Thymulin is a nonapeptide (9-amino acid) hormone produced exclusively by thymic epithelial cells — the only thymic hormone with a fully identified primary structure and a specific receptor on T-lymphocytes. Unlike Thymogen (a stimulatory dipeptide) or Thymosin Alpha-1 (a T-cell maturation hormone), Thymulin functions as a specific thymic signal that promotes the final maturation steps of T-lymphocyte differentiation — the conversion of pre-T cells into fully functional, antigen-responsive mature T cells capable of executing adaptive immune responses.

Thymulin requires zinc as an obligate cofactor for biological activity: the zinc-complexed form (Zn-Thymulin) is the active hormone, while the unbound peptide is inactive. This zinc dependence makes Thymulin one of the most informative biomarkers of thymic function — serum Thymulin levels fall dramatically with thymic involution and aging, can be partially restored by zinc supplementation in zinc-deficient elderly, and serve as a measurable index of residual thymic activity. The decline in Thymulin is measurable from adolescence and parallels the progression of immunosenescence.

The biological role of Thymulin centers on T-cell terminal differentiation and phenotypic identity. Thymulin induces expression of T-cell surface markers (CD3, CD4, CD8, TCR components) that define T-cell identity and function, enhances T-cell responsiveness to mitogenic stimulation, supports the development of regulatory T-cells (Tregs) that prevent autoimmunity, and modulates the balance between Th1 and Th2 T-cell subsets that determines immune response character. These activities are distinct from and complementary to Thymogen's T-cell activation effects and Thymosin Alpha-1's broader thymic hormone roles.

Khavinson group and broader thymus research has documented Thymulin's immunological effects extensively. In aging animal models with depleted Thymulin levels, exogenous Thymulin administration restored T-cell surface marker expression, improved T-cell mitogenic responses, and reduced autoimmune markers — outcomes consistent with restored T-cell terminal differentiation. Clinical studies in immunosuppressed patients (HIV, chemotherapy-induced lymphopenia, elderly with documented immunosenescence) showed improved T-cell counts and function, reduced infection rates, and normalized T-cell subset ratios following Thymulin treatment courses.

A noteworthy application of Thymulin is autoimmune modulation. By supporting Treg development, Thymulin contributes to the immune self-tolerance mechanisms that prevent autoimmune activity — an increasingly relevant consideration in aging populations where declining Thymulin is associated with increased autoimmune dysregulation and inflammaging. This immunomodulatory balance role distinguishes Thymulin from purely stimulatory immunological peptides.

Protocol: 1–2mg/day for 10 consecutive days, subcutaneous injection, 2–4 times per year. Zinc co-supplementation (15–30mg elemental zinc/day) during and between courses is recommended to ensure cofactor availability for Thymulin bioactivity.