Augmentation of parathyroid 201Tl/99Tcm scanning by infusion of trisodium edetate

A significant number of patients with primary hyperparathyroidism have negative preoperative 201Tl/99Tcm subtraction localization scans. In this study an attempt was made to improve scan localization by creating a period of relative hypocalcaemia and increased parathyroid hormone (PTH) secretion before scanning. Six patients with primary hyperparathyroidism were studied (mean serum calcium 2.80 mmol l-1; range 2.70-2.95). All had had a negative standard 201Tl/99Tcm scan carried out within the 6 months prior to this study. Patients were commenced on an intravenous infusion of the calcium chelating agent trisodium edetate at a dose of 24 mg kg-1 h-1 given in 500 ml 0.9% saline over 90 min. Immediately thereafter a 201Tl/99Tcm scan was carried out in the usual way. Three patients showed areas of discordant thallium uptake consistent with the presence of a parathyroid adenoma. Two of these patients had surgery and an adenoma was found at the site corresponding to the scan appearances. It would appear that creating relative hypocalcaemia and increasing PTH secretion may allow increased thallium uptake, possibly secondary to the increased cellular metabolic activity, and thus creating a positive scan.