Evaluation of bone resorption markers in patients with normocalcemic hyperparatyhroidism

Normocalcemic hyperparathyroidism is a condition that is characterized by elevated parathormone levels, despite normal serum totals and ionized calcium levels, after the exclusion of secondary causes that may result in elevated parathormone levels, such as renal failure, vitamin D deficiency, the use of thiazide and loop diuretics, lithium and malignancies. Osteoporosis is a common clinical presentation of normocalcemic hyperparathyroidism. The present study compares the femoral neck, lumbar vertebral and distal radius bone mineral density, and the type 1 collagen N-telopeptide and deoxypyridinoline levels in 24-hour urine samples, of 44 patients with normocalcemic hyperparathyroidism and 17 patients with hypercalcemic hyperparathyroidism. The aim in this regard is to identify the degree of osteoporosis in patients with normocalcemic hyperparathyroidism, and to determine the value of deoxypyridinoline and type 1 collagen N-telopeptide in the early diagnosis of osteoporosis. When type 1 collagen N-telopeptide and deoxypyridinoline levels measured in the 24-hour urine samples of the two groups were corrected by proportioning to 24-hour urinary creatinine, the median type 1 collagen telopeptide/creatinine ratio was 2.39 (0.65–5.12) nmol/mmol/l in the hypercalcemic group and 1.82 (0.41–4.65) nmol/mmol/l in the normocalcemic group. The median deoxypyridinoline/creatinine ratio was 16.7 (3.4–25.8) nmol/mmol/l in the hypercalcemic group and 12.2 (1.6–32.6) nmol/mmol/l in the normocalcemic group. The group comparison showed no significant difference in the type 1 collagen n-telopeptide/creatinine and deoxypyridinoline/creatinine ratios (p >0.05). The median bone mineral density of the femoral neck was -1.3 SD and 0.7 g/cm² in the hypercalcemic group and -1.2 SD and 0.72 g/cm² in the normocalcemic group; the median bone mineral density of the lumbar vertebrae was -1.9 SD and 0.9 g/cm² in the hypercalcemic group and -1.2 SD and 0.94 g/cm² in the normocalcemic group, and the median bone mineral density of the distal radius was -3 SD and 0.54 g/cm² in the hypercalcemic group and -2.5 SD and 0.56 g/cm² in the normocalcemic group. A comparison of the two groups identified no statistically significant difference in these three parameters (p > 0.05). Normocalcemic hyperparathyroidism affects bone mineral density as much as hypercalcemic hyperparathyroidism, and is not so harmless a condition that follow-up can be neglected. In terms of osteoporosis, it is absolutely necessary to follow lumbar vertebra, femoral neck and distal radius bone mineral density as well as bone turnover markers. Further comprehensive prospective studies are needed to understand better the other clinical presentations of normocalcemic hyperparathyroidism and the long-term clinical course.