Treatment failure in hyperthyroid cats after radioiodine (I-131) injection

Abstract

Background: There is limited published information on the outcome for cats where total thyroxine concentration (TT4) remains elevated after treatment with radioactive iodine (RAI). Objective: To determine the frequency of, and predictors for, subsequent treatment failure in cats for which TT4 remains elevated at hospital discharge, and to report clinical outcomes for cats requiring repeat treatment. Animals: One hundred twenty-one cats with TT4 ≥40 nmol/L after treatment with RAI (out of an original, treated study sample of 959 cats). Methods: Retrospective study. Data regarding signalment, weight, TT4 concentration (before RAI treatment, at discharge, and percentage change), day of sampling, and I-131 dose were acquired. Logistic regression was performed to evaluate predictors of treatment failure. Results: In the 87 cats for which classification was possible, 35 (40%) became euthyroid without further treatment. All TT4 variables and weight normalized RAI dose were independently predictive of subsequent treatment failure. In multivariate analysis, TT4 concentration at discharge (P 150 nmol/L at discharge after RAI might be candidates for immediate repeat treatment.