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.