High Rate of Occult Urolithiasis in Normocalcemic Primary Hyperparathyroidism

<b><i>Introduction:</i></b> Normocalcemic primary hyperparathyroidism (NPHPT) is characterized by elevations in serum parathyroid hormone levels in the presence of normal serum calcium concentrations after exclusion of secondary hyperparathyroidism. We have previously demonstrated no differences in the prevalence of clinically active urolithiasis between NPHPT and hypercalcemic asymptomatic PHPT, and that it is significantly higher in postmenopausal osteoporotic women with NPHPT in comparison to women with normal serum PTH and calcium concentrations. Few studies have addressed the occurrence of silent or occult kidney stones in asymptomatic hypercalcemic PHPT, but no data are available for NPHPT. <b><i>Objective:</i></b> To determine the presence of occult urolithiasis in NPHPT patients using routine abdominal ultrasonography. <b><i>Methods and Results:</i></b> We studied 35 patients with NPHPT (mean age 63.2 ± 10.7 years, 96% women; serum PTH 116.5 ± 39.2 pg/mL, 25OHD 38.5 ± 6.82 ng/mL, total calcium 9.1 ± 0.56 mg/dL; albumin 4.02 ± 0.37 g/dL; BUN 34.35 ±10.23 mg/dL; <i>p</i> = 3.51 ± 0.60 mg/dL; estimated glomerular filtration rate 88.44 ± 32.45 mL/min/1.73 m², and 24-h urinary calcium excretion 140.6 ± 94.3 mg/24 h). The criteria for the diagnosis of NPHPT were as follows: serum PTH above the reference range (11–65 pg/mL), normal albumin-corrected serum calcium concentrations, normal 24-h urinary calcium excretion, serum 25OHD above 30 ng/mL, estimated GFR (MDRD) above 60 mL/min/1.73 m² (with the exclusion of medications such as thiazide diuretics, lithium, bisphosphonates, and denosumab), a history of clinical symptoms of urolithiasis, and a family history of kidney stones. Thirty-five patients were evaluated and 25 of them met the inclusion criteria. Five patients presented nephrolithiasis corresponding to 20% of the study population. There were no statistically significant differences in any of the clinical or laboratory variables studied between patients with or without urolithiasis, although mean serum PTH levels were higher in patients with stones (180.06 ± 126.48 vs. 100.72 ± 25.28 pg/mL, <i>p</i> = 0.1). The size of the stones ranged from 0.6 to 0.9 cm and all of the stones were located in the renal pelvis. <b><i>Conclusion:</i></b> We found a high prevalence of occult kidney stones in NPHPT patients, similar to what is observed in clinically manifested urolithiasis, in hypercalcemic PHPT.