The results of univariate cox regression showed that the 11 factors included except ER were independent factors for secondary thyroid cancer in breast cancer patients, and the results of multivariate cox regression showed that age, ethnicity, location of primary tumor, whether or not to have radiotherapy, N-stage, and M-stage were independent factors for secondary thyroid cancer in breast cancer patients. This evidence concerns the gene ESR1 and thyroid gland carcinoma.