PTH is a key regulator of bone turnover. PTH can be oxidized in vivo, which impacts biological activity. Variable PTH oxidation may account for the rather poor correlation of PTH with indices of bone turnover. The present study tested the hypothesis that non-oxidized PTH (n-oxPTH) is superior to total PTH (tPTH) as marker of bone turnover.

Thirty-one patients with End Stage Renal Disease (ESRD) were selected from an ongoing prospective observational bone biopsy study in such a way that they covered the whole spectrum of bone turnover. ROC curves, Spearman correlation and regression analysis of iPTH, n-oxPTH and bone turnover markers (bone-specific alkaline phosphatase, P1NP and TRAP5b) were used to assess the capability of n-oxPTH vs tPTH to discriminate low from non-low and high from non-high bone turnover.

n-oxPTH and tPTH were strongly correlated (rho=0.92; p<0.001). Histomorphometric parameters of bone turnover, as well as circulating bone turnover markers showed similar correlation coefficients with n-oxPTH and tPTH. The AUROC values for discriminating between low/non-low turnover for n-oxPTH and tPTH were 0.82; p<0.01 and 0.79; p<0.01, respectively. For high/non-high turnover the AUROC was 0.76; p<0.01 and 0.80; p<0.01, respectively. Hence, measuring n-oxPTH using the currently available method provides no added value compared to tPTH in the assessment of bone turnover in ESRD patients.