Access of reproductive steroids to target tissues

The access of natural and synthetic steroids to their target cells is influenced by anatomic, physiologic, and biochemical variables that are tissue-specific and in many instances is also influenced by physiologic state. They include differences in vascular supply, the anatomic position of target cells relative to blood vessels, the cellular and extracellular matrix composition of tissues in which target cells are located, and the interaction between steroids and their plasma transport proteins. These variables are often overlooked in the design or evaluation of treatments involving natural or synthetic steroids, which are being used increasingly as contraceptives by young women or for hormonal replacement in the elderly. This article examines these issues with special emphasis on the role that plasma steroid-binding proteins play in regulating the metabolism and actions of steroids that influence reproductive tissues in women.