Effect of the Menstrual Cycle on Calcium-Regulating Hormones in the Normal Young Woman*

To examine the effect of estrogens on circulating levels of calcium-regulating hormones, total serum calcium, ionized calcium, inorganic phosphate, immunoreactive parathyroid hormone, calcitonin, 1,25-dihydroxyvitamin D, and estradiol (E2) levels were assayed on menstrual cycle days 3 and 13 in 12 healthy young women. Despite a 3-fold increase in serum E2 levels by midcycle, no differences in the other factors were observed. Furthermore, iv infusion of calcium (3 mg/kg over a 10-min period) produced similar changes in the serum levels of immunoreactive parathyroid hormone and calcitonin on days 3 and 13 in 8 women. Our findings suggest that in normal menstruating women, endogenous increases in E2 neither directly nor indirectly stimulate 1,25-dihydroxyvitamin D production, nor do they affect circulating levels of the hormones known to influence calcium homeostasis in man.