# Thymulin: The Zinc-Dependent Thymic Nonapeptide, Built From the Research > Thymulin is a nine-amino-acid thymic hormone, biologically active only when bound to one zinc ion. A plain-English, cited digest of the research record — mechanism, zinc dependence, and the studies. A nine-amino-acid chain plus one zinc ion equals an active hormone — and a research literature worth reading. Here it is, snapped together piece by piece and cited to source. ## The short version Thymulin (a tiny hormone made by the thymus, the immune-training gland behind the breastbone) is a chain of nine amino-acid building blocks — a nonapeptide. Here is the one fact everything else hangs on: it only switches on when a single zinc atom clicks into place. No zinc, no activity. With zinc bound, thymulin helps T cells (the immune system's trained defender cells) mature, and it quiets inflammation in animal studies. It is studied as a research peptide. It is not FDA-approved, not a supplement, and not the same molecule as thymosin alpha-1. ## What is thymulin? Thymulin is a zinc-dependent nonapeptide hormone — nine amino acids, one bound zinc ion — produced exclusively by thymic epithelial cells (the gland's lining cells that build it) [1]. It was first known as serum thymic factor (FTS, from the French *facteur thymique serique*). In 1982, Dardenne and colleagues showed that the zinc-bound, biologically active form is a distinct entity and named it thymulin [1]. Its identity is unusually clean for a peptide hormone: a defined sequence, a single metal cofactor, and a single gland of origin [2]. ### What is thymulin peptide? The thymulin peptide is the linear nonapeptide pyroGlu-Ala-Lys-Ser-Gln-Gly-Gly-Ser-Asn [2]. On its own — zinc-free — it is biologically inactive; this apo-form is called apothymulin. Bind one zinc ion in a 1:1 ratio and the chain folds into the specific three-dimensional shape that makes it a working hormone [1]. Everything thymulin is known to do, it does in that zinc-bound conformation. ### What does thymulin do in the body? Endogenously, zinc-bound thymulin participates in T-cell differentiation and immune modulation, and it acts as a hypophysiotropic peptide (one that signals to the pituitary gland) within a two-way thymus-neuroendocrine axis [4]. These are described as physiological roles in the research literature — what the molecule does inside its own animal models — not treatment effects in people. ### Is thymulin the same as serum thymic factor (FTS)? Yes. Serum thymic factor (FTS) is the original name; the zinc-bound, biologically active form was named thymulin, or FTS-Zn, in 1982 [1]. When older papers say FTS and newer ones say thymulin, they are describing the same peptide — the difference is whether the zinc is in the picture. ## Thymulin Peptide: A Zinc-Dependent Thymic Nonapeptide The reason to start with thymulin's chemistry is that the chemistry *is* the biology. The molecular formula is C33H54N12O15, the molecular weight is about 858.86 Da, and the CAS number is 63958-90-7 [2]. Treat the peptide with a zinc chelator — a molecule that strips metal ions out, such as Chelex 100 — and its activity in the classic rosette bioassay vanishes; add zinc back and activity returns, with a 1:1 metal-to-peptide ratio giving the cleanest activation [1]. Other metals can substitute weakly, but zinc is the natural and most effective partner [1]. This is why thymulin reads as a *modular* hormone: a fixed nine-block backbone plus one zinc block equals function. The zinc does not just sit there — it creates a conformational shape (a structure that only exists when the metal is in place) that nuclear-magnetic-resonance studies have detected [2]. That single structural fact threads through every page of this digest, and it is the subject of our dedicated page on the [zinc-dependence of thymulin](/zinc-dependence). ## Thymulin and Aging: The Decline of Thymic Function Circulating thymulin is not constant across a lifetime. A cross-sectional human study of healthy subjects found that serum thymulin peaks in childhood and declines progressively from adolescence into older age [11]. Animal work has gone a step further and shown *why* much of that decline happens: in old (22-month) mice, low thymulin reflected reduced peripheral zinc saturation of the peptide rather than a thymus that had simply failed, and one month of oral zinc supplementation produced full recovery of thymic function with organ regrowth plus partial restoration of mitogen responses and natural-killer-cell activity [9]. That link — between a zinc-dependent hormone and the slow weakening of immunity with age (immunosenescence) — is where current research sits. A 2025 review argues that age-related zinc deficiency is a key driver of immunosenescence and chronic low-grade inflammation ('inflammaging') [12]. Thymulin is the hormone at the center of that story, and we walk through it in detail on [thymulin, zinc status, and immune aging](/zinc-status-and-aging). ## Where the research goes from here The thymulin literature is real, reproducible in its core mechanism, and honest about its edges. The zinc-dependence is settled across decades of work [1][2]. The immune and anti-inflammatory effects are documented across animal and in-vitro models [6][8]. The gap — the next chapter, not a verdict on the first — is large modern human efficacy data on the native peptide, which does not yet exist [4]. This site keeps the proven parts and the missing parts equally visible. Start with the [thymulin research findings](/research), then read the [thymulin dosage in the literature](/dosage), and check any claim against the [thymulin references and citations](/references). --- RX Thymulin assembles the zinc-dependent thymulin record block by block — the 1:1 zinc switch logged before any effect, the T-cell and anti-inflammatory findings snapped to their own studies, and the empty human-efficacy slot left open rather than filled; a research build console, never a clinic, a pharmacy, or a prescription.