2017 ACVIM Forum Research Abstract Program

The purpose of this study was to compare a novel method of radioiodine dose determination that utilized thyroid scintigraphy (novel dose group) with a standard fixed dose method (fixed dose group) for the treatment of hyperthyroidism in cats. Specifically, this study focused on determining radioiodine dose by measuring thyroid gland size and percent of 99m TcO 4 uptake using thyroid scintigraphy. Hyperthyroidism was diagnosed in all cats by finding a serum T4 concentration above the reference interval and characteristics diagnostic of hyperthyroidism on nuclear scintigraphy. Scintigraphy was performed by obtaining images 30 minutes after IV administration of 3.0 mCi 99m TcO 4 . In cats in the novel dose group, percent dose uptake was calculated by drawing a region of interest around the thyroid gland lobe(s) and dividing the corrected counts (CPM) in the thyroid gland by the 99m TcO 4 dose (CPM). Cats with a percent dose uptake < 5%, 5-10%, and > 10% were calculated to receive 3 mCi, 3.5 mCi, or 4.5 mCi of 131 I, respectively administered subcutaneously. Thyroid size was assessed by a ratio of the thyroid ROI size (# of pixels) to the salivary ROI size (# of pixels) (TS). Cats with a TS ratio of <5, 5-10, and >10 were calculated to receive 3 mCi, 3.5 mCi, and 4.5 mCi of 131 I. The dose of 131 I to be given was first determined by measuring percent dose uptake. If the thyroid size fell into a higher dosing category than percent uptake, the dose was increased accordingly. Cats in the fixed dose group received a mean of 4.3 mCi of 131 I subcutaneously. Cats were evaluated 6 months after treatment to determine the functional status of the thyroid gland. Hypothyroidism was defined as having a serum T4 concentration below the reference interval and persistent hyperthyroidism was defined as having a serum T4 concentration above the reference interval 6 months post treatment. Fifty-eight cats were included in the novel dose group and the fixed dose group consisted of 25 cats treated prior to those in the novel dose group. Univariate analysis using Chi-square and Fisher's exact analysis was used to determine associations between dose determination method and outcome with the level of significance set at P < 0 .05.