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

Background: Radioiodine ( 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 I versus higher, standard-dose I as treatment for hyperthyroidism.

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

Conclusions And Clinical Importance: Low-dose 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.