It turns out that vitamin D receptors are present in most if not all cells in the body. Additionally, experiments using cultured cells have demonstrated that vitamin D has potent effects on the growth and differentiation of many types of cells. These findings suggest that vitamin D has physiologic effects much broader that a role in mineral homeostasis and bone function. As one example, many immune cells not only express vitamin D receptors, but are capable of synthesizing active vitamin D, and deficiency in vitamin D has been associated with increased incidence of autoimmune disease and susceptibility to disease.
Progestins , the most important of which is progesterone , are the other type of female sex hormone and are named for their role in maintaining pregnancy (pro- gestation ). Estrogens and progestins are secreted cyclically during menstruation . During the menstrual cycle , the ruptured ovarian follicle (the corpus luteum ) of the ovary produces progesterone, which renders the uterine lining receptive to the implantation of a fertilized ovum . Should this occur, the placenta becomes the main source of progesterone, without which the pregnancy would terminate. As pregnancy progresses, placental production of progesterone increases, and these high doses suppress ovulation , preventing a second conception . The contraceptive quality of progesterone led to the development of structurally modified progestins and estrogens—the oral contraceptives known as birth-control pills, used by women to prevent unwanted pregnancy.