Breast cancer and thyroid cancer, two seemingly unrelated diseases may bring you double damage. A study published in the December 2015 issue of Thyroid suggests that 4.3% of thyroid cancer patients go on to develop breast cancer and 2.6% of breast cancer patients go on to develop thyroid cancer. The probability of both occurring together is more than two times higher than the expected incidence of a single cancer in the general population. Why do breast cancer and thyroid cancer “go hand in hand”? The possible reason is the increased disease surveillance in women who already have one type of cancer. In addition, the expression of estrogen and progesterone receptors is significantly higher in women with both cancers than in women with only breast cancer, suggesting that there is a correlation between the molecular mechanisms underlying the development of the two cancers, which may also contribute to the predisposition of the two diseases to occur in the same patient. Which patients are likely to have both diseases together? The average time to develop breast cancer after a diagnosis of thyroid cancer is 5.2 years. Patients with thyroid cancer are significantly more likely to develop breast cancer than the general population, with a standard incidence rate of 2.45. Patients with thyroid cancer are more likely to develop breast cancer later in life, so routine mammography is currently recommended for patients with thyroid cancer. However, because of the lack of clinical evidence and the lack of clinical predictors of thyroid cancer after breast cancer; screening for thyroid cancer is not recommended regardless of the age of the breast cancer patient at disease diagnosis. In people with thyroid cancer diagnosed first and then breast cancer diagnosed later, breast cancer is more likely to be intraductal carcinoma in situ, while its incidence of invasive ductal carcinoma may be relatively low. Regardless of the disease, regular physical examination, close observation, ready consultation and early detection can effectively improve the prognosis and quality of life.