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Thymulin

A zinc-dependent thymic nonapeptide that regulates T-cell differentiation and, at low doses, has surprisingly strong anti-inflammatory and analgesic effects in animal models.

Also known as FTS, facteur thymique serique, serum thymic factor, zinc-thymulin

Animal data only — Rodent or other animal studies. Dose translation to humans is genuinely uncertain.

Decades of solid basic science on thymulin biology and zinc dependence, plus consistent rodent data on anti-inflammatory and analgesic effects. Human interventional evidence is essentially absent - the age-related decline in thymulin is well documented, but nobody has shown that replacing it does anything measurable in people.

How it works

Thymulin is pGlu-Ala-Lys-Ser-Gln-Gly-Gly-Ser-Asn, and it is one of the cleanest examples of an obligate metallopeptide: without a bound Zn2+ ion the apopeptide is biologically inert. Zinc-bound thymulin promotes differentiation of immature thymocytes, restores T-cell subset balance in zinc-deficient and aged animals, and modulates IL-1, IL-6 and TNF-alpha release from macrophages. A separate and well-replicated line of rodent work shows that low doses of thymulin and its analogue PAT produce marked anti-hyperalgesic and anti-inflammatory effects in the periphery and CNS, apparently independent of the classic thymic-hormone action. Circulating thymulin falls steeply with age in parallel with thymic involution, which is why it appears in longevity stacks; that association is real but the intervention data in humans are not.

Targets: Thymocyte differentiation, Zinc-dependent T-cell signalling, IL-1 / IL-6 / TNF-alpha, Hair follicle keratinocytes (topical zinc-thymulin)

Dosing

ProtocolDoseFrequencyRoute
Common research-use protocolUsually evening, alongside other thymic peptides.250 mcg – 1 mgonce dailysubcutaneous
Short pulsed courseTen consecutive days, then a long gap.500 mcg – 1 mgonce daily for 10 dayssubcutaneous
  • · There is no established human dose. This range is what research suppliers and clinic protocols use, extrapolated from rodent work at roughly 1-10 mcg/kg. Treat the numbers as convention, not evidence.
  • · Mirrors the Khavinson-style pulsing used with other thymic peptides; repeated two to three times a year.

Titration

Adequate zinc status matters more than dose escalation - the peptide is inactive without it. Correcting a zinc deficiency first is more rational than raising the peptide dose.

Cycling

Most protocols run 10-20 days and then stop for several months rather than dosing continuously. Nobody has established what chronic administration does in humans.

Work out your exact syringe units →

Pharmacology

Half-life
Cleared from plasma within minutes; no human pharmacokinetic profile has been properly published.
Onset
Immune-marker changes in animal work take one to three weeks; no reliable human onset data exist.
Routes
subcutaneous, intramuscular, topical
Molecule
Synthetic nonapeptide (zinc-dependent thymic hormone)
Sequence length
9 amino acids
Molecular weight
858.9 Da

Handling

Diluent
Bacteriostatic water
Typical mix
2 or 3 mL
Vial sizes
5, 10 mg
Lyophilised
Refrigerate; freezer for storage beyond a few months.
Reconstituted
Refrigerated, use within about 21-30 days.
Light sensitive
Yes — keep it out of the light

Mixing

A 10 mg vial in 2 mL gives 5 mg/mL, so 500 mcg is 10 units on a U-100 syringe. Swirl gently; do not shake.

Side effects

  • commonInjection-site irritation— Typical of any subcutaneous peptide.
  • commonUnknown long-term profile— Honest entry: there is no human safety database of any size for this compound.
  • uncommonTransient fatigue— Reported anecdotally in the first few days.

Do not use if

  • Active autoimmune disease - the T-cell effects are unpredictable in that setting.
  • Organ transplant recipients on immunosuppression.

Combining it

  • synergyZinc supplementation — Not optional in a real sense - thymulin requires bound zinc to have any activity at all.
  • redundantthymosin-alpha-1 — Overlapping thymic T-cell maturation targets.
  • conflictImmunosuppressants — Opposing pharmacology.

