We studied secretory parathyroid function in 16 newborn infants, aged 34–137 hr, before, during, and after exchange transfusion with a radioimmunoassay recognizing mainly intact human parathyroid hormone-(1–84) (hPTH-(1–84)). Before the exchange transfusion, serum calcium was normal or decreased, whereas parathyroid hormone (PTH) was within or above the range of normal adults. Serum PTH approached a maximum at decreased serum ionized and total calcium levels which were close to the normal range. During the early period of the exchange transfusion, serum ionized calcium was decreased as a result of the administration of considerable amounts of phosphate and citrate in the donor-blood and PTH was raised to levels seen in hypocalcemic infants before the exchange transfusion. Thereafter, serum ionized calcium remained low, whereas mean PTH concentrations were below basal levels. These inappropriately low PTH levels in relation to lowered serum-ionized calcium concentrations are probably the result of inadequate PTH secretion, which is enhanced by withdrawal of large amounts of presumably biologically active PTH-(1–84) during the exchange transfusion. Serum total calcium, on the other hand, was raised during the exchange transfusion probably as a result of calcium mobilization, and the net calcium balance expressed as the difference of calcium administered and in the withdrawn blood was negative. Even though serum ionized calcium decreased to levels below 2.5 mg/dl during the exchange transfusion, tetany was never observed, iv injections of calcium during the exchange transfusion caused temporary increases in total, but not in ionized calcium levels and shortening of Q-oTc intervals lasting for less than 1 min. We, therefore, advocate that iv calcium injections at regular time intervals during the exchange transfusion with donor blood preserved in citrate and phosphate should no longer be recommended as a prophylactic measure for preventing the decrease in serum-ionized calcium.
Parathyroid secretory function is inversely related to serum calcium concentrations in newborn infants. Serum PTH secretion is maximal at serum-ionized and total calcium concentrations close to the normal range despite further reduction to more markedly hypocalcemic levels. Transitory hypoparathyroidism occurs during exchange transfusion as a result of the removal of considerable amounts of biologically active PTH-(1–84).