Triiodothyronine and breast cancer

Core Tip

Core tip: Breast cancer (BC) is a malignant tumor occurring much more frequently in women than in men; worldwide, the incidence of BC has increased markedly in recent years. It is estimated that 1.7 million women will be diagnosed with BC in 2020, marking an increase of 26%, compared to the current incidence: 1.35 million new cases annually. Countless environmental risk factors, pathological conditions, and physiological agents, as well as thyroid hormones (THs), have been involved in the development of BC. Various lines of evidence suggest tumor-promoting effects of THs. The literature contains controversial reports regarding the relationship between thyroid diseases and BC; furthermore, studies reporting both an excess of and a lack of THs may affect breast development and progression to cancer. Epidemiologically, many studies suggest that hyperthyroidism is a factor in the development of BC. Furthermore, experimental studies have shown that high levels of THs reduce the interval of multiplication of BC cell lines. Therefore, the influence of THs on BC is unclear. However, the majority of BC research suggests a relationship, primarily, when the molecular aspects of these hormones are considered in the progression of this type of tumor.