What to monitor

  • · Serum zinc, since the peptide is zinc-dependent.
  • · CBC with differential if using it for immune restoration.
  • · No validated biomarker of thymulin activity is available outside research labs.

Legal status

Not approved anywhere as a drug; sold as a research chemical only.

References

  • Interactions between zinc and thymulin, review (PMC2364880) (review)
  • Safieh-Garabedian and colleagues, thymulin and PAT analogue anti-inflammatory and analgesic rodent studies (preclinical)
  • Bach and Dardenne, discovery and characterisation of facteur thymique serique (preclinical)

Mechanism in depth

Two separate pharmacologies live in this molecule and conflating them is the main error people make. The first is the classical thymic hormone action: zinc-bound thymulin promotes differentiation of immature thymocytes and restores T-cell subset balance, and it is measured by the rosette bioassay that Bach and Dardenne built the field on. Circulating levels fall steeply with age in parallel with thymic involution, and they also fall in zinc deficiency - which is the crucial point, because a low thymulin level in an older person is often a zinc problem rather than a peptide-supply problem. The second pharmacology, from Safieh-Garabedian's group, is dose-paradoxical and has nothing obvious to do with thymic hormone function. At high local doses thymulin is pro-hyperalgesic, working through IL-1 beta, nerve growth factor and prostaglandin E2. At low doses, and given systemically or intracerebroventricularly, it does the opposite - it reverses inflammatory hyperalgesia and cuts the proinflammatory cytokine rise induced by endotoxin. That biphasic dose-response is well replicated in rodents and is the single most interesting thing about the compound. It is also the reason 'more is better' reasoning fails badly here: the anti-inflammatory window is at the bottom of the dose range, and pushing past it flips the sign of the effect.

What usually goes wrong

The dominant failure is spending money on an inert injection because serum zinc was never checked. The second is dose escalation in the wrong direction: the rodent anti-inflammatory effect sits at low doses and the pro-hyperalgesic effect appears at higher local concentrations, so someone who feels nothing at 250 mcg and doubles it may be walking toward the wrong arm of a biphasic curve. The third is the honest one - there is no human safety database of any size, no human pharmacokinetics, and no validated biomarker, so nothing about tolerance or long-term effect can be predicted from the literature. The Core entry's contraindications for autoimmune disease and transplant immunosuppression are reasoned from mechanism, not from observed harm, because nobody has looked.

Bloodwork worth running

MarkerWhenWhy it matters
Serum zinc (and ideally a red cell zinc or zinc:copper ratio)Before you start, and again at four to six weeks if you were low.This is not an optional add-on. Thymulin without bound zinc is an inert peptide. If your zinc is low, correcting it will raise your own thymulin and will do more than injecting the peptide into a zinc-deficient system.Act if: Serum zinc below the laboratory reference range means fix the zinc first and reassess before spending money on the peptide. Note that serum zinc is a poor marker of total body zinc and falls with any acute inflammation, so interpret it alongside CRP.
CBC with differential and, if available, a lymphocyte subset panelBaseline and at the end of a ten to twenty day course.The only objective readout of the classical thymic action. Without it you are running a peptide with no human onset data and no way to tell whether it did anything.Act if: No defined threshold exists. Treat any change smaller than normal biological variation as noise.
hs-CRPBaseline and end of course.The rodent anti-inflammatory data are the most robust part of this compound's file. CRP is the cheapest human analogue of that endpoint, even though nobody has shown thymulin moves it in people.Act if: A rise on treatment in someone with joint or nerve pain is a reason to stop - the high-dose arm of the biphasic response is pro-inflammatory.
CopperAt three months if taking more than 40 mg elemental zinc daily.Only relevant if you are supplementing zinc aggressively to support thymulin activity. Sustained high-dose zinc induces metallothionein and drives copper deficiency, which causes anaemia and neutropenia.Act if: Falling copper with a normal or high zinc means reduce the zinc.

Pharmacokinetics

Metabolism
Peptidase hydrolysis. The more relevant chemistry is not metabolism but zinc occupancy: the apopeptide and the zinc-bound peptide are interconverting species in plasma, and only the zinc-bound form is biologically active.
Elimination
Peptide catabolism and renal handling of fragments.

