A prospective study of calciotropic hormones in pregnancy and post partum: Reciprocal changes in serum intact parathyroid hormone and 1,25-dihydroxyvitamin D

Objective: The purpose of this study was to examine the hormones regulating calcium homeostasis longitudinally in pregnancy and post partum. Study Design: Twenty-three women with normal pregnancies were studied in the second and third trimesters and post partum. At each time blood was analyzed for ionized calcium, vitamin D metabolites, and intact parathyroid hormone, and a 24-hour urine specimen was analyzed for creatinine, calcium, and sodium. Results: Urinary calcium excretion was 250% to 300% higher during pregnancy than post partum (p<0.00001). 1,25-Dihydroxyvitamin D levels were equivalent in the second and third trimesters but were twofold higher than postpartum values (p<0.01). Ionized calcium was similar at all time points. Intact parathyroid hormone in the second and third trimesters was 50% of postpartum levels (p<0.001). Conclusion: Pregnancy is associated with an increase in the levels of 1,25-dihydroxyvitamin D and a concomitant reciprocal fall in intact parathyroid hormone levels. The increase in serum 1,25-dihydroxyvitamin D values appears to be a key factor in providing for the increase in maternal calcium requirements during pregnancy.