Feature Article
Radioactive iodine (RAI) treatment for feline hyperthyroidism Jing-Lu Teh, BVM&S, MRCVS
Introduction
Feline hyperthyroidism (FHT) is currently the most diagnosed endocrine disease among cats worldwide (Carney et al. 2016; Peterson 2020). Until the exact cause is determined, and prevention becomes possible, the prevalence of FHT is likely to increase (Rijnberk and Kooistra 2010a; Carney et al. 2016; Peterson 2020). FHT arises most frequently from adenomatous hyperplasia, rarely malignant carcinoma and results in deleterious effects on kidney, heart, gastrointestinal tract and systemic blood pressure (Rijnberk and Kooistra 2010a; Carney et al. 2016; Peterson 2020). With proper treatment and monitoring hyperthyroid cats can be cured or controlled; and live >2 years (Carney et al. 2016; Peterson 2020). Currently there are four treatment methods available: radioiodine (RAI), thyroidectomy, medical management (e.g. methimazole) and dietary management (Rijnberk and Kooistra 2010a; Carney et al. 2016; Peterson 2020). This article focuses on RAI, now widely considered as the gold standard treatment and discusses its pros, cons, pre- and post-treatment management. Targeted ablation of the abnormal thyroid tissue is achieved via selective uptake and concentration of I-131 by the hyperfunctional thyroid tissue (Peterson 2020). Normal or atrophied thyroid tissue does not absorb I-131 as much as the hyperactive thyroid tissue thus remains unaffected (Rijnberk and Kooistra 2010).
Pros and efficacy of radioactive iodine treatment
Radioactive iodine treatment has a high cure rate (85–95%) (Slater et al. 1994; Rijnberk and Kooistra 2010a; Volckaert et al. 2016; Peterson 2020). In most cases the concentration of serum total T4 (TT4) reduced significantly within 1 month and continued to reduce gradually up to 6 months post-treatment (Mooney 2005; Peterson 2006). Successfully RAI-treated cats lived a median of 4 years after treatment, twice as long compared to cats undergoing methimazole treatment (Milner et al. 2006; Rijnberk and Kooistra 2010a). RAI targets specifically hyperfunctional thyroid tissue while sparing the surrounding healthy tissue (Peterson 2006; Rijnberk and Kooistra 2010a; Carney et al. 2016). It is more efficacious than surgery in treating ectopic thyroid, which was reported in 3.9–23% of hyperthyroid cats (Mullowney et al. 2021). RAI is a simple, anaesthesia-sparing procedure, administered by injection or orally (Carney et al. 2016; Peterson 2020). Contact: Chatswood Vet Clinic Willoughby, NSW, Australia. j.l.teh83@gmail.com
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The cons and side effects of radioactive iodine treatment Radioactive iodine has a higher upfront but lower longterm cost compared to other treatment (Carney et al. 2016; Peterson 2020). RAI may not be easily accessible in some areas as the procedure must be carried out at a licensed facility (Peterson 2006, 2020). The requirement for hospitalisation post-treatment may be stressful for some cats and owners (Peterson 2020). After treatment, clients need to adhere to regional rules and regulation to minimise human exposure to radiation (Mooney 2005; Rijnberk and Kooistra 2010a; Peterson 2020). Transient fever and dysphagia have occasionally been reported soon after treatment (Peterson 2006, 2020). Cases of iatrogenic hypothyroidism post-treatment have been reported when a standard high fixed-dose protocol (4–5 mCi) was used frequently in the past (Lucy et al. 2017). The incidence
Companion Quarterly: Official Newsletter of the Companion Animal Veterinarians Branch of the NZVA | Volume 33 No 1 | March 2022
