Vitamin D status and parathyroid hormone relationship in adolescents and its association with bone health parameters: Analysis of the Northern Ireland Young Heart's Project

In girls, a plateau in parathyroid hormone (PTH) was observed at a 25-hydroxyvitamin D (25(OH)D) concentration of approximately 60 nmol/1. In boys, there was no plateau in PTH concentrations as 25(OH)D concentration increased. A 25(OH)D threshold of 60 nmol/1 appears to have implications for bone health outcomes in both girls and boys. Introduction Our objective was to investigate if there is a threshold 25(OH)D concentration where a plateau in PTH concentration is evident and to examine the impact of this relationship on bone mineral density (BMD) and bone turnover in a representative sample of adolescents. Methods We conducted a cross-sectional analysis among 1,015 Northern Irish adolescents aged 12 and 15 years. Serum 25(OH)D, PTH, osteocalcin, type 1 collagen cross-linked C-telopeptide (CTx), and BMD of the nondominant forearm and heel were measured. Nonlinear regression analysis was used to model the association between 25(OH)D and PTH. Results In girls, a plateau in PTH was observed at a 25 (OH)D concentration of approximately 60 nmol/1 (PTH= 47.146+370.314×exp^{(-0.092×25(OH)D)}) while no plateau in PTH was observed in boys (PTH=42.144+56.366× exp ^{(-0.022×25(OH)D)}) Subjects with 25(OH)D levels <60 nmol/1 had significantly higher osteocalcin concentrations (P<0.05) compared with those who had ≥60 nmol/ 1, while no significant (P>0.05) differences were noted for CTx concentrations. In girls only, nondominant forearm BMD but not heel BMD was significantly higher (P= 0.046) in those with 25(OH)D concentrations ≥ 60 nmol/1. Conclusions Serum 25(OH)D levels above 60 nmol/1 in Northern Irish adolescent girls prevent an increase in serum PTH levels and maintaining 25(OH)D >60 nmol/1 in both girls and boys may lead to improved bone health outcomes.