Predicting outcomes in hyperthyroid cats treated with radioiodine

Abstract Background Radioiodine (131I) is the treatment of choice for cats with hyperthyroidism. After 131I, however, euthyroidism is not always achieved, with 5% to 10% of cats remaining persistently hyperthyroid and 20% to 50% developing iatrogenic hypothyroidism. Objectives To identify pretreatment factors that may help predict persistent hyperthyroidism and iatrogenic hypothyroidism after treatment of cats using a novel 131I dosing algorithm. Animals One thousand and four hundred hyperthyroid cats treated with 131I. Methods Prospective, before‐and‐after study. Pretreatment predictors (clinical, laboratory, scintigraphic, 131I dose, 131I uptake measurements) of treatment failure or iatrogenic hypothyroidism were identified by multivariable logistic regression analysis. Results Cats that developed iatrogenic hypothyroidism were more likely to be older (odds ratio [OR] = 1.10; 95% confidence interval [CI], 1.04‐1.17; P = .001), female (OR = 2.04; 95% CI, 1.54‐2.70; P < .001), have detectable serum thyroid‐stimulating hormone (TSH) concentrations (OR = 4.19; 95% CI, 2.0‐8.81; P < .001), have bilateral thyroid nodules (OR = 1.57; 95% CI, 1.19‐2.08; P < .001), have homogeneous, bilateral distribution of 99mTc‐pertechnetate uptake (OR = 2.93; 95% CI, 2.05‐4.19; P < .001), have milder severity score (OR = 0.62; 95% CI, 0.49‐0.79; P < .001), and have higher 131I uptake (OR = 2.40; 95% CI, 1.75‐3.28; P < .001). In contrast, cats remaining persistently hyperthyroid were more likely to be younger (OR = 0.81; 95% CI, 0.72‐0.92; P < .001), have higher severity score (OR = 1.87; 95% CI, 1.51‐2.31; P < .001), and have lower 131I uptake (OR = 3.50; 95% CI, 1.8‐6.80; P < .001). Conclusions and Clinical Importance Age, sex, serum TSH concentration, bilateral and homogeneous 99mTc‐pertechnetate uptake on scintigraphy, severity score, and percent 131I uptake are all factors that might help predict outcome of 131I treatment in hyperthyroid cats. Cats with persistent hyperthyroidism had many predictive factors that directly contrasted those of cats that developed 131I‐induced hypothyroidism.