Thymulin

What it is

Thymulin is a nonapeptide — nine amino acids long, sequence pyroGlu-Ala-Lys-Ser-Gln-Gly-Gly-Ser-Asn — secreted by thymic epithelial cells. The peptide itself is biologically inactive until it binds a single zinc ion; the zinc-bound form is responsible for the hormone's effects on T-cell maturation. The molecule was originally called "facteur thymique sérique" (FTS, serum thymic factor) by the Bach group before the active zinc complex was identified and the name thymulin came into use.

Thymulin is unusual among thymic peptides because its sequence is short and its mechanism centrally involves a metal cofactor.

History

Jean-François Bach and Mireille Dardenne at Hôpital Necker in Paris isolated FTS from porcine serum in 1977 and reported its primary amino acid sequence soon after. Through the early 1980s the Bach group and collaborators showed that the molecule was biologically inactive in the absence of zinc and that recombinant or chemically synthesized thymulin combined with zinc could restore T-cell function in thymectomized mice and in models of immune deficiency. The 1984 Dardenne paper in European Journal of Immunology directly tested marginally zinc-deficient mice and showed that the active zinc-bound form was sharply reduced in serum, with restoration after zinc supplementation.

Through the 1990s, the Mocchegiani group in Italy extended the work to aged rodents and to small human studies, reporting that age-related thymic involution corresponded to falling levels of zinc-bound thymulin and that zinc supplementation could partly restore active hormone levels in older adults. A small number of clinical trials in primary and secondary immunodeficiency were conducted in the 1990s and 2000s but the molecule never advanced to a registered Phase III program. Research interest has continued in academic settings, particularly in animal models of inflammation, pain, and neuroprotection.

Regulatory status

Thymulin is not approved as a drug in any country. No regulator has reviewed a marketing application. The compound is not on the FDA's 503A bulk substances list and cannot be compounded for patients in the United States. Material sold as "thymulin" online is sold strictly as a research chemical with not-for-human-use labeling.

The World Anti-Doping Agency has not specifically listed thymulin, but as a non-approved peptide hormone it would fall under WADA's S2 (peptide hormones) category in competition.

How researchers describe its action

The published mechanism papers describe thymulin as a zinc-dependent thymic hormone with the following actions in preclinical models:

1. T-cell maturation. Thymulin acts on immature T cells, supporting expression of late-stage differentiation markers, particularly in models of thymic deficiency.
2. Zinc dependence. The peptide must be bound to a single Zn²⁺ ion to display biological activity; the unbound peptide is functionally inert.
3. Anti-inflammatory effects in rodent models. Several published studies report reductions in inflammatory cytokines and improvements in inflammatory pain models, though these have not been replicated in controlled human trials.
4. Age-related decline. Active zinc-bound thymulin levels fall with age in parallel with thymic involution; zinc supplementation can partially restore measurable serum activity.

These mechanisms have been described in cell-culture and rodent work. None has been characterized in a large human trial.

Half-life and dosing intervals

The published terminal half-life of thymulin in circulation is short — approximately 1 hour in rodents and humans. The hormone is normally secreted episodically by thymic epithelial cells, and zinc availability is rate-limiting for the active form.

There is no validated human therapeutic dose. In the small human studies that have been conducted, thymulin has typically been administered at microgram-level doses (often 1 to 50 mcg) subcutaneously two or three times per week, often paired with oral zinc supplementation. These figures come from individual academic protocols rather than from a regulatory dose-finding program.

Reconstitution example

Research-grade thymulin is typically supplied lyophilized in 1 mg vials, sometimes with zinc co-formulated, sometimes as the bare peptide requiring separate zinc supplementation.

A 1 mg vial reconstituted with 2 mL of bacteriostatic water yields 0.5 mg/mL (500 mcg/mL). On a 1 mL U-100 insulin syringe, 2 units (0.02 mL) contains 10 mcg — within the range described in the small academic studies. Vial's calculator handles the unit-to-mcg conversion automatically.

What to know

• Long historical record, limited modern trial data. Thymulin has been studied for nearly five decades but has never been pushed through a registered Phase III program.
• Zinc cofactor matters. The peptide alone is inactive; preparations not supplied with zinc rely on endogenous zinc availability, which can be limiting in deficiency or in older adults.
• No approved use anywhere. No regulator has approved thymulin and US compounding pharmacies cannot legally prepare it.
• Banned in competition under WADA S2 as a non-approved peptide hormone.
• Storage. Lyophilized peptide is stable refrigerated and protected from light. Once reconstituted, refrigerate and use within the stability window specified by the supplier.