Endocrine Abstracts

Introduction: 99m/Tc-MIBI parathyroid scintigraphy (MIBI) is the most common test used for pre-operative localization of parathyroid adenoma in primary hyperparathyroidism. We evaluated the influence of serum calcium and parathyroid hormone (PTH) levels on sensitivity of MIBI imaging in successful parathyroid localisation in patients identified as having primary hyperparathyroidism.

Methods and material: Retrospective review of 403 patients who had MIBI scan as part of pre-operative assessment for localization of adenoma were included in this study. The relationships between serum calcium and PTH and sensitivity of MIBI were examined using the Student t-test for continuous variables and Chi-square test for categorical variables between the study groups. P<0.05 was considered significant.

Results: Of 403 patients, 279 (69.2%) were females and 124 (30.8%) were male. Mean age was 56 years (Range: 17–87). MIBI identified (positive scan) parathyroid adenoma in 260/403 (65%). The average corrected calcium concentration in patients with positive MIBI was 2.91 mmol/l (normal range 2.15–2.55 mmol/l); the value was 2.84 mmol/l in those with a negative MIBI scan (P<0.01). In this population none had a positive MIBI scan with serum calcium level <2.60 mmol/l. Similarly the average [PTH] in patients with a positive MIBI scan was 258 ng/l (normal range 10–65 ng/l), and 179 ng/l in those with a negative scan (P=0.1). The lowest [PTH] with a positive scan was 31 ng/l. Age and gender were not related to positivity of MIBI scan.

Conclusion: The likelihood of positive MIBI localisation increased with rising serum calcium but not PTH level. In ‘mild’ primary hyperparathyroidism with serum calcium <2.6 mmol/l MIBI did not identify an adenoma. We found that calcium is a better predictor of MIBI localisation than PTH. The performance of other imaging modalities (USS, PET, 4D CT) should be evaluated in ‘mild’ primary hyperparathyroidism.