Evaluation of Thyroid to Background Ratios in Hyperthyroid Cats

Bettencourt A.E., Panciera D.L., Larson M., et al.

Conference Proceedings, (2013). American College of Veterinary Radiology, Savannah, GA: p.75


IntroductionlPurpose: Thyroid scintigraphy has been used to diagnose, quantify and characterize disease, and predict treatment outcome in hyperthyroid cats. Scintigraphic methods to quantify thyroid function include thyroid-to-salivary (T:S) ratio, % dose uptake, and thyroid-to-background (T:B) ratio. Pertechnetate, when injected intravenously equilibrates in the vascular/extracellular fluid space. Background radioactivity should be a predictor of pertechnetate plasma concentration. Pertechnetate, an iodide analog, is transported by the Nal symporter into the follicular cells of the thyroid where it is concentrated. Therefore, the T:B ratio should be a measure of the ability of the thyroid gland to concentrate pertechnetate above plasma levels and should provide a good measure of thyroid function. The T:B ratio has recently been used to predict treatment response. However, the most suitable background ROI has not been elucidated. The purpose of this study was to determine the location of a background ROI that is the best predictor of pertechnetate concentration in plasma. Subsequently, T:B was compared to T:S and % dose uptake to determine which scintigraphic measurement correlates to serum T4 in the hyperthyroid cat.

Methods: Fifty-five hyperthyroid cats were prospectively enrolled. In this study, serum T4

was used as the standard measure of thyroid function. Pre-treatment serum T4ITSHand

renal chemistry values were obtained in each cat prior to thyroid scintigraphy and 1311

therapy. Blood samples were collected at the time of the thyroid scintigraphy and

radioactivity within the sample was measured using a Nal well detector. The plasma

radioactivity was compared to the background ROI count densities using linear regressions

analysis in 8 different anatomic regions. Thirty-nine cats were included in the second portion

of the study in which T:B, T:S, and % dose uptake on thyroid scintigraphy were compared to

pre-treatment serum T4 by linear regression analysis. Sixteen cats were excluded because

of methimazole administration within 2 weeks of imaging, YID diet use, and incomplete

data. Treatment response was evaluated as separate events. Chemistry, UA, and T~SH


were collected at 1, 3, and 6 months following therapy with 4.5 mCi of subcutaneously.


Results: The heart ROI was the background ROI that correlated the best to plasma pertechnetate activity (r = 0.70). The % dose uptake correlates best to serum T4(r = 0.74), followed by T:S ratio (r = 0.65), followed by the T:B ratio using the heart ROlon a ventral image (r = 0.58).

Discussion/Conclusion: Placing an ROI over the heart is the best method of quantifying plasma radioactivity. T:B ratio using the heart ROI as the background is a good predictor T4 but percent dose uptake or the traditional T:S ratio proved to be a better predictors of T4 than any of the T:B ratios. Data collection of post treatment T4ITSH will be complete by the end of the summer. The influence of the YID diet on thyroid uptake will also be discussed.