Starting in the late twentieth century there was considerable overdiagnosis of thyroid cancer, especially papillary thyroid carcinoma. Intriguingly, thyroid cancer researchers have suggested that knowledge of the history of thyroid cancer would have helped prevent this problem. Their intuition is that history has an epistemic role to play in justifying contemporary medical knowledge. This conflicts with an opposing intuition that history is irrelevant to the justification of contemporary knowledge. This chapter provides a Fleckian analysis of the history of thyroid cancer. It describes the development of a fluctuating network of active and passive elements of knowledge, out of which knowledge of cancer, malignancy, and papillary carcinoma emerge. What these objects are is shown to depend on this history. Whether physicians see the various forms of thyroid cancer as homogeneous kinds or as composite objects is shown to depend on this history. Therefore, the observations made of these objects also depend on this history. Justification of knowledge claims is a function of observations made of these objects, and the observations made of these objects is a function of their history. Therefore, justification of knowledge is a function of this history, which gives history an epistemic role to play in medical practice.