Hormones and hormone receptors in the etiology of breast cancer.

Many epidemiologic features of breast cancer suggest that endogenous hormones are of importance in the genesis of this disease. Endocrinologic studies based on total levels of various hormones in serum and urine have provided etiologic clues, but have failed to yield a clear understanding of the hormonal aberrations that promote the development of breast cancer. One reason for this is probably that neither urinary hormones, nor total levels of serum hormones accurately reflect biologically active levels to which the cells of origin are exposed. Levels of hormones unbound to serum sex binding globulin may be of more etiologic relevance, and future studies utilizing measurements of such hormone fractions may provide new information of etiologic importance. Some risk factors for breast cancer, especially age at birth of first child, may influence risk by altering the susceptibility of mammary ductal epithelial cells to estrogens or other endogenous hormones by altering the level of hormone receptor proteins in these cells. Additional studies to identify the determinants of hormone receptors in the breast are needed to test this hypothesis.