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

BACKGROUND: Radioiodine ((131)I) 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 (131)I versus higher, standard‐dose (131)I as treatment for hyperthyroidism. ANIMALS: A total of 189 client‐owned cats undergoing (131)I treatment for mild‐to‐moderate hyperthyroidism (serum T(4) ≥ 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) (131)I. Serum T(4), thyroid‐stimulating hormone (TSH), and creatinine concentrations were measured after 1, 3, and 6 months to determine persistent hyperthyroidism, overt hypothyroidism (low T(4), high TSH), subclinical hypothyroidism (normal T(4), 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 (131)I. No difference in incidence of azotemia existed between groups, but cats treated with standard‐dose (131)I had higher creatinine concentrations (P < .05) and higher percent rises in creatinine (P < .0001). CONCLUSIONS AND CLINICAL IMPORTANCE: Low‐dose (131)I 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.