LAUSR

Simple Summary Breast and thyroid cancer are commonly diagnosed in women. Physicians have recognized and evaluated the phenomenon of two cancers occurring synchronously or asynchronously. The potential mechanisms are complex and various. Hormone, autoimmune attack and genetic predisposition are significant and shared factors influencing two cancers. Medical interventions for the first cancer and other life-related factors are reviewed and discussed, as well. This article aims to expound the relationship between breast and thyroid cancer, and suggests that physicians should monitor for the second cancer appropriately whenever one cancer occurs. Abstract Breast and thyroid glands are two common sites of female malignancies. Since the late 19th century, physicians have found that the cancers in either thyroid or mammary gland might increase the risk of second primary cancers in the other site. From then on, many observational clinical studies have confirmed the hypothesis and more than one theory has been developed to explain the phenomenon. Since the two glands both have secretory functions and are regulated by the hypothalamic–pituitary axis, they may share some common oncogenic molecular pathways. However, other risks factors, including medical interventions and hormones, are also observed to play a role. This article aims to provide a comprehensive review of the associations between the two cancers. The putative mechanisms, such as hormone alteration, autoimmune attack, genetic predisposition and other life-related factors are reviewed and discussed. Medical interventions, such as chemotherapy and radiotherapy, can also increase the risk of second primary cancers. This review will provide novel insights into the research designs, clinical managements and treatments of thyroid and breast cancer patients.