It has been well established that estrogen plays an important role in the progression and treatment of breast cancer. However, the role of triiodothyronine (T₃) remains controversial. We have previously shown its capacity to stimulate the development of positive estrogen receptor breast carcinoma, induce the expression of genes (PR, TGF-alpha) normally stimulated by estradiol (E₂), and suppress genes (TGF-beta) normally inhibited by E₂. Since T₃ regulates growth hormones, metabolism, and differentiation, it is important to verify its action on other genes normally induced by E₂. Therefore, we used DNA microarrays to compare gene expression patterns in MCF-7 breast adenocarcinoma cells treated with E₂ and T₃. Several genes were modulated by both E₂ and T₃ in MCF-7 cells (Student's t-test, P < 0.05). Specifically, we found eight genes that were differentially expressed after treatment with both E₂ and T₃, including amphiregulin, fibulin 1, claudin 6, pericentriolar material 1, premature ovarian failure 1B, factor for adipocyte differentiation-104, sterile alpha motif domain containing 9, and likely ortholog of rat vacuole membrane protein 1 (fold change > 2.0, pFDR < 0.05). We confirmed our microarray results by real-time PCR. Our findings reveal that certain genes in MCF-7 cells can be regulated by both E₂ and T₃.