Peptides & growth hormone

THYMULIN

Updated · 24 sources · Editorial policy

Thymulin is a small hormone made by the thymus, the gland behind the breastbone where T cells (a type of white blood cell) mature. It is a nine-amino-acid peptide that only becomes biologically active when it binds one zinc ion. It was first isolated in 1977 under the French name facteur thymique sérique (FTS, “serum thymic factor”), and the synthetic peptide was later studied as a drug under the name nonathymulin.

Today synthetic thymulin circulates as a research peptide, and a homeopathic product carrying its name is labeled for “immune support.” The single most important fact: it is not an approved drug anywhere we could find, and the only controlled human trials are a few small studies from the 1980s, in rheumatoid arthritis and multiple sclerosis. No human study has tested it for muscle, recovery, cognition or performance.

This page summarizes what the original biochemistry, the human studies and regulators report. It is for education only and is not medical advice.

Property Detail
Class Natural thymic hormone; zinc-binding nonapeptide (metallopeptide)
Other names Facteur thymique sérique (FTS), serum thymic factor, FTS-Zn; synthetic form studied as nonathymulin
Chemistry C₃₃H₅₄N₁₂O₁₅, 858.9 g/mol for the zinc-free peptide (PubChem); active form binds zinc 1:1
Source in the body Made only by thymic epithelial cells
Studied doses (humans) Nonathymulin 1, 5 and 10 mg/day in rheumatoid arthritis trials (1987)
Half-life No published human pharmacokinetic data found
Approval Not an FDA-approved drug; not on the US 503A compounding bulks list; one “unapproved homeopathic” 9C product is listed in DailyMed
WADA Not named on the 2026 list; non-approved substances fall under S0, prohibited at all times

What it is

Discovery and structure

French researchers led by Jean-François Bach and Mireille Dardenne isolated a “circulating thymic factor” from 1,000 liters of pig serum and showed it was a nonapeptide, a chain of nine amino acids (Dardenne et al., J Biol Chem 1977). A companion paper reported its sequence, which begins with a pyroglutamic acid (a ring-closed form of glutamic acid). A synthetic copy had the same biological activity as the natural peptide (Pléau et al., J Biol Chem 1977).

In 1982 the same group found that FTS lost its activity when metals were stripped from it and regained it when zinc was added back. The best activation came at one zinc ion per peptide. They proposed the name “thymulin” for the active, zinc-bound form (Dardenne et al., PNAS 1982). The zinc-free peptide is biologically inactive.

In serum, part of the circulating thymulin is carried on a larger protein, roughly the size of albumin, rather than travelling free (Dardenne et al., Eur J Immunol 1980).

What it does in the body

Thymulin is produced only by the epithelial cells of the thymus. It is involved in several steps of T-cell differentiation, both inside and outside the thymus (Reggiani et al., Ann N Y Acad Sci 2009). Its developers described it as a “zinc-dependent hormone” that enhances several T-cell functions in normal or partly thymus-deficient recipients (Bach & Dardenne 1989).

Thymulin also sits inside a two-way loop with the hormone system (Reggiani et al. 2009):

  • Hormones that raise or lower thymulin. Growth hormone (GH), prolactin and thyroid hormone stimulate thymulin release in animal and cell studies. In children with congenital GH deficiency, low thymulin levels rose toward normal on GH therapy, and people with acromegaly (GH excess) had higher levels than age-matched controls.
  • Thyroid status in people. Hyperthyroid patients had higher thymulin levels than healthy subjects, hypothyroid patients had lower levels, and both changes reversed with treatment (Fabris et al., JCEM 1986).
  • Effects on the pituitary, in rats. In rat pituitary cells and tissue, thymulin changed the release of LH, prolactin, ACTH and other hormones. These are laboratory findings and have not been shown in people.

