Abstract: Determining thymic function in adults has attracted increasing interest in the context of diseases that affect the immunological system and, in particular, diseases that produce lymphopenias. The invention relates to a set of primers for determining the functionality of the human thymus by calculating the quotient between two types of TREC content, as well as to a method based on multiplex nested PCR which uses the aforementioned primers and can be used to determine the TREC quotient measured in samples of human blood or tissue. The invention is advantageous over methods known in the prior art in terms of reducing experimental errors and the time and costs involved.