Inverse relationship between 25 hydroxy Vitamin D and parathormone: Are there two inflection points?

Context: There is no consensus about the inflection point for 25 hydroxy vitamin D below which the intact PTH level increases. Objective: Determine the relationship/inflection point between 25 hydroxy vitamin D and parathormone levels. Materials and Methods: We performed a population-based analysis on a nonobese cohort (n = 405). Results: Prevalence of vitamin D deficiency was 58.76% (n = 228). Vitamin D insufficiency was found in 34.56% (n = 140). An inverse relationship between 25 hydroxy vitamin D (25(OH)D) and intact PTH exist, but strength of such relationship is weak (r = −0.16, P = 0.018). With respect to the 25(OH)D cut-off of 16 ng/mL by IOM (EAR linked), proportion of persons with high intact PTH was higher in the group with lower 25(OH)D compared with higher 25(OH)D group (P = 0.005) and it was similar for RDA linked cut-off of 20 ng/mL also (P = 0.017). LOWESS method revealed two inflection points at which PTH levels change. A less conspicuous inflection point was found at 32 ng/mL (95% CI, 27–36), which reasonably corroborates with the current cutoff of definition of vitamin D sufficiency, and the second, steeper inflection point was found at 16.5 ng/ml (95% CI, 14.9–18.8) which corroborates with the IOM supported EAR linked value of 25(OH)D level in general population and possible definition of vitamin D deficiency. Conclusions: There are possibly two inflection points at which PTH levels change in relation to 25(OH)D levels