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 < .001) and weight normalized RAI dose (P = .04) remained in the final model. All 28 cats with TT4 concentration ≥150 nmol/L at discharge ultimately failed treatment, compared with 13/40 (32.5%) and 11/19 (57.9%) cats with TT4 concentrations of 40-100 nmol/L and 100-150 nmol/L, respectively. Of the 52 cats that failed treatment, 14 were subsequently managed medically, 12 underwent thyroidectomy (4 with carcinoma), 14 had repeat RAI treatment which was successful in 12/14 (86%) cats, and 13 had no further treatment.

Predicting outcomes in hyperthyroid cats treated with radioiodine

“…Hyperthyroid cats with more severe disease (based on higher 131 I dose or severity score) were more likely to fail treatment and remain persistently hyperthyroid. This finding agrees with other reports in cats 3 , 5 , 6 , 43 , 44 , 45 and humans 26 , 28 , 46 , 47 , 48 , 49 in which individuals with higher serum T 4 concentration, larger goiters, or more severe hyperthyroidism were more likely remain hyperthyroid. In contrast, hyperthyroid cats with milder disease were more likely to develop 131 I‐induced hypothyroidism, similar to other reports in 131 I‐treated cats.…”

“… 1 , 2 , 3 , 4 , 5 Conversely, 5% to 10% of hyperthyroid cats fail 131 I treatment and remain persistently hyperthyroid, requiring retreatment with 131 I or alternative treatment with methimazole or surgical thyroidectomy. 1 , 2 , 3 , 4 , 5 , 6 The optimal 131 I dose that will both maximize the chance of curing hyperthyroidism but minimize the risk of developing hypothyroidism is highly variable among individual cats and, therefore, is difficult to calculate. We recently published the results of a large cohort of hyperthyroid cats treated with individualized 131 I doses using a novel algorithm, calculated to administer the lowest effective dose possible.…”

Hyperthyroid cats and their kidneys: a literature review

“…The likelihood of treatment failure appears to increase with disease severity as defined by the TT4 concentration at either baseline (pre‐treatment) or time of discharge post treatment. In a recent study of cats receiving radioiodine treatment, all 28 cats with TT4 concentration >150 nmol/L at discharge remained hyperthyroid long term, unless treated again 82 …”

“…After ablation of the autonomous hypertrophic or neoplastic thyroid follicular cells, unscathed normal thyroid follicular cells are stimulated by TSH and resume function over time 80 . Effects of radioiodine treatment are rapid, with up to 90% of cats having TT4 concentrations within or below the RI by hospital discharge, which can be up to 3 weeks following treatment depending on local radiation safety laws 81,82 . Treatment failure as defined by persistent hyperthyroidism is uncommon, seen in an average of 4% of treated cats based on feline radioiodine treatment studies of the last 20 years 83–89 .…”

“…80 Effects of radioiodine treatment are rapid, with up to 90% of cats having TT4 concentrations within or below the RI by hospital discharge, which can be up to 3 weeks following treatment depending on local radiation safety laws. 81,82 Treatment failure as defined by persistent hyperthyroidism is uncommon, seen in an average of 4% of treated cats based on feline radioiodine treatment studies of the last 20 years. [83][84][85][86][87][88][89] The likelihood of treatment failure appears to increase with disease severity as defined by the TT4 concentration at either baseline (pre-treatment) or time of discharge post treatment.…”