1117 DECREASED SENSITIVITY OF SERUM PARATHYROID HORMONE (PTH) TO HYPERCALCEMIA IN NEWBORNS

Abstract

The supression of neonatal PTH by calcium has not been studied. Theoretically, decreased neonatal parathyroid glandular response to hypocalcemia may be associated with decreased PTH response to hypercalcemia. In 29 “exchange” transfusions (ET) with citrated blood in 25 neonates, we studied the supressibility of PTH from Ca bolus at the 100ml mark of ET. Infants had birth wt (mean±SD) 1732±656g, gestation 32± 4wks, postnatal age 5.2±3.8d and 5 min Apgar 6.8±2.6. There was a significant rise of serum PTH (ΔPTH1) in response to decrease in ionized Ca of 1.4mg/dl during the first 100ml ET. At the 100ml mark of ET after Ca 9mg bolus, ionized Ca rose by 2.6mg/dl. Serum PTH (normal adults <180μl-Eq/ml) dropped from 218±16 SE to 184±16, 167± 13 and 169±11 at 1,2 and 3 min postCa respectively (paired t, p< .01). The nadir of PTH occured at 1 min in 20/29 occasions. Δ2 PTH (postCa change of PTH from 0-min to nadir) was correlated with Δ1-PTH, change of PTH in response to hypocalcemia (Spearman r=.47, p<.02). Lesser response of Δ2-PTH occured with lower pre-exchange serum Ca (r=0.59, p<.01) and Mg (r=0.63, p< .01); no correlation of Δ2PTH was obtained with postnatal age, gestational age and Apgars. Thus, infants with low serum Ca and Mg have lower PTH responsivity to hypercalcemia; infants with low PTH response to hypocalcemia also have low response to hypercalcemia; we speculate that the parathyroids of infants with hypocalcemia or hypomagnesemia are less responsive to changes in calcium.