Characteristics of study participants

Two hundred and twenty-one patients were enrolled in the present analysis. The mean age was 43.4 ± 14.3 years; 56.6% were male. The mean body mass index (BMI) was 21.8 ± 3.1 kg/m2. In addition, 215 patients (97.3%) had been on dialysis (131 hemodialysis, 84 peritoneal dialysis), and 6 patients were given conservative treatment without dialysis. The median dialysis duration was 61.0 months (range 0–186 months). The median level of iPTH was 852.0 pg/ml (range 113.4–3864.0 pg/ml). Osteoporosis was present in 27.6% (61/221) of patients, parathyroid hyperplasia in 69.7% (154/221) of patients, and vascular calcification in 51.1% (113/221) of patients.

Factors associated with osteoporosis

The characteristics of ESRD patients categorized by osteoporosis status are shown in Table 1. Patients with osteoporosis were more likely to be older (p < 0.001) and female (p = 0.01) and to have a longer dialysis duration (p < 0.001). The level of serum albumin was lower (p = 0.002), and the levels of bone metabolism markers, including intact PTH (p < 0.001), sALP (p < 0.001), ß-CTX (p = 0.01), and P1NP (p < 0.001), were significantly higher in osteoporosis patients. Compared to nonosteoporosis patients, the incidence of parathyroid hyperplasia (93.4% vs. 60.6%, p < 0.001) and vascular calcification (75.4% vs. 41.9%, p < 0.001) was higher in osteoporosis patients.

Table 1 Demographics and parameters of mineral metabolism in ESRD patients categorized according to osteoporosis and vascular calcification status

In multivariable binary logistic regression analysis, osteoporosis was significantly positively associated with age and the level of iPTH (Table 2). According to ROC analysis, the cutoff values of age and iPTH were 47.5 years and 646.7 pg/mL, respectively.

Table 2 Association of osteoporosis and vascular calcification with demographic data and biochemical variables determined with multivariate logistic regression analysis (binary) and further ROC analysisFactors associated with vascular calcification

The clinical and biochemical parameters of the study participants by vascular calcification status are shown in Table 1. Similar to osteoporosis, the ESRD patients with vascular calcification were older (p < 0.001), were more likely to be on hemodialysis (p = 0.02), had a longer dialysis duration (p < 0.001) and had a reduced level of serum albumin (p < 0.001). The levels of intact PTH, sALP, ß-CTX, P1NP (p < 0.001), OC (p = 0.005) and 25(OH)D (p = 0.04) were significantly higher in patients with vascular calcification. Compared to patients without vascular calcification, patients with vascular calcification had a higher incidence of parathyroid hyperplasia (89.4% vs. 49.1%, p < 0.001) and osteoporosis (40.7% vs. 13.9%, p < 0.001).

Vascular calcification was significantly positively associated with age and the levels of 25(OH)D and iPTH according to multivariate analysis (Table 2). The cutoff values of age and iPTH were 41.5 years and 727.3 pg/mL, respectively.

Changes in biochemical parameters among different iPTH groups

To investigate the changes in biochemical parameters associated with the degree of hyperparathyroidism, study participants were divided into three groups based on iPTH levels: 130–599 pg/ml, 600–1499 pg/ml, and ≥ 1500 pg/ml (Table 3). The dialysis duration was significantly longer among those with iPTH 600–1499 pg/ml and iPTH ≥ 1500 pg/ml groups. Serum albumin and hemoglobin levels decreased as hyperparathyroidism severity increased, whereas serum ALP, calcium-phosphorus product, and bone turnover markers rose significantly (Fig. 1). Furthermore, the proportions of parathyroid hyperplasia, osteoporosis, and vascular calcification were higher in the groups with iPTH 600–1499 pg/ml and iPTH 1500 pg/ml.

Table 3 Demographics and parameters of study patients according to iPTH levelFig. 1

Nutritional and mineral metabolism parameters according to iPTH level. A Serum albumin; B hemoglobin; C serum alkaline phosphatase; and D calcium–phosphorus product. *p < 0.05, **p < 0.001 vs. iPTH 200–600; #p < 0.05, ##p < 0.001 vs. iPTH 600–1500

Correlations of serum markers

Table 4 shows the results of univariate correlation analyses between bone turnover markers and demographic variables. The levels of the bone turnover marker osteocalcin, P1NP, and β-CTx correlated positively with age. Both formative and resorptive markers correlated positively with intact PTH, sALP and serum phosphate levels. Serum albumin and hemoglobin correlated negatively with bone turnover markers. There were no correlations between 25(OH)D and bone turnover markers. Serum calcium had a weak negative correlation with ß-CTX.

Table 4 Univariate correlations between bone turnover markers and characteristics of ESRD patients