Increased fracture risk in normocalcemic postmenopausal women with high parathyroid hormone levels: a 16-year follow-up study.

High PTH levels increase bone turnover and decrease bone mineral density (BMD). Low plasma 25-hydroxyvitamin D (25OHD) levels cause secondary hyperparathyroidism, but the relative contribution of low 25OHD and high PTH levels on risk of fracture is largely unknown. Within the cohort of women (n = 2,016) included in the Danish Osteoporosis Prevention Study (DOPS), we studied risk of fracture according to parathyroid status. Analyses were performed on effects of high PTH levels (i.e., in the upper tertile, ≥4.5 pmol/L) on risk of incident fractures at different 25OHD levels during 16 years of follow-up. Incident fractures were assessed using a nationwide hospital discharge register. In addition, effects of high PTH levels on BMD and vertebral fractures were assessed by DXA scans and spinal X-ray examination after 10 years of follow-up. High PTH levels were associated with a decreased body mass index, adjusted BMD, and an increased risk of any fracture (HR = 1.41, 95% CI 1.11-1.79) as well as an increased risk of osteoporotic fractures (HR = 1.59, 95% CI 1.20-2.10). Plasma 25OHD levels per se did not affect fracture risk, but high PTH levels were associated with an increased fracture risk only at 25OHD levels <50 nmol/L and 50-80 nmol/L. High PTH levels did not increase risk of fracture at 25OHD levels >80 nmol/L. In conclusion, PTH levels in the upper part or above the upper level of the reference interval increase risk of fracture in the presence of low vitamin D levels.