AIM: Determining the physiologic activity (hormone production) of individual parathyroid glands by measuring their contained radioactivity is extremely accurate and provides tremendous guidance during parathyroidectomy. METHODS: A single-surgeon's experience with over 8,000 radioguided parathyroidectomies, 7,500 of which were for primary, sporadic, non-MEN hyperparathyroidism (HPT). All operations were within 2.25 hours of sestamibi scanning. Ex-vivo measures of parathyroid glands, thyroid nodules, lymph nodes, thymus, and fat were obtained from all patients, constituting over 42,000 specimens. RESULTS: Parathyroid glands occur in three distinct groups according to their hormone production which very accurately indicates the type of pathology present. Adenomas (N.=7 943) contained 59+/-35% of background radioactivity; hyperplastic glands (N.=876) contained 17+/-5%; and normal glands (N.=13 821) contained 4+/-0.1%, all P<0.00001). Fat and lymph nodes were always less than normal glands (P<0.001). There was no overlap between different tissue types in any individual (P<0.001). Contained radioactivity was a better predictor of cure than histology (P<0.001). CONCLUSIONS: Measures of sequestered radioactivity is an extremely accurate estimate of individual parathyroid gland hormone production allowing near 100% distinction between normal, hyperplasic, and adenomatous glands as well as distinguishing parathyroids from other neck tissues (fat, lymph nodes, thyroid). This instantaneous measure can eliminate frozen sections and parathyroid hormone assays in nearly all patients undergoing parathyroid surgery, providing insight that allows the operation to progress rapidly.