Endocrine Abstracts

Background and aim: The 25OHD concentration at which intact parathyroid hormone (iPTH) is maximally suppressed and below which PTH begins to rise (inflection point) has been used to define optimum 25OHD concentration. While it is not known if the lower 25OHD levels in obese children are associated with a PTH increase in the same manner as it is in normal weight children, we aimed to study the relation between circulating PTH and 25OHD levels and to search for a 25OHD threshold associated with a significant PTH increase.

Methods: This cross-sectional study was conducted on 198 boys and 178 girls aged ≥6 and ≤13 years with BMI ≥ 1SD (according to WHO criteria) recruited from primary schools of Tehran, the capital of Iran. Adjusted iPTH for BMI z-score, pubertal status, and dietary calcium intake were used. Restricted cubic splines showed a nonlinear relationship between iPTH and 25OHD concentration. Nonlinear regression was used to model the relationship between 25OHD and iPTH and identify a suppression point in 25OHD where iPTH reached a plateau and was maximally suppressed. Piecewise regression analysis models with a single knot for all possible values of 25OHD were fitted. The optimal threshold value was chosen based on adjusted R², the F statistic, model standard error, and the t value and associated P value for the threshold variable.

Result: The mean age(SD) of girls and boys was 9.3(SD) and 9.1(SD) years, respectively. Median 25OHD and iPTH were 13.8ng/mL and 38.9 pg/mL in boys and 9.9 ng/mL and 43.5 ng/mL in girls, respectively. The final equation in girls was: iPTH (pg/mL) =43.91+59.41 exp [(−0.188*25OHD]. The point for near maximal suppression of iPTH by 25OHD for girls occurred at a 25OHD concentration of 20ng/mL (95% confidence interval: 28–48 ng/mL). No point of maximal suppression was found for boys. We also found a 25OHD threshold of 11 ng/mL for girls (f: 9.8) by linear piecewise regression modeling of adjusted iPTH. We noted a significant negative estimated slope above this threshold of 25OHD levels in girls (β=−6.2, 95% CI: −9.3 to −3.2). No significant inflection point for boys was observed.

Conclusion: In overweight/obese girls in this cohort, when the concentration of 25OHD was higher than 20 ng/mL, an iPTH mean plateau level was reached. When 25OHD concentrations approached 11 ng/mL, the slope in iPTH concentration accelerated.