Receptor targets

  • Zn2+ coordination site (Asp/Ser/Asn residues of the peptide itself) — Zinc binding is the activation switch rather than a target - the metal-free apopeptide has no measurable biological activity in the rosette assay

    Confers the active conformation. This is why serum zinc status determines whether an injected dose does anything at all.

  • Thymocyte differentiation pathway (specific receptor not molecularly identified) — No receptor has been cloned or a Kd published - this is a genuine gap in a fifty-year-old literature

    Drives immature thymocytes toward mature T-cell phenotypes and normalises CD4/CD8 balance in zinc-deficient and aged animals.

  • IL-1 beta, TNF-alpha and IL-6 release from macrophages — Not applicable - modulated output, not a binding site

    Suppressed at low systemic doses, which is the basis for the anti-hyperalgesic effect. Increased locally at high intraplantar doses, which is the basis for the paradoxical pro-hyperalgesic effect.

  • Nerve growth factor and prostaglandin E2 (peripheral) — Not applicable

    Mediators of the high-dose local hyperalgesic response, identified by Safieh-Garabedian in 1997.

What to expect, and when

Minutes: the peptide is gone from plasma. Whatever it does, it does by triggering, not by occupancy. Days 1-3: nothing established; anecdotal reports of transient fatigue. Weeks 1-3: this is where the rodent immune-marker changes appear, and it is the basis for the ten to twenty day course structure. Months: no human data at all. The compound has never been followed longitudinally in people.

Stacking and comparisons

Zinc is not a stack, it is a prerequisite. Running thymulin in a zinc-deficient person is pharmacologically pointless. Beyond that, pairing it with thymosin alpha-1, thymalin, thymopentin or Vilon is redundant - they all claim thymic T-cell maturation and only one of them has phase 3 data. The interesting and untested pairing is with a genuine anti-inflammatory peptide such as KPV, because thymulin's best-replicated preclinical effect is anti-hyperalgesic rather than immunostimulant, and the two act by completely different routes. There is no human data on any combination involving this compound.

Thymulin is the most mechanistically elegant compound in this class and the least usable. Against thymosin alpha-1 it has a fraction of the evidence and no human dosing. Against thymopentin it shares the same problem - a plasma half-life measured in minutes and a triggering rather than occupancy mode of action - but thymopentin at least has randomised human trials behind it. The one place thymulin genuinely stands apart is the analgesic and anti-hyperalgesic rodent work, which no other peptide in this class replicates, and which nobody has ever taken into a human trial. If your actual goal is thymic support in an older adult, correcting zinc status has better evidence behind it than the peptide does.

Rough cost

$40–$90/month. A 10 mg vial runs a research course at 250-1000 mcg daily for ten to twenty days comfortably, and pulsed dosing means most users buy two to three vials a year rather than monthly. The monthly figure here is the cost during an active course, not an annualised average.

Genuinely uncertain

  • No human pharmacokinetic study of exogenous thymulin exists by any route. Tmax, bioavailability, volume of distribution, clearance and protein binding are all genuinely unknown, not merely unreported here.
  • The receptor through which thymulin drives thymocyte differentiation has never been molecularly identified. There is no cloned receptor and no published binding affinity, which is remarkable for a molecule described in 1977.
  • The 250-1000 mcg daily dose range in circulation is extrapolated from rodent work at roughly 1-10 mcg/kg. Whether the low-dose anti-inflammatory window in rodents maps onto that range in humans is unknown, and given the biphasic dose-response this is not a trivial question.
  • Whether thymulin crosses the blood-brain barrier after peripheral administration is unresolved. The central effects in the Safieh-Garabedian work were shown with intracerebroventricular injection, which sidesteps the question.
  • The molecular weight of 858.9 Da quoted here is for the zinc-free nonapeptide. Zinc-thymulin is roughly 924 Da. Suppliers rarely state which form they are selling or whether zinc is present at all.
  • No human trial of any kind has been run. The trials array is empty because it is genuinely empty, not because nothing was found.

Papers