Is the Serum Vitamin D-Parathyroid Hormone Relationship Influenced by Obesity in Children?

Background/Aims: Serum 25 hydroxyvitamin D [25(OH) D] and parathyroid hormone (PTH) levels correlate inversely. The 25(OH) D level at which PTH is maximally suppressed and below which PTH begins to rise (inflection point) has been used to define optimum 25(OH) D levels. While serum 25(OH) D levels are lower in obese individuals, likely due to sequestration of vitamin D in the adipose tissue, it is not known if the lower 25(OH) D levels in obese children are associated with a PTH increase in the same manner as it is in normal weight children. Methods: A retrospective record review of children between 2 and 18 years of age (n = 269) undergoing simultaneous measurement of serum 25(OH) D and PTH levels was performed. Results: The level of serum 25(OH) D below which PTH begins to rise was significantly lower (p < 0.001) in overweight/obese children compared to normal weight children [12.4 (95% CI 9.8-15.0) vs. 17.0 ng/ml (95% CI 11.4-22.6)]. Conclusions: In overweight and obese children, the PTH axis is activated at much lower 25(OH) D levels than in normal weight children. These findings are a consequence of similar ionized calcium levels in these groups despite lower serum 25(OH) D levels in obese children. (C) 2013 S. Karger AG, Basel