Thymulin is a nine amino acid hormone produced by the thymus gland that requires zinc to become active, which is a genuinely interesting detail since its activity falls when zinc is deficient. It was characterised decades ago and studied for its role in the maturation of T cells, the immune cells that coordinate much of the adaptive immune response. Research interest largely faded and no modern development programme exists. There is no completed randomised controlled trial, and the human literature consists of older immunology studies rather than clinical trials of it as a treatment. It is worth distinguishing from thymosin alpha-1, a different thymic peptide with a substantially larger evidence base and approval in many countries.