EVALUATION OF THYROID TO BACKGROUND RATIOS AND COMPARISON OF VARIOUS SCINTIGRAPHIC MEASUREMENTS AND THEIR CORRELATION TO SERUM T4 IN HYPERTHYROID CATS

Thyroid‐to‐salivary ratio and percent dose uptake are the most widely recognized scintigraphic measurements. Recently, the thyroid‐to‐background ratio has been proposed as an alternate method. However, this method has not been validated. The purpose of this observational, cross‐sectional, prospective study was to determine the location of a background region of interest (ROI) that is most reflective of blood pool activity. We also hypothesized that the thyroid‐to‐background ratio using this background ROI would be a better predictor of thyroid function. Fifty‐six cats presented to the Virginia‐Maryland College of Veterinary Medicine seeking radioiodine therapy for hyperthyroidism were enrolled in this cross‐sectional study to evaluating thyroid‐to‐background ratio. A blood sample for measuring plasma radioactivity was collected at the time of scintigraphy. The plasma radioactivity was compared to the background ROIs in eight anatomic regions. Scintigraphic measures of thyroid‐to‐background and thyroid‐to‐salivary ratios, and percent dose were then compared to serum T₄. The heart ROI was most closely correlated with plasma pertechnetate activity (r = 0.70). Percent dose uptake was most closely correlated with serum T₄ (r = 0.74), followed by thyroid‐to‐salivary ratio (r = 0.66) and thyroid‐to‐background ratio using the heart ROI (r = 0.59). Thyroid‐to‐background ratio using the heart background ROI is a good predictor T₄ but percent dose uptake and thyroid‐to‐salivary ratio proved to be better predictors of T₄ than any of the thyroid‐to‐background ratios.