Role of lipid peroxidation and oxidative stress in the association between thyroid diseases and breast cancer

Evidence is accumulating for a role of the thyroid in the natural history of breast cancer, although no plausible mechanism has been advanced to explain this association. We believe that the thyroid disease–breast cancer relationship provides a unique opportunity to find out the causes of breast cancer. Both diseases are female predominant, with specifically identified biological pathways and genetic and environmental determinants, and seeing them in concert provides an opportunity to identify the most relevant mechanistic pathways. In this communication, we advance a plausible mechanism to explain the thyroid disease–breast cancer relationship. We specifically propose that the reduction in risk associated with hyperthyroidism or increased levels of thyroid hormones, or iodine, may derive from the pro-oxidant properties of these compounds, i.e., from its ability to generate oxidative stress-induced apoptosis. Conversely, the increased risk from hypothyroidism may derive from its ability to inhibit this stress-mediated apoptotic process.