Investigation of a novel variable dosing protocol for radioiodine treatment of feline hyperthyroidism

• Published in: Journal of veterinary internal medicine 2018-11, Vol.32 (6), p.1856-1863
• Main Authors: Morré, Wendy A., Panciera, David L., Daniel, Gregory B., Monroe, William E., Werre, Stephen
• Format: Article
• Language: English
• Subjects: Animals; Cat Diseases - drug therapy; Cats; Drug Dosage Calculations; feline; Female; hyperthyroid; Hyperthyroidism - drug therapy; Hyperthyroidism - veterinary; Iodine Radioisotopes - administration & dosage; Iodine Radioisotopes - therapeutic use; Male; radioiodine; scintigraphy; SMALL ANIMAL; Treatment Outcome
• ISSN: 0891-6640; 1939-1676
• DOI: 10.1111/jvim.15296

Background Radioiodine is the treatment of choice for hyperthyroidism in cats. The ideal method of dose determination of radioiodine remains controversial. Objective To compare a method of radioiodine dose determination that utilized thyroid scintigraphy with a standard fixed dose for treatment of hyperthyroidism. Animals Fifty‐seven and 23 client‐owned hyperthyroid cats in the variable and fixed dose groups, respectively. Methods Cats with a percent dose uptake using 99mTc‐pertechnetate uptake on thyroid scintigraphy 10% were to receive 3, 3.5, or 4.5 millicuries (mCi) of radioiodine, respectively, administered SC. Radioiodine dose was adjusted according to thyroid gland size as determined by the thyroid:salivary size ratio and categorized as 10:1. If the thyroid size fell into a higher dosing category than the percent dose uptake, the dose was increased accordingly. Cats in the fixed dose group received 4.5 mCi. Six months after treatment, cats were determined to be euthyroid, hypothyroid, or hyperthyroid based on serum thyroxine and thyroid stimulating hormone concentrations. Results No difference in outcome was found between the variable and fixed dose treatment groups. Euthyroidism, hypothyroidism, and persistent hyperthyroidism developed in 61, 30, and 9% of cats in the fixed dose group compared to 58, 26, and 16%, respectively, in the variable dose group. Conclusions A variable dosing method of radioiodine based on percent dose uptake primarily and thyroid gland size secondarily did not improve outcome compared to a standard fixed dose method.