Efficacy of Low-Dose (2 Millicurie) Versus Standard-Dose (4 Millicurie) Radioiodine (131I) Treatment for Cats with Mild-to-Moderate Hyperthyroidism

Lucy J., Peterson M., Randolph J.F., et al. (2015). ACVIM Forum. Indianapolis, Indiana: 563.


Radioiodine (131I) is an effective treatment for feline hyperthyroidism, but optimal dosing to restore euthyroidism without inducing hypothyroidism remains unclear. Post-treatment hypothyroidism can lead to azotemia and reduced survival. The objective of this prospective, longitudinal study was to compare the frequency of persistent hyperthyroidism and iatrogenic hypothyroidism in cats with mild-to-moderate hyperthyroidism (thyroxine [T4] > 4 but < 13 μg/dL) treated with either a low-dose (2 milliCuries [mCi]; n=131) or a standard-dose (4 mCi; n=37) of 131I. Serum concentrations of T4, thyroid stimulating hormone [TSH], and creatinine were assessed at 0, 3, and 6 months after treatment. Cats were categorized at each time interval as having persistent hyperthyroidism, euthyroidism (normal T4 [0.9-4.0 μg/dL] and TSH [≤ 0.3 ng/mL]), overt hypothyroidism (low T4, high TSH), or subclinical hypothyroidism (normal T4 but high TSH) (Table). Categorical and continuous data were analysed using Chi-Square and Wilcoxon Rank Sum, respectively, with P < 0.05 considered significant. At baseline, there were no differences in sex, age, or serum concentrations of T4, TSH, or creatinine between treatment groups. Cats treated with 2 mCi 131I were significantly (P ≤ 0.01) less likely to develop overt hypothyroidism at 3 and 6 months compared to cats treated with 4 mCi. There was no significant difference in persistent hyperthyroidism between groups. Cats dosed with 4 mCi 131I had significantly (P ≤ 0.007) higher median creatinine concentrations at 3 (1.9 mg/dL) and 6 months (2.0 mg/dL) than did cats receiving 2 mCi (1.6 mg/dL and 1.7 mg/dL, respectively). In conclusion, most cats with mild-to-moderate hyperthyroidism were effectively treated with 2 mCi 131I with reduced risk of overt hypothyroidism compared to cats dosed with 4 mCi.