Unsuppressed parathyroid hormone in patients with autoimmune/inflammatory rheumatic diseases: implications for vitamin D supplementation.

Objectives: To verify if autoimmune/inflammatory rheumatic disease (ARD) patients were more refractory to PTH suppression by 25(OH) vitamin D (VITD).

Methods: Data from 105 consecutive ARD patients (including RA, PMR, spondyloarthritis and other CTDs) attending a tertiary-level immuno-rheumatology clinic and 1542 subjects tested at our central laboratory from 2008 to 2010 (controls) were collected. After exclusion of patients with renal failure, primary hyperparathyroidism and hypercalcaemia (n = 522), plasma VITD, PTH, calcium and phosphate concentrations were compared between these two groups.

Results: Plasma VITD concentrations were <25 nmol/l in 257 patients (severe deficit, 22.8%), ≥25 nmol/l but <75 nmol/l in 661 (mild deficit, 58.8%) and ≥ 75 nmol/l in 207 (normal, 18.4%). Despite similar median age, plasma VITD, calcium and phosphate values (P = 0.96, 0.30, 0.94, respectively), PTH was higher in ARD {73.0 [interquartile range (IQR) 54.2-93.7] pg/ml} than in controls [61.4 (46.9-80.3), P < 0.0002], also in all above-defined VITD categories (P < 0.05). Suppressed PTH was observed in 96.9% (95% CI 95.8%, 98.0%) of controls with VITD ≥ 75 nmol/l. However, PTH was increased more frequently in ARD vs controls. At multiple linear regression analysis, plasma VITD, age and the presence of an ARD (partial correlation coefficients -0.21, 0.15, 0.12, respectively, P < 0.0001) were independent predictors for increased PTH.

Conclusions: Patients with ARD had, on average, an increased PTH concentration for any plasma VITD range, suggesting an impaired vitamin D metabolism. Therefore, vitamin D supplementation to ARD patients may be targeted to reach PTH suppression and not simply to obtain VITD concentrations considered optimal in other categories of patients.