Abstract Context Primary hyperparathyroidism (PHPT) is commonly diagnosed in the setting of hypercalcemia, whereas normocalcemic primary hyperparathyroidism (NHPT) may be misdiagnosed. Objective Our objective was to compare hypercalcemic hyperparathyroidism (HPHPT) versus NHPT hypercalciuric renal stone patients. Design and Setting We took advantage of a routine calcium load test performed in hypercalciuric renal stone patients to assess retrospectively among PHPT patients, prevalence and characteristics of NHPT and HPHPT under a calcium restricted diet Results Among 1671 hypercalciuric patients included, 91 patients have a final diagnosis of PHPT(post load ionized calcium (iCa)>1.31 mmol/L and PTH>30 pg/ml). Prevalence of NHPT is 40% of all PHPT, however according to total serum calcium 4/35 NHPT and 7/56 HPHPT would have been misclassified in the other group. 18/35 NHPT and 40/56 HPHPT underwent parathyroidectomy. No significant characteristics related to parathyroid weight, stone composition or bone remodeling biomarkers is detected between groups. Whereas iCa is higher in HPHPT in fasting state and after calcium load, we found no difference for calcium diet, 24-hour calciuria, or calcitriol. Of notice, renal calcium excretion (FECa) post load increases by 303% in NHPT but only 176% in HPHPT (p=0.01) likely explained by a lesser PTH decrease (p=0.02). However, a strong negative association (p<0.0001) detected between pooled pre and post load iCa and PTH only in NHPT group suggests a persistent efficient PTH-CaSR control within parathyroid glands in this group. Conclusion Our data show the relevance of dynamic tests to unmask NHPT in hypercalciuric renal stone patients.