In this thesis, I develop a history of thyroid hormone treatment (THT) that centers on the bodies of animals and women between 1890 and 1970. This history contextualizes the current debate between two forms of THT, desiccated and synthetic. Drawing on the discourses present in biomedical journals, I trace how medical practitioners used the animals and women to demonstrate and make sense of THT's effectiveness over time. As such, I study what Catherine Waldby terms the "biomedical imaginary" or the speculative fabric of scientific thought, to demonstrate how an "ordinary" medical technology crosses and reinforces the conceptions of gender and animality. THT emerged in the 1890s as an organotherapy, or a medicine made from animal organs. Like other organotherapies, general physicians used THT for a wide variety of ailments that had not been scientifically proven through the practices of vivisection or animal experimentation. From its emergence, THT served as a site of tension between scientific researchers and general practitioners. This tension only increased when a synthetic form of THT was invented in the 1920s, when scientific researchers embraced synthetic THT and general practitioners continued using desiccated THT. At the center of the controversy were the productive and subversive relationships of animals and women to biomedical meaning-making. Over the twentieth century, methods of defining THT's effectiveness and purity were defined in opposition to these bodies. These chemical measures combined the specialist and physician's measurement of THT's clinical effectiveness, which led to a preference for synthetic THT. / Master of Science