Background: MicroRNAs (miRNAs) are short non-coding RNA molecules that play a critical role in mRNA cleavage and translational repression, and are known to be altered in many diseases including breast cancer. MicroRNA-10a (miR-10a) has been shown to be deregulated in various cancer types. The aim of this study was to investigate miR-10a expression in breast cancer and to further delineate the role of retinoids and thyroxine in regulation of miR-10a.Methods: Following informed patient consent and ethical approval, tissue samples were obtained during surgery. miR-10a was quantified in malignant (n = 103), normal (n = 30) and fibroadenoma (n = 35) tissues by RQ-PCR. Gene expression of Retinoic Acid Receptor beta (RAR beta) and Thyroid Hormone receptor alpha (THR alpha) was also quantified in the same patient samples (n = 168). The in vitro effects of all-trans Retinoic acid (ATRA) and L-Thyroxine (T-4) both individually and in combination, on miR-10a expression was investigated in breast cancer cell lines, T47D and SK-BR-3.Results: The level of miR-10a expression was significantly decreased in tissues harvested from breast cancer patients (Mean (SEM) 2.1(0.07)) Log(10) Relative Quantity (RQ)) compared to both normal (3.0(0.16) Log(10) RQ, p < 0.001) and benign tissues (2.6(0.17) Log(10) RQ, p < 0.05). The levels of both RAR beta and THR alpha gene expression were also found to be decreased in breast cancer patients compared to controls (p < 0.001). A significant positive correlation was determined between miR-10a and RAR beta (r = 0.31, p < 0.001) and also with THRa (r = 0.32, p < 0.001). In vitro stimulation assays revealed miR-10a expression was increased in both T47D and SK-BR-3 cells following addition of ATRA (2 fold (0.7)). While T-4 alone did not stimulate miR-10a expression, the combination of T-4 and ATRA was found to have a positive synergistic effect.Conclusion: The data presented supports a potential tumour suppressor role for miR-10a in breast cancer, and highlights retinoic acid as a positive regulator of the microRNA.