Primary hyperparathyroidism masquerading as immobilization hypercalcemia: report of an illustrative case

We report a male who exhibited the Landry-Guillain-Barré syndrome and hypercalcemia. He initially exhibited normocalcemia, followed by hypercalcemia which developed during tetraplegia and the recovering phase of the syndrome. The administration of prednisolone, saline, calcitonin, etidronate, and indomethacin failed to normalize the serum calcium level. Since, with mobilization, the serum calcium level gradually became normal, the calcium abnormality was misdiagnosed as immobilization hypercalcemia. However, among 6 different parathyroid hormone (PTH) assays used, including a two-site immunoradiometric assay, only a mid-region specific PTH (mPTH) assay showed high levels in both hypercalcemic and normocalcemic periods, and a high level of mPTH was not suppressed by calcium infusion in the normocalcemic period. Neck exploration disclosed a parathyroid adenoma weighed 100 mg. This case illustrates the hypercalcemia-inducing effect of immobilization on mild type primary hyperparathyroidism. A high level mPTH assay, its unsuppressibility by the calcium infusion test, and ineffectiveness of oral etidronate for hypercalcemia were valuable in differentiating hypercalcemia due to primary hyperparathyroidism from that resulting solely from prolonged immobilization.