Positive 99mTc-MIBI and the subtraction parathyroid scan are related to intact parathyroid hormone but not to total plasma calcium in primary hyperparathyroidism.
To correlate PTH and calcium serum levels with the percentages of positive results on the parathyroid scintigraphy, we retrospectively analyzed 194 patients who performed parathyroid scintigraphy. The distribution of the parathyroid scintigraphy results was visually analyzed in a scatter plot, being calcium (Y axis) and PTH (X axis) serum levels the axes. Six groups of patients were defined: 1) calcium > or = 12 mg/dL; 2) 11 mg/dL calcium < 12 mg/dL; 3) 9.9 mg/dL < or = calcium < 11 mg/dL with PTH > 120 pg/mL; 4) 9.9 mg/dL < or = calcium < 11 mg/dL with 65 pg/mL < PTH < or = 120 pg/mL; 5) 9.9 mg/dL < or = calcium < 11 mg/dL with PTH < or = 65 pg/mL; and 6) calcium < 9.9 mg/dL. The percentage of positive scintigraphy in these groups were respectively: 10/10 (100%), 18/29 (62%), 7/9 (78%), 18/45 (40%), 2/21 (10%) and 1/80 (1%). We conclude that in patients with suspected primary hyperparathyroidism, parathyroid scintigraphies performed before surgery on those with calcium level above 11 mg/dL are frequently positive. In patients with calcium levels between 9.9 mg/dL and 11 mg/dL the pertinence of performing the scintigraphy will depend on the PTH levels, and it will be high for patients with PTH serum level above 120 pg/mL and very low for patients with PTH level below 65 pg/mL. Patients with calcium level below 9.9 mg/dL rarely present positive results on parathyroid scintigraphies.