Is low plasma 25-(OH)vitamin D a major risk factor for hyperparathyroidism and Looser's zones independent of calcitriol?

Background. Recent reports suggest that calcitriol might not be the sole active metabolite of vitamin D and that plasma concentrations of 25-(OH)vitamin D (25OHD) are often abnormally low in hemodialysis patients. We have therefore evaluated plasma 25OHD as a risk factor for parathyroid hormone (PTH) hypersecretion and radiological bone disease. We carried out a cross-sectional study during the month of September in an Algerian dialysis center of 113 patients who were not taking supplements of alphacalcidol or calcitriol. Methods. Plasma 25OHD, calcitriol, PTH, calcium, phosphate, bicarbonate, and aluminum were measured, and x-rays of the hands and pelvis were obtained for evaluation of subperiosteal resorption and Looser's zones. Results. The median plasma 25OHD was 47.5 nmol/liter (range 2.5 to 170.0). Univariate analysis showed that plasma PTH was correlated positively with months on maintenance dialysis and negatively with plasma 25OHD, calcitriol, calcium, bicarbonate and aluminum, but not with that of phosphate. plasma 25OHD was positively correlated with calcium and calcitriol. Using multiple regression analysis, only plasma 25OHD (negative) and the duration on maintenance dialysis (positive) were independently linked to plasma PTH. The prevalence of isolated subperiosteal resorption (ISR) was 34%, and that of the combination of resorption with Looser's zones (CRLZ) was 9%; thus, only 57% of the patients had a normal x-ray appearance. These groups were comparable with regards to age, gender, and duration on dialysis. When the biochemical measurements of the patients with CRLZ were compared with those from patients without radiological lesions, plasma 25OHD was the only parameter to show a statistically significant difference, being significantly lower in the CRLZ group (26 +/- 18 vs. 57 nmol/liter, ANOVA, P < 0.004). Plasma 25OHD was also significantly lower in the ISR group (44, P < 0.05) than in the normal x-ray group, and plasma Ca (P < 0.003) and bicarbonate (P < 0.02) were lower. Logistical analysis showed that the presence of resorption was independently linked only with plasma PTH. Looser's zones and subperiosteal resorption were not seen in patients with plasma 25OKD of more than 40 (Looser's zones) and more than 100 nmol/liter (subperiosteal resorption). The optimal range for intact PTH in hemodialysis patients with mild aluminum overload is 10 to 25 pmol/liter. We found that plasma PTH was inappropriately high only when plasma 25OHD was less than 100 nmol/liter. With a plasma 25OHD of between 100 and 170 nmol/liter, hypercalcemia was present with a plasma PTH of less than 10 pmol/liter in only one case. Conclusions. This cross sectional study shows that low plasma 25OHD is a major risk factor for hyperparathyroidism and Looser's zones. In dialysis patients, we suggest that the plasma levels of 25OHD are maintained around the upper limit of the reference range of sunny countries.