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

Abstract

The purpose of this study was to examine the hormones regulating calcium homeostasis longitudinally in pregnancy and post partum. 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. 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). 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.