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.