Immune & antimicrobial

Thymulin

Also called Facteur thymique sérique (FTS), Serum thymic factor, Zinc-thymulin (FTS-Zn), Nonathymulin (synthetic form used in trials)

Thymulin is a small hormone made only by epithelial cells of the thymus, a gland in the chest that helps T cells (a type of white blood cell) develop ⁴⁵. It is a chain of nine amino acids that becomes active only when it binds zinc ²⁵. A few small human studies of the synthetic peptide were run in the 1980s, with mixed results. It is not an approved medicine, and most of what is known comes from lab and animal research ¹¹¹²¹³.

Early human dataSome small human studies exist; evidence is limited.

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All printable flashcards

Illustrative chain. The exact sequence has not been published in a source we could verify.

At a glance

Type: Peptide
Size: 9 amino acids (binds one zinc ion)
Origin: Natural nonapeptide hormone produced only by thymic epithelial cells [4]. Its sequence is pGlu-Ala-Lys-Ser-Gln-Gly-Gly-Ser-Asn, where the first residue is pyroglutamic acid [5]; the sequence field is left empty because a plain one-letter code would misrepresent this residue. The peptide alone is biologically inactive; binding zinc in a 1:1 ratio gives it its activity [5]. The sequence was first reported by Bach and colleagues in 1977 [1]. Formula, molecular weight and CAS number are left blank because they could not be confirmed from an authoritative database.
Evidence: Early human data
US FDA status: No FDA-approved drug containing thymulin was identified. Thymulin acetate appears on FDA's list of bulk drug substances nominated for compounding under section 503A, which is not an approval ¹⁴.

Overview

How it works

Established: thymulin's biological activity depends on zinc being bound to the peptide ²⁵. In lab and animal work it is involved in several steps of T-cell development, both inside and outside the thymus ⁵. Its production and release are stimulated by other hormones such as growth hormone and prolactin ⁵. Proposed (mainly from animal studies): thymulin may also act on the pituitary gland, for example on hormones that control reproduction ³⁴⁵, and thymulin or a related synthetic analogue may calm inflammation and pain signals in nerves and the brain ⁶⁷⁸. How it does this is not settled; the authors of one mouse study wrote that they could only speculate about the mechanism ⁵.

What research shows

Human case series (n=3 children with immune deficiency, 1982): after intravenous synthetic FTS, the authors reported fewer and milder infections, better T-cell test results and the appearance of IgA antibodies within 4 weeks; gains faded when treatment was paused in two children and returned when it restarted. Uncontrolled and very small ¹¹.
Human randomized, double-blind, placebo-controlled trials (two trials in rheumatoid arthritis, 1987; total n not stated in the abstract): 5 mg/day of synthetic nonathymulin worked best, with 56% of treated patients improved on global assessment versus 17% on placebo; side effects were described as minimal and immune lab tests did not clearly change ¹².
Human randomized, double-blind, placebo-controlled pilot trial (n=40, multiple sclerosis, 1989): six months of subcutaneous nonathymulin did not differ from placebo on disability or walking scores, and the authors concluded it was not effective for this group; no significant side effects were recorded ¹³.
Human zinc studies (indirect evidence): a pilot crossover study in very old nursing-home residents reported that two months of zinc-fortified milk raised thymulin activity in the blood; thymulin itself was not given ¹⁰.
Animal study (mice, 2004): thymulin activity in the blood fell sharply in old mice compared with young mice, and the authors linked this to reduced zinc availability inside thymus cells ⁹.
Animal studies (rats and mice, 1996 and later): thymulin reduced the increased pain sensitivity caused by injecting a bacterial toxin (endotoxin) that triggers inflammation, especially at higher doses, while low doses increased pain sensitivity in some experiments ⁶⁷.
Animal studies (mice and rats): thymulin and its analogue lowered inflammatory signaling molecules and eased 'sickness behavior' after body-wide inflammation ⁷⁸.
Animal gene-therapy study (mice, 2007): blocking thymulin in newborn mice lowered reproductive hormones later in life, and a single gene-therapy injection that restored thymulin prevented this drop in thymus-lacking mice ⁵.

Doses studied in people

Study or label	Dose used	Source
Two randomized, double-blind, placebo-controlled trials of synthetic nonathymulin in adults with rheumatoid arthritis (1987); route and duration not given in the abstract	1, 5 or 10 mg per day compared; 5 mg per day was reported as the most effective dose	¹²

The only dose listed comes from a 1980s research trial; thymulin is not an approved medicine, so no official dose exists ¹²¹⁴.

Read this first: these are the amounts given to participants in published studies or written on official prescribing labels, reported for reference. They are not recommendations. Trial doses were chosen for specific patients under medical supervision, and a dose that is safe in one person can be harmful in another.

