The aim of this study was to characterise the scintigraphic findings in a large population of hyperthyroid cats in order to determine the location of thyroid pathology in newly diagnosed hyperthyroid cats and those that had previously undergone thyroidectomy. A specific aim was to identify the proportion of cats with ectopic hyperfunctional thyroid tissue and characterise the scintigraphic and clinical features Of this Subset of cats. Nearly one in hyperthyroid cats was identified to have multiple areas of hyperfunctional thyroid tissue and/or intrathoracic hyperfunctional thyroid tissue where surgical thyroidectomy would not be curative. In addition, this Study demonstrated that scintigraphy cannot reliably distinguish between thyroid carcinoma and adenoma. Owners should always be warned about the possibility of ectopic thyroid tissue before thyroidectomy is performed. In this Study, intrathoracic hyperfunctional thyroid tissue and multiple areas of increased radionuclide uptake (IRU) were a Common feature of benign thyroid disease and responded well to treatment with low dose radioiodine. (C) 2008 ESFM and AAFR Published by Elsevier Ltd. All rights reserved.