This project examines the history of constitutional medicine in twentieth-century Spain, Ecuador, and Chile to analyze changing ideas about race, gender, and sexuality in medical science that produced new models of human subjectivity and political identity. Constitutional medicine measured an individual's health and susceptibility to disease according to his or her position within a grid of different "human types." Practitioners of constitutional medicine established these categories by correlating body traits with mental characteristics using the work of anatomists, physiologists, and psychologists. Constitutional medicine is an understudied topic in twentieth-century histories of science and medicine based on prevailing historical narratives that regard psychoanalysis and cultural anthropology as fundamental revisions of sex and race in the early twentieth century. This dissertation shows that constitutional medicine not only challenges prevailing assumptions about the history of science in the twentieth century but also decenters Western Europe and the United States as dominant sites of scientific research. By analyzing the scientific work of Gregorio Maranan (Spain), Agustfn Cueva Tamariz (Ecuador), and Alejandro Lipschutz (Chile), this dissertation maps the transregional circulation of constitutional medicine while distinguishing its development in particular national contexts. It also considers how constitutional medicine reconfigured ideas about sex and race through endocrinological models of glands and hormones and how knowledge of "hormonal bodies" became part of nation-building efforts in these regions.