Predicting outcomes in hyperthyroid cats treated with radioiodine

Background Radioiodine (I-131) is the treatment of choice for cats with hyperthyroidism. After I-131, 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 I-131 dosing algorithm. Animals One thousand and four hundred hyperthyroid cats treated with I-131. Methods Prospective, before-and-after study. Pretreatment predictors (clinical, laboratory, scintigraphic, I-131 dose, I-131 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 Tc-99m-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 I-131 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 I-131 uptake (OR = 3.50; 95% CI, 1.8-6.80; P < .001). Conclusions and Clinical Importance Age, sex, serum TSH concentration, bilateral and homogeneous Tc-99m-pertechnetate uptake on scintigraphy, severity score, and percent I-131 uptake are all factors that might help predict outcome of I-131 treatment in hyperthyroid cats. Cats with persistent hyperthyroidism had many predictive factors that directly contrasted those of cats that developed I-131-induced hypothyroidism.