LEVEL 00 — THE ZINC-BOUND HORMONE
Thymulin is the zinc-dependent nonapeptide your thymus assembles to school its T-cells.
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.
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.
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, then read the thymulin dosage in the literature, and check any claim against the thymulin references and citations.