Thymulin is not the same as other thymic peptides

Several unrelated peptides from the thymus are sold under similar names. They differ in size, structure and evidence:

Peptide What it is Library page
Thymulin Nine-amino-acid peptide that needs bound zinc for activity This page
Thymosin alpha-1 (thymalfasin) A 28-amino-acid peptide, sold as Zadaxin in some countries (Stincardini et al. 2018) Thymosin Alpha-1
Thymosin beta-4 / TB-500 Tβ4 is a 43-amino-acid actin-binding peptide (Low et al., PNAS 1981); TB-500 is a synthetic 7-amino-acid fragment of it TB-500 (Thymosin Beta-4)
Thymalin A mixture of natural peptides extracted from calf thymus, developed in Russia (Morozov & Khavinson 1997) Thymalin
Thymagen (Thymogen) A synthetic dipeptide (glutamic acid–tryptophan) first isolated from thymalin (Morozov & Khavinson 1997) Thymagen

Results for one of these cannot be assumed to apply to another.

How it behaves in the body

Levels across life

Thymulin levels change with age. In 93 healthy people from birth to age 80, thymulin was detectable at birth, peaked in children aged 5–10, then fell from adolescence. It reached its lowest level around age 36 and stayed at that level up to age 80, the oldest person tested (Consolini et al., Clin Exp Immunol 2000). This roughly tracks the shrinking of the thymus with age.

Zinc status controls active thymulin

Because the hormone needs zinc, measured thymulin activity falls when the body is short of zinc:

  • Mild zinc deficiency. In volunteers made mildly zinc deficient by diet, and in mildly deficient adults with and without sickle cell anemia, serum thymulin activity fell and was corrected by zinc supplementation (Prasad et al., J Clin Invest 1988).
  • Early marker. In the experimental human model, thymulin activity dropped within 8–12 weeks of a zinc-deficient diet, before lymphocyte zinc (20 weeks) or plasma zinc (24 weeks) fell (Prasad, J Immunol Res 2020).
  • Older adults. In 13 zinc-deficient older adults, zinc supplementation significantly increased serum thymulin activity (Prasad et al., Nutrition 1993).
  • Crohn’s disease. In a 3-month randomized trial in 27 patients in remission, 200 mg/day zinc sulfate restored low plasma zinc and thymulin, while 60 mg/day and placebo did not (Brignola et al., Aliment Pharmacol Ther 1993).

These studies raised the body’s own active thymulin through zinc. They did not give thymulin itself.

Pharmacokinetics

We found no published study of how injected thymulin is absorbed, distributed or cleared in humans. Its half-life in people is unknown.

Regulatory status

  • Not an approved drug. We found no FDA-approved thymulin product in Drugs@FDA or DailyMed. The only DailyMed listing is a sublingual homeopathic pellet labeled “Thymulinum 9C,” marketed for “immune support” and classed as an unapproved homeopathic product (DailyMed). Homeopathic dilutions were not the preparation tested in the trials below.
  • Compounding. Thymulin is not on the list of bulk substances US pharmacies may use under section 503A (21 CFR 216.23). It is also not among the peptides on FDA’s list of substances that may present significant safety risks in compounding, which does include thymosin alpha-1 and TB-500 (FDA, updated April 22, 2026). Absence from that list is not evidence of safety.
  • Sport. The 2026 WADA Prohibited List does not name thymulin. Its S0 class prohibits at all times any pharmacological substance not covered elsewhere on the list “with no current approval by any governmental regulatory health authority for human therapeutic use” (WADA 2026 Prohibited List). Because we found no regulatory approval for thymulin, it would fall under this class.

What the research shows

Rheumatoid arthritis

The most positive human data come from two randomized, double-blind, placebo-controlled trials in rheumatoid arthritis that compared nonathymulin at 1, 5 and 10 mg/day (Amor et al., Ann Rheum Dis 1987):

Measure Nonathymulin 5 mg/day Placebo
Improved on global assessment (all enrolled patients) 56% 17% (p under 0.02)
Objective measures Four improved significantly —
Adverse effects Minimal —

The 5 mg dose was the most effective of the three. The treatment produced no clear changes in measured immune markers. These were small trials run by the peptide’s developers, and we found no later trial that confirmed the result or led to approval.

