Predicting outcomes in hyperthyroid cats treated with radioiodine

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