To investigate the effect of triiodothyronine (T3) on cell proliferation, we utilized the breast cancer cell line T47D, which is estrogen receptor-positive (ER+), progesterone receptor-positive (PR+), HER2-negative luminal B subtype, and the non-tumorigenic, immortalized mammary epithelial cell line MCF10A. This evidence concerns the gene ESR1 and breast cancer.