Urinary calcium excretion and bone turnover in osteoporotic patients

Introduction: It is well documented that patients with osteoporosis (OP) have high incidence of hypercalciuria (HC). However, the mechanism of HC in patients with OP is not well established. It is thought to be the result of high bone turnover (HBT) with excessive bone resorption. OP also frequently presents with low bone turnover (LBT). At this time, it is not clear whether OP with LBT is also associated with hypercalciuria. Purpose: The purpose of this study is to evaluate urinary calcium excretion in osteoporotic patients with HBT and LBT. Materials and methods: This is a retrospective study of 132 patients with osteoporosis who underwent bone biopsy at the University of Kentucky between January 2010 and December 2012. Based on bone biopsy results, patients were divided into HBT or LBT groups. Demographic data, medical history, bone mineral density, serum creatinine, calcium, phosphorus, estimated glomerular filtration rate (eGFR), filtered calcium load, fractional excretion of calcium and phosphorus, 25-hydroxy vitamin D levels, and 24-hour urinary calcium excretion and creatinine were obtained from the patients' medical records. Also, intact parathyroid hormone (iPTH), serum osteocalcin, bone-specific alkaline phosphatase, N-telopeptide of type I collagen, and urine pyridinium levels were measured. Results: Hypercalciuria was present in approximately half of the patients in both the HBT and LBT groups. Patients with HBT OP were significantly younger than those with LBT OP (p = 0.013). There was no difference between HBT and LBT patients in 24-hour urinary calcium excretion, serum creatinine, calcium, phosphorus, eGFR, filtered calcium load, and fractional excretion of phosphorus. Mean values of serum osteocalcin and serum N-telopeptide of type I collagen were significantly lower in the LBT compared to the HBT group (p = 0.000 and 0.0152, respectively). There was a significant correlation between filtered calcium load and urinary calcium excretion in HBT patients but not in patients with LBT. Fractional excretion of calcium significantly correlated with urinary calcium excretion in both groups. There was no correlation between kidney function and 24-hour urinary calcium excretion. There was no correlation between dual-emission X-ray absorptiometry T-scores and 24-hour urinary calcium excretion. Conclusion: HC is frequently present in patients with OP regardless of the underlying bone turnover status. This may suggest the presence of a bone-derived renal calcium regulating factor(s). Further studies are needed to understand the exact mechanism and the potential consequences of HC in OP patients.