Multiple sclerosis

A double-blind pilot trial randomized 40 patients with active, moderately disabling multiple sclerosis to subcutaneous nonathymulin or placebo for 6 months, then followed them for another 6 months. There was no difference between groups in disability scores, walking or functional scales, and no significant side effects. The authors concluded that nonathymulin “is not effective” in this setting (Roullet et al., Acta Neurol Scand 1989).

Pain and inflammation: animal studies

Most recent work on thymulin is in rodents, and the results depend on dose (Dardenne et al., Ann N Y Acad Sci 2006):

  • Low doses increased pain, in rats. As little as 0.5 ng injected into a rat’s paw caused hyperalgesia (heightened pain sensitivity) and raised the inflammatory signals IL-1β and nerve growth factor (Safieh-Garabedian et al., Brain Behav Immun 1997).
  • Higher doses reduced inflammatory pain, in rats. Systemic doses of 1–25 µg of thymulin, or of a synthetic analogue called PAT, reduced pain and cytokine levels after endotoxin injection. Injected into the brain, thymulin reduced endotoxin-induced hyperalgesia.

None of this has been tested in people.

Gene therapy and hormones: animal studies

Researchers built a synthetic gene for a thymulin analogue (metFTS) and delivered it with a viral vector. In thymus-deficient (nude) mice, a single injection at birth restored circulating thymulin and prevented the low LH and FSH levels and ovarian damage these mice normally develop (Reggiani et al. 2009). This is experimental gene therapy in mice, not injected peptide in people.

Muscle, recovery, hair and cognition

We found no human study of thymulin for muscle mass, strength, injury recovery, hair growth, cognition or athletic performance, and no registered trial of thymulin as a treatment on ClinicalTrials.gov.

Side effects and risks

What has been reported in humans

The 1980s trials reported minimal adverse effects in rheumatoid arthritis and no significant side effects over 6 months in multiple sclerosis (Amor et al. 1987; Roullet et al. 1989). These were small studies with limited follow-up, and the published abstracts give no event rates. There is no modern safety database.

Dose-dependent effects on pain and inflammation

In rats, low doses of thymulin increased pain sensitivity and inflammatory mediators, while higher doses had the opposite effect (Safieh-Garabedian et al. 1997; Dardenne et al. 2006). Whether this applies to people is unknown, but it shows the peptide is not simply “anti-inflammatory.”

Product identity and quality

Thymulin sold as a research chemical comes from outside the regulated drug supply. Because the active hormone requires bound zinc, a product’s content of the zinc-free peptide, its zinc content and its purity all matter, and none is controlled for research-grade products. There are no data in pregnancy, breastfeeding, children, people with autoimmune disease or cancer, or people taking other medicines.

Key takeaways

  • A real thymic hormone that needs zinc. Thymulin is a nine-amino-acid peptide made only by the thymus, and it is active only when bound to one zinc ion.
  • Levels fall with age and with zinc deficiency. Thymulin peaks in childhood and falls through early adulthood. Zinc supplementation restored it in zinc-deficient adults, without giving the peptide.
  • Thin and old human evidence. One pair of small 1980s trials suggested benefit in rheumatoid arthritis at 5 mg/day; a 40-patient multiple sclerosis trial found no effect. No later trials followed.
  • No data for performance uses. No human study has tested thymulin for muscle, recovery, hair growth or cognition, and its human half-life is unknown.
  • A different molecule from other thymic peptides. Thymosin alpha-1, thymosin beta-4/TB-500, thymalin and thymagen are separate substances with separate evidence.
  • Unapproved, and covered by WADA’s catch-all ban. It is not an FDA-approved drug or an allowed 503A compounding substance. WADA does not name it, but as a substance with no regulatory approval it would fall under S0, prohibited at all times.

Sources

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  22. 21 CFR 216.23. Bulk drug substances that can be used to compound drug products in accordance with section 503A of the Federal Food, Drug, and Cosmetic Act. eCFR.
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