Normocalcemic Hyperparathyroidism: Study of its Prevalence and Natural History

CONTEXT: Normocalcemic hyperparathyroidism (NPHPT) is characterized by persistently normal calcium levels and elevated parathyroid hormone (PTH) values, after excluding other causes of secondary hyperparathyroidism. The prevalence of the disease varies greatly and the data on the natural history of this disease are sparse and inconclusive. OBJECTIVES: The objectives of this study are to describe the prevalence of NPHPT and its natural history in a referral population and to compare the variability of serum calcium with a group of patients with primary hyperparathyroidism (PHPT). DESIGN: A retrospective study was conducted over 5 years. SETTING: The setting for this study was a metabolic bone referral center. PATIENTS: A total of 6280 patients were referred for a bone mineral density measurement (BMD). MAIN OUTCOME MEASURES: The prevalence and natural history of NPHPT and variability of calcium were the main outcome measures. RESULTS: We identified NPHPT patients using data from the day of the BMD measurement. We excluded patients with low estimated glomerular filtration rate (eGFR) or vitamin D, or with no measurements available. Based on the evaluation of their medical files, we identified 11 patients with NPHPT (prevalence 0.18%). Only 4 patients had consistent normocalcemia throughout their follow-up, with only 2 also having consistently high PTH. None had consistently normal eGFR or vitamin D. Intermittent hypercalcemia was present in 7 of the 11 NPHPT patients. The mean adjusted calcium was found to be significantly lower in the NPHPT group compared with the PHPT group but higher than the control group. PTH was similar for NPHPT and PHPT. These 2 groups had similar variability in serum calcium. CONCLUSIONS: NPHPT patients often have episodes of hypercalcemia. We believe that NPHPT is a mild form of PHPT.