The genesis and development of both TC and BC are quite complicated. Some other factors may play a role in the co-existence of TC and BC. Radioactive iodine therapy is a routine therapy for differentiated TC in Western countries, and it is increasingly being used in China. As mentioned above, mammary gland cells also express NIS, thus, the breast tissue may also absorb radioactive iodine, and the absorption of a high dose of radioactive substances may induce carcinogenesis [
52]. The results of a retrospective single-center study in Portugal suggest that the risk of developing second primary cancer is increased after radioactive iodine therapy, particularly for activities > 200 mCi [
53]. However, because of the lack of statistics, this hypothesis requires further support. Additionally, in a 2015 long-term follow-up study, radioactive iodine (RAI) therapy did not significantly increase the occurrence and recurrence of subsequent BC [
54]. In 2015, Zhang YJ et al. from Peking Union Medical College conducted a meta-analysis that included 6 cohort studies, involving 17,914 patients. The results suggested that the risk of secondary primary BC in TC survivors treated with RAI did not increase compared with TC survivors not treated with RAI [
55]. Folate metabolism, which plays an essential role in DNA synthesis, is another important aspect of carcinogenesis, and it was recently shown to be involved in the increased incidence of TC and BC. As shown in a study by Zara-Lopes T, an alteration in the methylenetetrahydrofolate reductase (MTHFR) gene that participates in folate metabolism, C677T, is significantly associated with the increased incidence of thyroid and breast cancer. These factors may be used as potential predictive and prognostic markers for both types of cancer [
56]. Obesity and a higher cancer risk have a well-established, strong association; weight, weight gain, and obesity are responsible for approximately 20% of all malignant neoplasms. TC and BC are no exception [
57]. A 2016 study in Korea reported a positive association between a high body mass index (BMI) and TC incidence, and prevention efforts, such as weight gain control, may reduce the burden of TC [
58]. Yunji Hwang et al. conducted a large-scale case-control study and suggested that middle-aged adults who gain weight have a higher risk of developing PTC. Although this study has some limitations, such as recall and detection bias, the results still suggest that weight gain control can decrease the incidence of TC [
59]. A study in France also supported this hypothesis. Clavel-Chapelon F et al. identified a significant dose-dependent association between the risk of developing TC and BMI, particularly in women who gained weight from menarche to adulthood [
60]. Meanwhile, the role of obesity or a high BMI in the development of breast cancer has been well-known for years [
61,
62]. Moreover, weight loss interventions are recommended for patients with BC [
63].