Circulating Levels of Biologically Active and Immunoreactive Intact Parathyroid Hormone in Human Newborns

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Abstract
We evaluated circulating levels of biologically active and immunoreactive intact parathyroid hormone [iPTH-(1–84)] in 47 newborns at birth and eight hypocalcemic preterm infants during the first 10 d of life. Use of two sensitive detection systems, the cytochemical bioassay and an immunoradiometric assay specific for intact parathyroid hormone, enabled us to compare plasma concentrations of PTH-like bioactivity (bioPTH) and iPTH-(1–84). Mean umbilical venous plasma bioPTH was elevated in nondiabetic term and preterm newborns [22.5 ± 3.1 (±SEM) and 15.8 ± 2.5 ng-equiv/L, respectively] compared with normal adult subjects (9.8 ± 2.6 ng-equiv/L; p < 0.01). Umbilical bioPTH was suppressed in five term infants of diabetic mothers (2.6 ± 0.4 ng-equiv/L). In contrast, iPTH-(1–84) was low in term and preterm non-diabetic infants' and term infants of diabetic mothers' umbilical samples (5.4 ± 1.5, 4.3 ± 1.5, and 2.4 ±1.0 ng/L, respectively). Umbilical venous bioPTH was highly correlated with the magnitude of the transplacental calcium gradient (r = 0.90; p < 0.05). In eight preterm infants studied longitudinally, by 24–36 h of life, declining plasma total and ionized calcium (1.71 ± 0.04 and 0.78 ± 0.03 mmol/L, respectively) were accompanied by a significant rise in both bioPTH (41.2 ± 6.3 ng-equiv/L) and iPTH-(1–84) (56.3 ± 11.6 ng/L). These data indicate that the 3rd trimester fetoplacental circulation contains levels of bioPTH several-fold higher than those of immunoreactive intact hormone. We also conclude that even hypocalcemic preterm newborn infants can significantly elevate circulating levels of PTH.

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