BACKGROUND Most commercial intact parathyroid hormone (intact PTH) assays cross-react with non-(1-84) PTH (likely 7-84 PTH). Using a whole-molecule PTH (whole PTH) assay that specifically measured only 1-84 PTH, we compared the kinetics of whole PTH and intact PTH after parathyroidectomy in patients with primary hyperparathyroidism (HPT) and secondary HPT. METHODS This study comprised 74 patients with primary HPT caused by a single adenoma and 18 patients with secondary HPT who underwent parathyroidectomy. Blood samples were drawn after anesthesia, just before excision of a single adenoma in primary HPT, and just before excision of the last parathyroid gland in secondary HPT, and at 5, 10, and 15 minutes after excision. The 7-84 PTH level was calculated by subtracting the whole PTH value from the intact PTH value. RESULTS There was a difference between the percentage of 7-84 PTH/intact PTH in plasma samples from patients with primary HPT and secondary HPT (28%+/-12% vs 35%+/-9%; P<.05). Plasma whole PTH decreased more rapidly than intact PTH after parathyroidectomy in patients in both the primary HPT (P<.0001) and secondary HPT groups (P<.0001). Decline of intact PTH was slower in patients with secondary HPT than in patients with primary HPT; however, there was no significant difference in the decline of whole PTH between the 2 groups. CONCLUSIONS The quick intact PTH assay is not used frequently during surgery in patients with secondary HPT; however, our results suggest that a quick whole PTH assay may be a more useful adjunct to parathyroidectomy in both secondary HPT and primary HPT.