Parathyroid hormone-ionized calcium dynamics over the first year of life

Background: Ionized calcium (iCa) is believed to be the principle determinant of parathyroid hormone concentration (PTH). However, previous studies contained few infants. Methods: This ancillary study from our vitamin D-3 doseresponse trial in healthy, breastfed infants measured calcium, phosphorus, PTH and 25(OH)D (25-hydroxyvitamin D) at 1, 2, 3, 6, 9 and 12 months of age. The relationship between iCa and PTH was assessed by Pearson correlation and a mixed effects regression model to account for repeated measures. Results: No significant correlations were observed between iCa and PTH at individual visits (p > 0.2). After accounting for repeated measures, PTH decreased with increasing iCa (slope -5.25; 95% confidence intervals (CI) -8.78 to -1.73), decreased with increasing 25(OH) D (slope -0.006; 95% CI -0.009 to -0.002, and increased with later visits (6-12 months, p < 0.001), Conclusions: We observed a weak negative relationship between iCa and PTH and an increase with age consistent with physiologic maturation.