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

Background Radioiodine (131I) is effective treatment for hyperthyroidism in cats, but optimal dose to restore euthyroidism without inducing hypothyroidism is unclear. Treatment‐induced hypothyroidism can lead to azotemia and reduced duration of survival. Objective To compare efficacy and short‐term outcomes of low‐dose 131I versus higher, standard‐dose 131I as treatment for hyperthyroidism. Animals A total of 189 client‐owned cats undergoing 131I treatment for mild‐to‐moderate hyperthyroidism (serum T4 ≥ 4.0 μg/dL and <13.0 μg/dL). Methods Prospective, nonrandomized, cohort study comparing treatment with either low‐dose (2 mCi, n = 150) or standard‐dose (4 mCi, n = 39) 131I. Serum T4, thyroid‐stimulating hormone (TSH), and creatinine concentrations were measured after 1, 3, and 6 months to determine persistent hyperthyroidism, overt hypothyroidism (low T4, high TSH), subclinical hypothyroidism (normal T4, high TSH), and azotemia. Results There was no significant difference in prevalence of cats with persistent hyperthyroidism between standard‐ and low‐dose treatment groups at 3 (0% versus 5.3%; P = .34) and 6 (0% versus 3.3%; P = .51) months. Overt (18% versus 1%; P = .0005) or subclinical (46% versus 21%; P = .004) hypothyroidism was more common in cats at 6 months after standard‐dose 131I. No difference in incidence of azotemia existed between groups, but cats treated with standard‐dose 131I had higher creatinine concentrations (P < .05) and higher percent rises in creatinine (P < .0001). Conclusions and Clinical Importance Low‐dose 131I is safe and effective for cats with mild‐to‐moderate hyperthyroidism, as evidenced by a cure rate of >95% with reduced frequency of iatrogenic hypothyroidism and azotemia.