Evaluation of fixed-dosage radioiodine treatment in feline hyperthyroidism at the Utrecht University for Companion Animals

Abstract

Introduction: Radioiodine therapy (I-131) is an effective treatment for feline hyperthyroidism. In 2023, the Utrecht University Clinic for Companion Animals resumed offering I-131 therapy. This study aimed to evaluate treatment success, the incidence of iatrogenic hypothyroidism, and the effects on renal function, in order to optimize the approach and provide valuable insights for other centers. Animals: Fifty-one client-owned cats were referred for radioiodine treatment. Methods: The cats were treated with a single 220 MBq I-131 subcutaneous injection. Patient records were analysed to collect data regarding clinical signs and plasma concentrations of TT4, TSH, creatinine, urea, ALP, and ALT. Results: 93% of the cats did not have hyperthyroidism anymore after the treatment. 25% developed overt hypothyroidism at 3 months post-treatment. A significant increase in renal parameters was observed, with 2 out of 14 cats having hypercreatinemia. Conclusion and clinical implications: I-131 therapy proved to be highly effective for treating feline hyperthyroidism, although the incidence of overt hypothyroidism was relatively high. The clinical relevance of the increased renal values is minimal, particularly in cats with low or low-normal creatinine concentrations. Further evaluation and optimisation of the treatment protocol, including the consideration of individualised dosages, is warranted.