Hyperthyroid cats that develop azotemia following successful radioiodine treatment have shorter survival times compared to cats that remain nonazotemic.

Objective: Many hyperthyroid cats (15% to 50%) have concurrent chronic kidney disease (CKD) that is "masked" and will not become azotemic until after treatment. Previous studies reported that mild-to-moderate azotemic CKD after methimazole or thyroidectomy does not adversely affect survival. Our objective was to determine whether hyperthyroid cats with masked CKD rendered euthyroid with radioiodine (131I) have shorter survival than 131I-treated euthyroid cats that remain nonazotemic.

Methods: We treated 1,047 hyperthyroid cats with 131I from 2013 to 2020 to render them euthyroid, as defined by normal serum concentrations of thyroxine and thyroid-stimulating hormone. Cats were subsequently classified as azotemic or nonazotemic at 6 to 12 months after 131I treatment (median, 6.1 months). Cats were then monitored at 6- to 12-month intervals (ie, complete physical examination, routine laboratory testing, and serum thyroid hormone concentrations) until death or end of study period (January 2024). Survival time was defined as the period from the date of 131I treatment to the date of death or last follow-up.

Results: After 131I treatment to restore euthyroidism, 128 of 1,047 (12%) cats developed azotemia, whereas 919 remained nonazotemic. Azotemic cats had a shorter median survival time than nonazotemic cats (2.8 vs 4.3 years).

Conclusions: In contrast to earlier studies that reported no shortening of survival in cats with mild-to-moderate azotemic CKD after treatment, our results show that euthyroid cats developing azotemia after 131I treatment have shorter survival, with median survival lessened by 1.5 years.

Clinical Relevance: Recognizing concurrent CKD in hyperthyroid cats is crucial, as it shortens survival and enables clinicians to tailor treatment for affected hyperthyroid cats.