Thymulin (also known as thymic factor or its old name facteur thymique serique) is a nonapeptide produced by two distinct epithelial populations in the thymus first described by Bach in 1977.

The hormone is believed to be involved in T-cell differentiation and enhancement of T and NK cell actions.Besides this rather paracrine or auto-organic effects on the thymus dependent immune system, thymulin seems to have neuroendocrine effects as well.

There exist bidirectional interactions between thymic epithelium and the hypothalamus-pituitary axis.