Urinary phosphate and cyclic AMP excretion following citrate-induced hypocalcemic stimulation of the neonatal parathyroid glands

Intact parathyroid hormone levels during pregnancy, in healthy term neonates and in hypocalcemic preterm infants

We measured parathyroid hormone levels in pregnant and nonpregnant women and at 1, 2 and 5 days of life in healthy term neonates and in hypocalcemic preterm infants using a new immunoradiometric assay which measures only biologically active intact parathyroid hormone and by a mid-molecule parathyroid hormone radioimmunoassay. During pregnancy intact and mid-molecule parathyroid hormone levels did not show any modification and were not different from parathyroid hormone levels of nonpregnant age-matched controls. Serum calcium and phosphorus levels did not vary during each trimester of pregnancy. In cord serum intact and mid-molecule parathyroid hormone values were low in both term and preterm infants. In term neonates intact and mid-molecule parathyroid hormone levels peaked on day 1; in preterm infants intact parathyroid hormone levels peaked on day 1 while mid-molecule parathyroid hormone values peaked on day 2. Intact parathyroid hormone levels showed a more marked increase in preterm (19-fold) than in term neonates (7.5-fold) on day 1. Our data do not confirm the previously reported "physiologic" hyperparathyroidism in pregnancy. Moreover we found a normal parathyroid gland responsiveness to decreasing serum calcium levels in the first days of life in term and preterm infants. Our results suggest that measurement of intact parathyroid hormone 1-84 by immunoradiometric assay in the first days of life is a more sensitive index of parathyroid gland secretory function than the measurement of middle or carboxyl-terminal parathyroid hormone fragments allowing the detection of the dynamic changes of parathyroid hormone which occur in hypocalcemic preterm infants.

Blood exchange transfusions in newborns, the effect on serum ionized calcium

Serum levels of ionized calcium (CaF) were determined in 35 newborn infants subjected to blood exchange transfusion with citrated blood. Eleven of these were supplemented with calcium injections during and after the transfusion. No symptoms or signs accompanied the extremely low calcium levels noted during the exchange transfusion. Intravenous calcium administration often rendered transitory high serum concentrations of ionized calcium. The lowest and highest calcium levels noted during transfusion were 0.28 and 5.16 mmol/l respectively. Calcium levels normalized within 1 day after exchange transfusion without calcium therapy. Repeated exchange transfusions brought about progressively lower calcium values.

Effect of phosphorus intake in total parenteral nutrition infusates in premature neonates

Phosphorus intake was evaluated in 27 appropriate weight for gestational age, critically ill neonates who required total parenteral nutrition for 2 weeks. All received approximately 30 mg/kg/d elemental calcium. The low P intake group (1.01 mmol/kg/d, 30 mg/kg/d) showed signs of phosphate depletion: hypercalciuria, hypophosphatemia, and absence of phosphaturia. The high P intake group (1.67 mmol/kg/d, 50 mg/kg/d) did not have signs of P depletion; however, they had high urinary cyclic adenosine monophosphate excretion and marked phosphaturia, suggesting secondary hyperparathyroidism. The moderate P intake group (1.34 mmol/kg/d, 40 mg/kg/d) had evidence of neither phosphate depletion nor secondary hyperparathyroidism. This phosphorus dose appears to be appropriate for the very sick, poorly growing infant receiving total parenteral nutrition.

Evidence for transient peripheral resistance to parathyroid hormone in premature infants

Serum immunoreactive parathyroid hormone (iPTH), ionized calcium, the urinary cyclic AMP/creatinine ratio (cAMP/Cr) and some indices of bone turnover (alkaline phosphatase (AP), serum osteocalcin, and the urinary total hydroxyproline/creatinine ratio (OH-P/Cr)) were measured in 26 preterm infants during the first 4 weeks of life. Despite of stimulated parathyroid gland activity cAMP/Cr, AP, osteocalcin and OH-P/Cr were low during the first week. Thereafter iPTH decreased, whereas cAMP/Cr, and the indices of bone turnover increased, reaching high-normal values (in comparison to full-term infants) during the second and third week of life. Serum iPTH was negatively correlated to cAMP/Cr in the first week (r = -0.61, p less than 0.01), whereas the relationship became positive during the second (r = 0.47, p less than 0.05) and third (r = 0.54, p less than 0.05) week of life indicating maturation of the renal response to PTH. The study supports the concept that in premature infants a transient pseudohypoparathyroid-like state is present during the first week of life reflecting an immaturity of renal and possibly bone response to PTH. This may be an etiological factor in hypocalcemia of prematurity.

Hypophosphatemia and hypercalciuria in small premature infants fed human milk: evidence for inadequate dietary phosphorus

Phosphorus and calcium balance was measured prospectively in stable premature infants (less than or equal to 1600 gm) fed human milk or a standard commercial formula. Throughout the study, the P and Ca intakes of the infants fed human milk were two to three times less than those of infants fed formula. Infants fed human milk showed low serum P and normal serum Ca concentrations, complete renal reabsorption of P, and elevated renal Ca excretion. The net effect in infants fed human milk was a 50% reduction in the P and Ca retention, compared with the formula-fed group. Despite the unfavorable P and Ca balance in the group fed human milk, the only evidence of rickets was elevated alkaline phosphatase activity. Nevertheless, based on the biochemical changes in these infants, low serum P values, and excess urinary calcium losses, we conclude that the stable small premature infant fed human milk exclusively is deficient in phosphorus and only slightly more sufficient in calcium.

Neonatal hypermagnesemia: effect on parathyroid hormone and calcium homeostasis

Twenty pre-eclamptic mothers treated with MgSO4 and their newborn infants were studied prospectively to determine the clinical and biochemical effects of hypermagnesemia. Maternal serum magnesium concentration rose to 4.4 mg/dl at delivery and was accompanied by a fall in maternal serum calcium concentration during labor. Neonatal serum Mg concentration remained elevated for the first 72 hours of life (mean at 72 hours = 3.0 mg/dl). Serum Mg concentration was higher in premature infants and in babies with birth asphyxia and/or hypotonia. Serum Ca concentration was higher and serum PTH was lower in hypermagnesemic study infants when compared to a retrospectively selected, matched froup of control infants. We speculate that elevated serum Mg values in these infants result in a shift of Ca from bone to plasma, and that elevated Mg and Ca concentrations further suppress neonatal parathyroid function.