In addition, it has been documented that: i) the expression of thyroid hormone (TH) receptors is deregulated in primary and metastatic breast cancer cells; ii) TH may bind and activate the estrogen receptor in breast cancer cells; iii) TH level positively correlates with breast cancer risk; iv) TH affect estrogen production as well as estrogen receptor levels [36–42]. This evidence concerns the gene ESR1 and breast cancer.