Investigation of a Novel Modified Fixed Dose Determination Protocol for Radioiodine Treatment of Feline Hyperthyroidism

Morre WA, Panciera D, Daniel G, et al.

American College of Veterinary Internal Medicine 2017.

While radioiodine is the treatment of choice for feline hyperthyroidism because of its safety and efficacy, dose determination of 131I in cats remains controversial. The goal is to achieve a euthyroid state with one treatment, avoiding persistent hyperthyroidism or hypothyroidism. No method of dose determination has been able to consistently achieve this goal. We tested the hypothesis that a radioiodine dose determination based on thyroid scintigraphy would be associated with reduction in hypothyroidism without an increase in persistent hyperthyroidism at 6 months post radioiodine treatment compared with a fixed dose of 131I.

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 99mTcO4- 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 99mTcO4. 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 99mTcO4 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 131I, 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 131I. The dose of 131I 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 131I 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 0.05.

Hypothyroidism developed in 32% of cats in the fixed dose group compared to 21% in the novel dose group (p=0.27). Persistent hyperthyroidism occurred in 8% of cats in the fixed dose group compared to 16% of cats in the novel dose group (p=0.35).

Results of this study showed no statistical difference in the number of cats remaining persistently hyperthyroid or becoming hypothyroid at 6 months post radioiodine treatment using the novel modified fixed dose protocol compared to the standard fixed dose protocol. More studies investigating this novel radioiodine dose determination are needed that incorporate additional doses of 131I in addition to the 3mCi, 3.5mCi, and 4.5mCi described in this study.