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

Radioactive iodine (¹³¹I) is the preferred treatment for feline hyperthyroidism but neither the optimal ¹³¹I dose nor consistent predictors of post-treatment azotaemia have been determined. The aims of the study were to evaluate the relationships between: (1) ¹³¹I dose and survival; and (2) pretreatment and post-treatment serum creatinine concentration.

Medical records of hyperthyroid cats treated with ¹³¹I at a single referral hospital were reviewed. Information regarding signalment, body weight, pretreatment and post-treatment serum total thyroxine concentration (TT4), serum creatinine concentration, ¹³¹I 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.

One hundred and ninety-eight (79 male, 119 female) cats were treated for hyperthyroidism with ¹³¹I (median dose 138 MBq; interquartile range 92-168). Median survival time was 1153 days (range 16-1871). Post-treatment serum creatinine (P <0.001) and age (P = 0.049) were significantly associated with survival. Every 10 µmol/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 μmol/l increase in pretreatment serum creatinine concentration was associated with an increase in post-treatment serum creatinine concentration of 0.7 μmol/l (SE 0.17; P <0.001). Conversely, every 1 nmol/l decrease in post-treatment serum TT4 concentration was associated with a 1.2 μmol/l (SE 0.61; P <0.001) increase in post-treatment serum creatinine concentration.

Post-treatment serum TT4 concentration was associated with post-treatment azotaemia, which was associated with survival. Although ¹³¹I dose was not directly associated with survival, dosing strategies that minimise post-treatment hypothyroidism and azotaemia could improve patient survival.