Background: We aimed to validate a nomogram for diagnosing primary hyperparathyroidism (PHP), particularly when normocalcemic PHP and vitamin D (VitD25) deficiency coexist. Methods: The nomogram calculates maximal upper limit of normal PTH unique for each person by maxPTH = 120 - [6*calcium] - [*VitD25] + [*age]. PHP is suspected when serum PTH exceeds maxPTH. Normocalcemic PHP (NCPHP) was defined as always normal serum calcium (8.5-10.5 mg/dL) with PTH >60 pg/mL preoperatively and VitD25 deficiency as <31 ng/mL. Results: A total of 477 patients had operatively and histologically proven PHP. Overall and including those with classical presentation (high serum levels of calcium and PTH), the nomogram predicted PHP in 97% patients. A total of 66 had NCPHP: 47 with low VitD25 levels (20 ± 0.4 ng/mL) made initial PHP diagnosis challenging; 19 had normal VitD25 status. Although the level of serum calcium concentrations were equivalent in these 2 groups (10.1 ± 0.4 mg/dL), PTH was greater in patients with concurrent VitD25 deficiency (129 vs 97 pg/mL, P =.04). However, when used to calculate maxPTH, the nomogram predicted PHP correctly in all 66 NCPHP patients (100%). Conclusion: The maxPTH nomogram functions as expected to classify patients with PHP and may aid in the diagnosis of NCPHP regardless of vitamin D status and repletion, reassuring primary providers and surgeons alike to embark on appropriate and timely PHP management.
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