Abstract
Background: Hyperthyroidism is the most frequent endocrinopathy in older cats. To date, there is no consensus on how to best calculate the dose of radioiodine to administer to hyperthyroid cats.
Aim: The goals of this study were to compare thyroid function, renal function and survival time between hyperthyroid cats receiving a fixed dose of radioiodine and those receiving an individualized dose calculated using a clinical scoring system.
Methods: Medical records of 110 cats treated with radioiodine therapy at the University of Bern between 2010 and 2020 were reviewed. Thyroid function, renal function and survival of cats treated with a fixed dose of radioiodine (2010-2015; n= 50) were compared to those of cats treated with an individualized dose (2015-2020; n= 60) at different timepoints after therapy.
Results: Treatment with a fixed dose of radioiodine (mean 168 ± 26 MBq) was associated with 69% of euthyroidism, 19% persistent hyperthyroidism and 12% hypothyroidism, whereas treatment with an individualized dose (mean 120 ± 30 MBq) led to 54% euthyroidism, 23% hyperthyroidism and 23% hypothyroidism (P=0.73).
More than twelve months after treatment, the incidence of azotaemia was comparable between cats treated with a fixed dose (37%) and those treated with an individualized dose (31%) (P= 0.77).
No factors were found to be predictive of treatment failure (hypothyroidism or hyperthyroidism) after therapy.
Median survival time after radioiodine therapy was 44 months. In a multivariate analysis, persistent hyperthyroidism was the only variable independently associated with a shorter survival time (HR= 6.24, P= 0.002).
Conclusion: The method of calculating the dose of radioiodine (fixed vs individualized) to treat feline hyperthyroidism does not appear to be decisive for post-treatment thyroid function, renal function or survival.

Key words: cat; dose; hyperthyroidism; radioiodine.