Thymulin

Definition

Thymulin is a nonapeptide produced by the thymus, and it is unusual among the peptides in this index in that its activity depends on a metal: the molecule is biologically active only when bound to zinc. That dependence is the reason its name appears throughout a literature that is not about it.

It is sold as research-use-only material on claims about immune function and thymic aging. The federal record and the published record are both thinner than that marketing implies, and the published record is thin in a way a count conceals rather than reveals.

Evidence

Across two name variants PubMed returns 498 indexed records, of which 226 carry the Humans subject heading, 8 the clinical-trial publication type, and 5 the randomized-controlled-trial type ^[1]. The publication-type filter was proved live in the same run against a control term returning 8,906 records under the clinical-trial type and 6,115 under the randomized type, the invalid tag returned zero for that control term, and a deliberately invalid record identifier errored in the same run ^[1].

Eight is small enough to itemize, so all eight were opened, and the reading reverses what the number suggests. Seven of the eight do not administer thymulin at all. They measure the concentration of the body's own thymulin as a readout of zinc status inside trials of zinc supplementation: in patients with Crohn's disease, where zinc restored plasma zinc and thymulin concentrations ^[2]; in young rural Gambian children ^[3]; in children with Down's syndrome; and as an adjunct to zidovudine therapy ^[1]. Others track thymulin against melatonin, against a zinc-fortified milk, and against prenatal and childhood exposures ^[1].

The eighth record does administer something, and it is not thymulin. A 1989 double-blind pilot study tested nonathymulin, a synthetic analogue, in multiple sclerosis ^[4]. That is the entire administered human record indexed under these names: one pilot study, of a different molecule, in a different field, thirty-seven years ago.

The correct statement is therefore not that thymulin has eight human trials. It is that thymulin has none, that a synthetic analogue has one pilot, and that the count exists because the peptide is a convenient biomarker for something else. The human trial record prints the count with that reading attached, which is the only form in which the count is not misleading.

Lawful access

There is no lawful United States supply route. Drugs@FDA holds no approved product with thymulin as an active ingredient; the same query form in the same session returned one application for bremelanotide and fourteen for liraglutide, and returned nothing for a deliberately invented ingredient name ^[5].

The section 503A bulks list FDA published May 14, 2026 lists Thymulin acetate in Category 3 — the file for substances nominated without adequate support to evaluate — and in neither of the other two categories ^[6]. Category 3 confers nothing. It is not the safety finding that Category 2 records, and it is not the permission during review that Category 1 grants; it records that the nomination itself was inadequate to evaluate.

The substance was not among the seven peptides the advisory committee reviewed in July 2026, so no vote, recommendation or pending rulemaking attaches to it ^[6]. What is sold is research-use-only material, outside pharmacy licensure and outside every quality obligation that attaches to a drug.

Related molecules

Thymulin joins thymosin alpha-1 among the immune peptides, and the two are worth reading together: one has a substantial non-United States clinical literature and a withdrawn nomination, the other has an inadequate nomination and a trial count that dissolves on reading. Neither has a lawful route. The nomination tracker files them under different headings for different reasons.

Elsewhere in this index