CIRCULATING LEVELS OF IMMUNOREACTIVE PARATHYROID HORMONE-RELATED PROTEIN AND INTACT PARATHYROID-HORMONE IN HUMAN FETUSES AND NEWBORNS

Undetectable or extremely low levels of circulating immunoreactive par athyroid hormone (PTH) have been reported in human newborns while PTH bioactivity was high, This prompted the hypothesis that the fetal calc emic hormone might be PTH-related protein, The purpose of this study w as to measure circulating immunoreactive PTH-related protein in human fetuses and newborns in order to investigate this hypothesis, Parathyr oid hormone-related protein (PTHrP(1-86)) and intact PTH were measured using two-site immunoradiometric assays in plasma obtained by cordoce ntesis from 23 fetuses (19-33 weeks of gestation), from 17 newborns at term (38-41 weeks), from their mothers and from 22 normal women of re productive age. Plasma PTHrP was detectable in all but one of the fetu ses and newborns and in all the mothers and the controls. The mean lev el was similar among fetuses (19-33 weeks) (0.43 +/- 0.18 pmol/l), new borns (0.48 +/- 0.12), mothers (0.48 +/- 0.14) and normal controls (0. 46 +/- 0.09), Plasma PTH was found to be significantly higher in fetus es at mid-gestation (1.0 +/- 0.99 pmol/l) than in the newborns (0.22 /- 0.21) (p < 0.0025); maternal PTH was significantly higher compared to fetal level at mid-gestation (2.1 +/- 1.0, p < 0.01) as well as at term (2.69 +/- 1.40, p < 0.001), In the control women PTH was 3.07 +/- 1.25 pmol/l, These results showed that plasma amino-terminal PTHrP-(1 -86)) is detectable during the second half of human fetal life and its level remains unchanged during this period of time, in contrast to ch anging levels of fetal plasma PTH. The relatively low PTHrP-(1-86) lev el that we found in the newborns is not responsible for the high PTH-l ike bioactivity found by some investigators in cord blood at term.