Regulation of PTH secretion by 25-hydroxyvitamin D and ionized calcium depends on vitamin D status: A study in a large cohort of healthy subjects

Introduction: Previous papers investigating vitamin D status have often outlined the significant relationships between serum parathyroid hormone (PTH) and 25-hydroxyvitamin D (25OHD), but the influence of ionized calcium levels has not been concomitantly considered. Design: Cross-sectional. Materials and methods: In 1050 healthy men (547) and women (503), serum ionized calcium (iCa), creatinine (Cr), albumin, 25OHD, and PTH were measured. After conventional analysis, a regression tree was fitted on the data set. Results: 25OHD and PTH values showed significant opposite seasonal changes. 25OHD levels negatively correlated with PTH, which in turn negatively correlated with iCa. A regression tree was fitted to the whole data set using PTH as the response variable and 25OHD and iCa as covariates. PTH concentration depended on that of iCa only in subjects with 25OHD levels>16.35. ng/mL, while for 25OHD<16.35. ng/mL it depended on 25OHD values. Conclusions: Our results indicated that PTH levels were highly conditioned by those of 25OHD in subjects with 25OHD values lower than 16.35. ng/mL and by those of iCa only for higher 25OHD concentration.