Introduction: A possible link between thyroid disorders and breast cancer has been a subject of debate, since thyroid extract was first used to treat breast cancer in 1896. Many researchers have suggested an increased risk of breast cancer in women with thyroid disorders and others have indicated a decreased risk. Purpose: To determine if there is a relationship between thyroid disorders and breast cancer in women. Method: An evidence based literature review using Medline, AbstractsFirst, and Cochrane using the terms: hypothyroidism, hyperthyroidism, thyroid dysfunction, breast cancer, and breast carcinoma. Criteria for article selection: publication in a peer reviewed journal, level 3 evidence or higher, and the relevance of the data to this study. Results: Twenty articles met criteria for this study: 11 show an association and 9 show no association. Conclusion: There is no definitive association between thyroid dysfunction and breast cancer. However, recent studies continue to suggest a biologic link between hypothyroidism and breast cancer etiology in post-menopausal women.