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Background: Patients with primary hyperparathyroidism (PHPT) treated surgically occasionally have normalized calcium, but persistently high parathyroid hormone (PTH). We hypothesized that a possible explanation for this phenomenon is an underlying hyperplasia rather than adenoma.Methods: Retrospective cohort of patients who underwent parathyroidectomy for PHPT with biopsy of a normal-appearing parathyroid gland were included. Cellularity level of each biopsy and of the adenoma's rim was determined.Results: Forty-seven patients were included. Of them, 19 (40%) had postoperative normocalcemia but elevated PTH. There was no correlation between cellularity either in the rim or of the normal-appearing parathyroid gland and postoperative PTH. The postoperative high PTH group had higher preoperative PTH (P = 0.001) and larger adenomas (P = 0.025).Conclusions: High PTH levels after successful parathyroidectomy in patients with primary hyperparathyroidism do not appear to result from underlying hyperplasia. A possible alternative explanation is that these patients have a higher preoperative burden of disease.cellularity, elevated PTH levels, parathyroid adenoma, parathyroid hyperplasia, parathyroidectomy | INTRODUCTIONPrimary hyperparathyroidism (PHPT) is diagnosed by an elevated parathyroid hormone (PTH) concentration in patients with hypercalcemia and hypercalciuria. Its most common clinical presentation is asymptomatic hypercalcemia. 1 It is recommended that patients with symptomatic PHPT (nephrolithiasis, symptomatic hypercalcemia) have parathyroid surgery, which is the only definitive therapy. 2,3 In some patients with asymptomatic disease, surgery is not mandatory. However, this is disputed as some claim that a truly asymptomatic PHPT patient is quite rare. 4 Whereas most asymptomatic patients do not exhibit disease progression, as defined by worsening hypercalcemia,