Predicting Outcomes in Hyperthyroid Cats Treated with Radioiodine

Source: J Vet Intern Med. 2022 Jan-Feb; 36(1): 49–58.

Hyperthyroidism is a common disease in cats, and mostly afflicts cats middle-aged and older. It is caused by the overproduction of thyroid hormone by the thyroid glands. Radioiodine (¹³¹I) is the treatment of choice to restore euthyroidism with a single dose of radiation without producing hypothyroidism for cats receiving the treatment. However, 30% to 50% of ¹³¹I‐treated cats develop iatrogenic hypothyroidism after treatment. Besides, 5% to 10% of hyperthyroid cats fail 131I treatment and remain persistently hyperthyroid.

To identify pretreatment factors that may help predict persistent hyperthyroidism and iatrogenic hypothyroidism after treatment of cats using a novel ¹³¹I dosing algorithm. Peterson, ME and Rishniw, M conducted a study involving one thousand and four hundred hyperthyroid cats treated with ¹³¹I. Cats underwent an evaluation that included a complete physical examination, routine laboratory testing (CBC, serum biochemical profile, complete urinalysis), determination of serum thyroid hormone concentrations (total thyroxine [T4], triiodothyronine [T3], and thyroid‐stimulating hormone [TSH]), and qualitative and quantitative thyroid scintigraphy.

Pretreatment predictors (clinical, laboratory, scintigraphic, ¹³¹I dose, ¹³¹I uptake measurements) of treatment failure or iatrogenic hypothyroidism were identified by multivariable logistic regression analysis. In conclusion, 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.