Predictors of Response to Radioiodine Therapy in Hyperthyroid Cats

Abstract

The objective of this retrospective study was to evaluate hyperthyroid cats for pretreatment factors that would predict response to radioiodine therapy. Hyperthyroidism was diagnosed in 193 cats based on elevated serum thyroxine levels and/or elevated thyroid to salivary gland ratios on thyroid scintigraphy. All cats were treated with an intravenous bolus of 4 mCi of radioiodine and follow-up serum thyroxine levels were evaluated at 1 week and 1, 3, 6, and 12 months post-therapy. There was a significant relationship between pretreatment thyroxine values and post-treatment thyroxine values at all of the follow-up time points (p < 0.001). There was also a relationship between thyroid to salivary gland technetium scan ratio results and serum thyroxine values at pretreatment and at 1 week post-treatment (p = 0.02, 0.005, respectively). A greater scan ratio was associated with higher thyroxine levels at these time points, but not at 1, 3, 6 or 12 months post-therapy. Ninety-eight cats pretreated with methimazole were analyzed for the effect of this drug on response to therapy. Methimazole was discontinued > or = 5 days before radioiodine therapy in 58 cats and < 5 days in 31 cats, in 9 cats the number of days off methimazole was unknown. There was no difference in response to radioiodine based upon when methimazole was discontinued (p = 0.70).

Efficacy of Low‐dose (2 millicurie) versus Standard‐dose (4 millicurie) Radioiodine Treatment for Cats with Mild‐to‐Moderate Hyperthyroidism

“…Cats with mild‐to‐moderate hyperthyroidism treated with 2 mCi 131 I were significantly less likely to develop iatrogenic hypothyroidism compared with cats receiving the higher, standard dose of 4 mCi. Furthermore, persistent hyperthyroidism in cats dosed with 2 mCi 131 I was rare (3.3%) and comparable to published rates for cats treated with conventional doses of radioiodine 5, 11, 12…”

“…Importantly, hypothyroidism induced by treatment for hyperthyroidism appears to contribute to the development of azotemia and reduced duration of survival in cats 16. Iatrogenic hypothyroidism, based on the finding of subnormal serum thyroxine (T 4 ) concentration 3 or more months after standard‐dose 131 I treatment, occurs in as few as 3%10 and up to as many as 79%17 of all treated cats, with most studies reporting a prevalence of ≤9% 9, 11, 12. No study has examined the prevalence of iatrogenic hypothyroidism in cats with milder degrees of hyperthyroidism treated with standard‐dose 131 I.…”

Effect of thyroid volume on radioiodine therapy outcome in hyperthyroid cats

“…Both in human and feline studies, multiple factors influencing radioiodine therapy outcome, for example the administration of antithyroid drugs and severity of disease, have been proposed and argued. 6,12,13,16,19,20,24,26,31,32 An interesting finding was the larger total thyroid volume in cats with a low TT4 outcome compared with those with a euthyroid outcome in dose categories 2 and 3 ( Table 1). The majority (32/40 [80%]) of these final low TT4 cats were either bilaterally affected or bilaterally affected with additional ectopic thyroid tissue.…”

Feline Hyperthyroidism

“…Other investigators have evaluated the efficacy of administration if a fixed dose of 131 I for treatment of feline hyperthyroidism (Meric and Rubin, 1990;Chun, 2002;Forrest et al, 1996;Craig, 1993). The most commonly utilized dose reported was 4 mCi.…”

“…Of 321 cats treated with 4 mCi, a good response to treatment was reported in 96% of cats and 7% were reported to become hypothyroid (Chun, 2002;Meric and Rubin, 1990;Craig, 1993).…”