Role of 17β-Hydroxysteroid Dehydrogenases in Sex Steroid Formation in Peripheral Intracrine Tissues

In postmenopausal women, almost 100% of active sex steroids are synthesized in peripheral target tissues from inactive steroid precursors and, in adult men, approximately 50% of androgens are made locally in target tissues. This new field of endocrinology has been called intracrinology. The last and key step in the formation of all estrogens and androgens is catalyzed by a series of substrate-specific, cell-specific and unidirectional 17β -hydroxysteroid dehydrogenases (17β -HSDs). To date, seven human 17β -HSDs have been cloned, sequenced and characterized. The 17β -HSDs provide each cell with the means of precisely controlling the intracellular concentration of each sex steroid according to local needs.