBaseline and at eight weeks.Not a target of the drug, but a useful sanity check that you are not driving inflammation upward in someone with subclinical autoimmune disease.Act if: A sustained rise from a normal baseline in someone with joint or skin symptoms is a reason to stop.
ANA and, if positive, an extractable nuclear antigen panelBaseline only, unless symptoms develop.The one predictable way this drug hurts someone is by unmasking or flaring autoimmunity. Knowing your ANA before you start makes that conversation possible.Act if: A high-titre positive ANA at baseline with symptoms is a reason not to start.

Pharmacokinetics

Tmax
2 h
Metabolism
Degraded by serum and tissue aminopeptidases and endopeptidases into constituent amino acids. There is no cytochrome P450 involvement, so the entire CYP interaction table that dominates cyclosporine dosing is irrelevant here.
Elimination
Peptide catabolism with renal handling of the fragments. No parent-drug excretion pathway of consequence.

Receptor targets

  • TLR9 (endosomal, plasmacytoid dendritic cells and macrophages) — Not quantified - no published Kd or EC50 for thymosin alpha-1 at TLR9

    Engages the MyD88/IRF7 pathway, driving type I interferon production and antiviral priming rather than NF-kB-driven inflammation. Demonstrated in murine cytomegalovirus infection in vivo.

  • TLR2 (myeloid dendritic cells, monocytes) — Not quantified

    Contributes to dendritic cell maturation and the IL-12 / interferon-gamma limb of the response.

  • Indoleamine 2,3-dioxygenase (induced, not bound) — Not applicable - this is an induced enzyme, not a receptor

    Tryptophan catabolism in dendritic cells generating kynurenines and regulatory T-cells. This is the tolerogenic counterweight that keeps the Th1 push from becoming autoimmunity in most people.

  • IL-2 receptor alpha (CD25) expression on T-cells — Not applicable - upregulated, not bound

    Raises T-cell responsiveness to IL-2, which is the proximate reason lymphocyte counts and proliferative responses improve in immunosuppressed states.

Trials

  • TESTS - The Efficacy and Safety of Thymosin alpha-1 for Sepsis Phase 3 · n=1106 · 4 weeks · 2025

    28-day all-cause mortality: 23.4% with thymosin alpha-1 versus 24.1% with placebo, hazard ratio 0.99, P=0.93. A clean, adequately powered null result in 552 versus 554 patients across multiple Chinese centres. This is the most important trial in the compound's history and it is negative.

  • Liu and colleagues, thymosin alpha-1 in severe COVID-19 (retrospective multicentre cohort, not randomised) Retrospective cohort · 2020

    Reduced mortality in severe COVID-19 associated with restoration of lymphocyte counts and reversal of T-cell exhaustion markers. Read this as hypothesis-generating: it is a retrospective comparison, not a randomised trial, and the subsequent randomised sepsis data did not support the mortality claim.

What to expect, and when

Hours: nothing you will feel, beyond a possible flu-like ache on the first two or three doses. Days 1-7: some people report the transient fatigue that goes with a Th1 shift; if it has not settled by day ten it is not this. Weeks 2-4: lymphocyte subsets begin to move if they were depressed at baseline - this is the first thing that is measurable. Weeks 4-12: the CD4 count, CD4:CD8 ratio and NK cytotoxicity changes are established, and this is the right window to retest. Months 6-12: the registered hepatitis endpoints - HBeAg seroconversion, sustained virological response - were measured here, not earlier. If you are running this for immune support and have seen nothing objective by three months, stop.

Stacking and comparisons

The one combination with actual outcome data behind it is thymosin alpha-1 plus interferon alfa in chronic hepatitis B, which outperformed interferon alone in the meta-analysed trials. Everything else is convention. Pairing it with LL-37 is common in chronic-infection protocols on the theory that you are covering innate and adaptive arms; there is no human data on the combination. Do not stack it with thymalin, thymopentin or Vilon - they all claim the same T-cell maturation endpoint and you are paying three times for one mechanism, with thymosin alpha-1 being the only one of the four with phase 3 data. The genuinely dangerous pairing is with any calcineurin inhibitor: cyclosporine and voclosporin exist to block IL-2 transcription, and this drug exists to raise it.

Against the rest of this class, thymosin alpha-1 is the only thymic peptide with a properly powered phase 3 trial, more than 30 national approvals and a meta-analysed indication. That makes it the reference compound - and it also means the negative sepsis result carries more weight than a positive result from any of the others, because the others have never been tested at that standard. Against thymopentin it is longer-lived (two hours versus thirty seconds), better characterised and dosed in micrograms rather than tens of milligrams. Against thymalin it is a defined single molecule rather than a bovine extract. Against Vilon and Crystagen it is a different category of evidence entirely and should not be discussed in the same register. If your goal is broad anti-inflammatory rather than immune restoration, this is the wrong compound - KPV or alpha-MSH act on NF-kB directly and thymosin alpha-1 does not.

Rough cost

$60–$250/month. Research-supplier pricing for 5 mg and 10 mg vials, which is what the market is now that the compound is off the US compounding pathway. At the registered 1.6 mg twice weekly you burn roughly 13 mg a month. Daily 450-900 mcg wellness dosing lands at a similar monthly total. These are typical observed ranges rather than a verified survey - vendor pricing in this space moves constantly.

Genuinely uncertain

  • Absolute subcutaneous bioavailability, volume of distribution, clearance and protein binding have never been published for thymalfasin. Every one of those fields is genuinely null rather than omitted for brevity.
  • The English Wikipedia article renders the sequence as SDAAVDTSSEITTKDLKEKKEVEEEAEN. That version is inconsistent with the PubChem molecular formula C129H215N33O55 and MW 3108.3, which resolves exactly to SDAAVDTSSEITTKDLKEKKEVVEEAEN. The VVEE version is used here on that basis, but the discrepancy is worth knowing about if you are ordering synthesis.
  • Whether thymosin alpha-1 crosses the blood-brain barrier is not established. No CSF penetration data were found.
  • The titration ladder given here is clinic convention. No trial has compared a ramped start with going straight to 1.6 mg.
  • There is no validated biomarker of thymosin alpha-1 pharmacodynamic effect. CD4 counts are a proxy, not a target-engagement measure.
  • The sepsis and COVID dosing at 1.6 mg once or twice daily is far above the registered twice-weekly exposure, and the safety of sustained daily dosing at that level outside a short course has not been studied.
  • Whether the negative TESTS result generalises beyond sepsis - to immunosenescence, post-viral states or chronic infection - is genuinely unknown. It is a null result in one hard population, not a null result for the molecule.

Papers