Breast cancer and thyroid malignancy are very common tumors in women. These tumors have high curative rates, therefore, a significant probability of both exists during a life-span. Epidemiological studies support the presence of some common risk factors. These data raise questions about biological mechanisms and changes at the molecular level of breast and thyroid cancer cells that may be a target for biological therapy in a common pathway. Here, we present a clinical case of a patient who had metastatic breast cancer and at the time of diagnosis another primary malignant tumor was revealed in the thyroid gland. The issue of treatment choice during two simultaneous active malignancies is always a challenging theme. Surprisingly, both tumors responded to a treatment protocol tailored to a single one. The treatment chosen was not chemotherapy but a special biological treatment combined with hormonal therapy. Further to the case presentation, we discuss molecular changes appearing in both tumors, malignant cell activity mechanisms accordingly, and a basis for biological treatment that may affect two processes simultaneously.