Radioactive iodine dose and survival in cats with hyperthyroidism (2015-2020).

Abstract

OBJECTIVES: Radioactive iodine (I) is the preferred treatment for feline hyperthyroidism but neither the optimalI dose nor consistent predictors of post-treatment azotaemia have been determined. The aims of the study were to evaluate the relationships between: (1)I dose and survival; and (2) pretreatment and post-treatment serum creatinine concentration. METHODS: Medical records of hyperthyroid cats treated withI at a single referral hospital were reviewed. Information regarding signalment, body weight, pretreatment and post-treatment serum total thyroxine concentration (TT4), serum creatinine concentration,I dose and survival were determined. Multivariable Cox proportional hazards analysis was used to identify variables associated with survival. Multivariable linear regression analysis was used to identify variables associated with post-treatment serum creatinine concentration. RESULTS: One hundred and ninety-eight (79 male, 119 female) cats were treated for hyperthyroidism withI (median dose 138 MBq; interquartile range 92-168). Median survival time was 1153 days (range 16-1871). Post-treatment serum creatinine (<0.001) and age (&#x2009;=&#x2009;0.049) were significantly associated with survival. Every 10&#x2009;&#xb5;mol/l increase in post-treatment serum creatinine concentration and every year increase in age was associated with a 1.07-fold (confidence interval [CI] 1.04-1.11) and 1.17-fold (CI 1.00-1.37) increase in the daily hazard of death, respectively. Pretreatment serum creatinine concentration was directly, and post-treatment serum TT4 concentration was inversely, associated with post-treatment serum creatinine concentration. Every 1&#x2009;&#x3bc;mol/l increase in pretreatment serum creatinine concentration was associated with an increase in post-treatment serum creatinine concentration of 0.7&#x2009;&#x3bc;mol/l (SE 0.17;<0.001). Conversely, every 1&#x2009;nmol/l decrease in post-treatment serum TT4 concentration was associated with a 1.2&#x2009;&#x3bc;mol/l (SE 0.61;<0.001) increase in post-treatment serum creatinine concentration. CONCLUSIONS AND RELEVANCE: Post-treatment serum TT4 concentration was associated with post-treatment azotaemia, which was associated with survival. AlthoughI dose was not directly associated with survival, dosing strategies that minimise post-treatment hypothyroidism and azotaemia could improve patient survival.