Positive Tc-MIBI and the subtraction parathyroid scan are related to intact parathyroid hormone but not to total plasma calcium in primary hyperparathyroidism.

OBJECTIVE

Primary hyperparathyroidism (pHPT) is characterised by increased parathyroid hormone (PTH) secretion and consequently increased plasma calcium. During the last few decades parathyroid scintigraphy (PS), is applied in almost all patients with pPHT before surgery and specifically before minimally invasive parathyroidectomy. The aim of this study was to find the best cut-off levels of total plasma calcium and intact PTH (iPTH) that correlate with positive technetium-99m-methoxy isobutyl isonitrile (Tc-MIBI) PS and with positive subtraction PS (SPS) in patients with pHPT and thus the positive diagnostic value of these PS.

SUBJECTS AND METHODS

We studied 50 patients, operated for pHPT, aged from 22-78 years, (median age 60 years), 45 female and 5 male, with a total number of 57 parathyroid glands (PG), (46 adenomas and 11 hyperplasias). All patients underwent SPS before surgery. Static scintigrams of the head, neck and chest were performed 15min after the intravenous (i.v.) injection of 740MBq of Tc-MIBI. Late scintigrams of the head, neck and chest were performed 2h and 3h after the injection of Tc-MIBI. Four to 24h after the washout of Tc-MIBI from the parathyroid and the thyroid glands, we injected i.v. 185MBq of Tc-pertechnetate (TcP) and after 15min we performed the PS. Normalization and motion correction of the early Tc-MIBI scan and the TcP followed. We then subtracted the TcP from the Tc-MIBI scan. The areas of increased uptake on the Tc-MIBI scan visible at the early and late or at the subtraction images represented the hyperfunctioning tissue of the enlarged and hyperfunctioning parathyroid glands. Scintigraphic findings were graded subjectively, from 1 to 5 depending on the degree of the uptake of the radiopharmaceutical. Normal iPTH levels were between 10.0-65.0pg/mL and normal total plasma calcium between 2.13-2.65mmoL/L.

RESULTS

Of all patients 12/50 and 38/50 had both PS positive grade 4 and very positive grade 5 findings respectively. In all patients iPTH levels ranged from 54 to 837pg/mL, median value 187.0±133.8pg/mL, and total plasma calcium ranged from 2.40 to 3.83mmoL/L, median value, 2.87±0.237mmoL/L. In 43 patients, both calcium and the iPTH levels were elevated. Strong positive correlation was found between scintigraphic findings and levels of iPTH: P=0.003. A significant relation between plasma calcium levels and different grades of scintigraphic findings was not found, although significant correlation was found between iPTH and plasma calcium levels (P=0.021).

CONCLUSION

In patients with pHPT, the Tc-MIBI PS and the subtraction PS showed a strong correlation to iPTH (P=0.003) but not to total plasma calcium levels indicating the importance of both the Tc-MIBI and the subtraction parathyroid scans to indicate pHPT.