BackgroundRadioiodine (131I) is effective treatment for hyperthyroidism in cats, but optimal dose to restore euthyroidism without inducing hypothyroidism is unclear. Treatmentâinduced hypothyroidism can lead to azotemia and reduced duration of survival.ObjectiveTo compare efficacy and shortâterm outcomes of lowâdose 131I versus higher, standardâdose 131I as treatment for hyperthyroidism.AnimalsA total of 189 clientâowned cats undergoing 131I treatment for mildâtoâmoderate hyperthyroidism (serum T4 â„ 4.0 ÎŒg/dL and <13.0 ÎŒg/dL).MethodsProspective, nonrandomized, cohort study comparing treatment with either lowâdose (2 mCi, n = 150) or standardâdose (4 mCi, n = 39) 131I. Serum T4, thyroidâstimulating 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.ResultsThere was no significant difference in prevalence of cats with persistent hyperthyroidism between standardâ and lowâdose 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 standardâdose 131I. No difference in incidence of azotemia existed between groups, but cats treated with standardâdose 131I had higher creatinine concentrations (P < .05) and higher percent rises in creatinine (P < .0001).Conclusions and Clinical ImportanceLowâdose 131I is safe and effective for cats with mildâtoâmoderate hyperthyroidism, as evidenced by a cure rate of >95% with reduced frequency of iatrogenic hypothyroidism and azotemia.