Vial math

Many research vials hold 5 or 10 mg of powder. This table shows how much ends up in each mark on a U-100 insulin syringe for common amounts of added liquid. It is arithmetic only, not a dose.

Vial	Liquid added	Concentration	Per unit (0.01 mL)
5 mg	1 mL	5 mg/mL	50 mcg	Open in calculator
5 mg	2 mL	2.5 mg/mL	25 mcg	Open in calculator
5 mg	3 mL	1.67 mg/mL	16.7 mcg	Open in calculator
10 mg	1 mL	10 mg/mL	100 mcg	Open in calculator
10 mg	2 mL	5 mg/mL	50 mcg	Open in calculator
10 mg	3 mL	3.33 mg/mL	33.3 mcg	Open in calculator

More sizes: all vial sizes · How this works: Syringes and units · Reconstitution guide.

Safety

In the small 1980s trials, side effects were described as minimal (rheumatoid arthritis) or not significant (multiple sclerosis), but these studies were small and short ¹²¹³.
Long-term safety in humans is unknown; no large or long-term human studies were found ¹¹¹²¹³.
Animal studies found that effects can depend on dose, with low doses increasing pain sensitivity in some experiments and high doses reducing it ⁷.
No independent data were found on the content or purity of thymulin products sold outside regulated channels.
Thymulin is not an approved drug; FDA lists thymulin acetate among bulk substances for compounding that were nominated without adequate support ¹⁴.

Report side effects of any drug or product to the FDA through MedWatch. For a possible poisoning in the US, call Poison Control at 1-800-222-1222.

Legal status

FDA: No FDA-approved drug containing thymulin was identified. Thymulin acetate appears on FDA's list of bulk drug substances nominated for compounding under section 503A, which is not an approval ¹⁴.
Compounding: On FDA's 503A nominations list (updated May 14, 2026), 'Thymulin acetate' is listed under Category 3: Bulk Drug Substances Nominated Without Adequate Support ¹⁴. Thymulin is not named on FDA's page of substances that may present significant safety risks (Category 2), content current as of April 22, 2026 ¹⁵.
Sports (WADA): Thymulin is not named in the WADA 2026 Prohibited List (in force from 1 January 2026). Section S0 bans at all times any pharmacological substance not covered elsewhere on the List that has no current approval by any government health authority for human therapeutic use, so athletes should confirm its status with their anti-doping organization ¹⁶.
Other: No approval by other regulators was identified during this review.

Rules change often. See Legal status of peptides for how these categories work.

History

French researchers Jean-François Bach, Mireille Dardenne and colleagues reported in Nature in 1977 the sequence of a nine-amino-acid serum factor whose presence depends on the thymus; it was found in the blood of several animal species ¹. It was called facteur thymique sérique (FTS), French for serum thymic factor ⁵. Later work showed that zinc is needed for its biological activity, and the zinc-bound form became known as thymulin ²⁵. Small clinical studies of the synthetic peptide followed in France during the 1980s ¹¹¹²¹³. Later research shifted toward its links with the nervous and hormonal systems, its anti-inflammatory effects in animals, and experimental gene therapy in mice ⁴⁵⁷.

Common questions

What is thymulin?

Thymulin is a nine-amino-acid hormone made by epithelial cells of the thymus ⁴⁵. It needs zinc attached to work, and in lab and animal studies it is involved in T-cell development ²⁵.

Who discovered thymulin?

Jean-François Bach, Mireille Dardenne and colleagues in Paris reported its sequence in 1977, calling it serum thymic factor (FTS) ¹⁵. Later work showed that zinc is needed for its activity ²⁵.

Has thymulin been tested in people?

Only in a few small studies from the 1980s. A case series in three children with immune deficiency reported immune improvements ¹¹; two placebo-controlled trials in rheumatoid arthritis reported benefit at 5 mg per day ¹²; and a 40-patient trial in multiple sclerosis found no benefit ¹³. No large modern trials were found.

Why is zinc important for thymulin?

The peptide on its own is inactive; binding zinc gives it its biological activity ²⁵. Blood thymulin activity drops with age in mice ⁹, and in a small human pilot study zinc-fortified milk raised thymulin activity in very old people ¹⁰.

Is thymulin FDA-approved or allowed in compounding?

No FDA-approved thymulin product was identified. FDA lists thymulin acetate as a compounding nominee in Category 3, 'nominated without adequate support' ¹⁴, and it is not named on FDA's Category 2 safety-risk page ¹⁵.

Is thymulin banned in sport?

It is not named on the WADA 2026 Prohibited List, but section S0 bans any substance without government approval for human therapeutic use that is not covered elsewhere on the List, so athletes should check with their anti-doping organization ¹⁶.

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Sources

Last reviewed 2026-10-28 · How we research · Found an error? See our corrections policy.