PTH and osteoporosis: There may be differences in the DRs of PTH and bone turnover markers in people with pathological conditions such as osteoporosis [28], [29] and diabetes [20] but this has so far not been shown for 1,25(OH)2D. Plasma concentrations of 1,25(OH)2D reflect the renal rather than the extra-renal production of 1,25(OH)2D. Other cell types which express the CYP27B1 and CYP24A1 may produce and catabolize 1,25(OH)2D intracellularly and thus may have an independent DR.