99m Tc-Pertechnetate Scintigraphy in Hyperthyroid Cats with Normal Serum Thyroxine Concentrations (Abstract)

Tomsa K., Hardegger R., Glaus T., et al.

Conference Proceedings, (2001). American College of Veterinary Internal Medicine:


99mTc-PERTECHNETATE SCINTIGRAPHY IN HYPERTHYROID CA TS WITH NORMAL SERUM THYROXINE CONCENTRATIONS. K. Tomsa1, R. Hardegger2, T. Glaus1, C. Reusch1. 1Clinic for Small Animal Internal Medicine, and 2Polyclinic for Nuclear Medicine, University of Zurich, Switzerland

We previously reported, that hyperthyroid cats suffering from severe non- thyroidal illness (sick hyperthyroid cats) were indistinguishable from sick euthyroid cats based on serum T4 and on thyrotropin-releasing hormone (TRH) stimulation test. Limitation of the TRH stimulation test was mainly poor specificity.

The purpose of this prospective study was to evaluate the usefulness of thyroid pertechnetate scintigraphy for assessment of thyroid function in a population of cats with clinically suspected hyperthyroidism, and normal serum T4 concentration and to evaluate its potential superiority over TRH stimulation test.

Inclusion criteria were: clinical suspicion ofhyperthyr oidism, serum T4 concentration < 3.5 μg/dl, and available thyroid histology. Complete blood count, serum chemistry, urinalysis and TRH stimulation test were performed in each cat. Qualitative pertechnetate thyroid scan (70 MBq 99m TcO4IV, imaging after 20 min, 150 000 counts) was acquired using high resolution parallel hole collimator. Scintigraphy was additionally performed in 3 clearly hyperthyroid cats (serum T4 > 3.5 μg/dl) as positive controls.

Up until now a total of 14 cats fulfilled the inclusion criteria (study cats). Serum T4 concentrations ranged from 0.6 to 3.0 μg/dl (median 1.7). Percentage of stimulation after TRH application ranged from –7 to 83% (median 29.5). Eight cats showed stimulation < 50%, 3 cats between 50-60%, and 3 cats > 60%. Thyroid scan was positive in all cats. Thyroid pathology, consistent with hyperthyroidism (nodular hyperplasia or adenoma), was evident on histology in 3 control cats and in 11 of 14 study cats. No thyroid pathology was found in 3 study cats. These were the cats with a stimulation of > 60% after TRH application.

Scintigraphy was a sensitive test for diagnosing hyperthyroidism in sick hyperthyroid cats. However, the positive results in 3 cats with normal thyroid histology raise suspicion about its specificity. Possible explanations for positive scintigraphy in these 3 cats are true false-positives, false-positives associated with iodine depletion, or true-positives but false-negative histopathology. Further studies are needed with larger number of cats, especially euthyroid cats with a lack of stimulation after TRH application.