A controversy exists in regard to thyroid function and breast cancer. Hypothyroidism has been suggested as being either protective from breast cancer or predisposing to the disease. It has been hypothesized that a deficiency in circulating thyroid hormones may hypersensitize the mammary glandular epithelium toward prolactin and estrogens, thus aiding the development of breast neoplasia. On the other hand, thyroid hormone replacement therapy has been connected with an increased risk of breast cancer, but this has been contested. At this time the American Thyroid Association recommends that, if indicated, hypothyroid patients should take their thyroid hormone medication. Hyperthyroidism has been associated with a decreased risk of breast cancer. Also, in hyperthyroid patients with inoperable breast cancer, the malignant growth is thought to be slowed. However, this, too has been disputed. Moreover, hyperthyroidism has been connected with the development of breast cancer in premenopausal women. At present no role of thyroid hormone in the pathobiology of breast cancer can be defined. It seems that the "thyroid-breast cancer controversy" can only be resolved by a prospective study preferably on postmenopausal women correlating thyroid (T3, T4, PBI), pituitary (TSH, TRH, Prolactin), and adrenocortical (androgens) function tests with the clinical examination of thyroid, breast, and genital apparatus and determination of the estrogen status (vaginal smear, plasma estrogens) as well.
