Investigation of a novel modified fixed dose determination protocol for radioiodine treatment of feline hyperthyroidism

Background: Radioiodine is the treatment of choice for feline hyperthyroidism. The ideal method of dose determination of 131I remains controversial.

Objective: To compare a method of radioiodine dose determination that utilized thyroid scintigraphy with a standard fixed dose for treatment of feline hyperthyroidism.

Methods: Fifty-seven and 23 cats were in the novel and fixed dose groups, respectively. Cats with a percent dose uptake as determined using 99mTcO4- uptake on thyroid scintigraphy <5%, 5-10%, and >10% were designated to receive 3 mCi, 3.5 mCi, or 4.5 mCi of 131I, respectively, administered subcutaneously. Radioiodine dose was adjusted by thyroid size, determined by evaluating the thyroid:salivary size ratio (T:S) and categorized as <5, 5-10, and >10. 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 4.5 mCi of 131I. Six months after treatment, cats were determined to be euthyroid, hypothyroid, or hyperthyroid based on serum T4 concentrations relative to an established reference interval. Univariate analysis using Chi-square was used to determine associations between treatment and outcome.

Results: There was no difference in outcome between the novel and fixed dose treatments. Euthyroidism, hypothyroidism, and hyperthyroidism developed in 61, 30, and 9% of cats in the fixed dose group, respectively compared to 58, 26, and 16% in the novel dose group.

Conclusions: A modified fixed dose method of radioiodine based upon thyroid size and percent dose uptake was ineffective in improving outcomes over a standard fixed dose method.