Role of Parathyroid Hormone in Bone Fragility of Postmenopausal Women with Vitamin D Insufficiency

Vitamin D insufficiency is related to an increase in PTH, which might be critical for an increase in bone fragility. However, the role of endogenous PTH in vitamin D insufficiency-induced fracture risk remains unclear. The present study was performed to examine the relationships among vitamin D insufficiency, bone fragility, and PTH in 202 Japanese postmenopausal women. Serum 25-hydroxyvitamin D (25[OH]D) levels were measured. The percentages of subjects with 25(OH)D levels below 10, 15, and 20 ng/ml were 5.0, 41.0, and 80.7%, respectively. Serum 25(OH)D levels were negatively related to age and serum levels of Cr and PTH; they were positively related to bone mineral density (BMD). In multiple regression analysis, BMD was significantly related to 25(OH)D levels when adjusted for age, body mass index (BMI), and serum levels of Cr and PTH. Multiple logistic regression analysis showed that lower 25(OH)D levels were significantly related to prevalent fracture risk when adjusted for age, BMI, serum levels of Cr and PTH, as well as femoral neck BMD. The proportion of subjects with prevalent fractures was significantly higher in the group with lower PTH and lower 25(OH)D than in the group with lower PTH and higher 25(OH)D or higher PTH and higher 25(OH)D. In conclusion, vitamin D insufficiency was found to be related to prevalent fracture risk independently of PTH. Functional hypoparathyroidism, rather than functional hyperparathyroidism, might be a risk factor for bone fragility in vitamin D insufficiency.