Hormone actions are determined by how much hormones available and their receptor expressed in the target cells. Therefore, numerous efforts have been made toward the precise measurement of hormones and their metabolites in urine and blood in the field of endocrinology. Estrogens play pivotal roles in the pathogenesis and development of estrogen dependent breast cancer patients with estrogen receptor expressed in carcinoma cells. However, there had been no differences of estrogens and their metabolites in urine and serum/plasma between estrogen dependent breast cancer patients and age matched controls of women, especially in post-menopausal settings.These results even led to the hypothesis that estrogens were not involved in the pathology of these breast cancer patients.
 However, in late 1980’s, estrogen concentrations in breast cancer tissue or adipose tissues around cancer turned out to be 5 to 50 times higher than adipose tissue of abdomen or buttocks of the same patients or benign breast disorders such as fibroadeoma. Post-menopausal breast cancer patients are actively converting from circulating adrenal androgens, precursors of estrogens, into estrogens through aromatase at the sites of estrogen actions. This mechanism allows breast cancer to
grow and invade independent of circulation estrogen levels in post-menopausal women.
 This mode of hormone action, i.e., the conversion or synthesis of active steroids from circulating precursors at the sites of actions, is termed “Intracrinology” in contrast to classical “Endocrinology”. These intracrine actions of hormones could allow hormone dependent biology regardless of circulating hormones and is involved not only in estrogen dependent breast cancer but also in atherosclerosis, osteoporosis, dementia and others in postmenopausal women.