Prevalence of iatrogenic hypothyroidism in hyperthyroid cats treated with radioiodine using an individualised scoring system

Abstract

Objectives The aim of this study was to report the prevalence of iatrogenic hypothyroidism, with or without azotaemia, based on the measurement of serum total thyroxine (T4), thyroid-stimulating hormone (TSH) and creatinine concentrations, in hyperthyroid cats undergoing radioiodine (131I) treatment where the 131I dose was calculated using a previously described scoring system. A secondary aim of the study was to determine the positive and negative predictive values of serum T4 and TSH concentrations obtained 19 days after treatment in order to predict the development of iatrogenic hypothyroidism 6–9 months after 131I treatment. Methods Serum T4, TSH and creatinine concentrations were measured 19 days and 6–9 months after 131I treatment. The prevalence of iatrogenic hypothyroidism was assessed with the results obtained 6–9 months after 131I treatment. Results The prevalence of overt and subclinical hypothyroidism 6–9 months after 131I treatment was 40.0% (22/55 cats) and 12.7% (7/55 cats). Overt hypothyroidism with azotaemia was diagnosed in 8/55 (14.5%) cats. The positive and negative predictive values for the prediction of the development of iatrogenic hypothyroidism 6–9 months after 131I treatment were 72.2% and 80.0%, respectively, for a low serum T4 concentration, and 75.0% and 44.6%, respectively, for an increased serum TSH concentration. Conclusions and relevance The use of an individualised scoring system is effective in determining the 131I dose for the treatment of hyperthyroid cats. However, the prevalence of overt hypothyroidism was higher in comparison with other studies using different dosing protocols. Further studies comparing the efficacy of individualised scoring systems and different fixed doses to determine which method is superior are warranted.

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

“…The success rate of repeat treatment with RAI after initial treatment failure has not been reported, although isolated case reports exist. 18 Results of our study are supportive of repeat treatment with RAI because this was successful in 10/12 cats treated for benign disease with standard (<250 MBq) doses of RAI, as well as both cats receiving a high (>1000 MBq) dose for treatment of thyroid carcinoma. Although these results are encouraging this might reflect careful case selection of cats that were considered likely to respond to standard doses.…”

“…The modal TT4 concentration measured 19 days after RAI treatment is reported as 10.0 nmol/L (IQR 10.0-13.6 nmol/L). 18 Furthermore, older cats with comorbidities are expected to have serum TT4 concentration lower than 40 nmol/L. 23 In this study sample, 121/959 (12.5%) cats had elevated TT4 concentration at the time of discharge from the hospital.…”

“…The modal TT4 concentration measured 19 days after RAI treatment is reported as 10.0 nmol/L (IQR 10.0‐13.6 nmol/L). 18 Furthermore, older cats with comorbidities are expected to have serum TT4 concentration lower than 40 nmol/L. 23 …”

“…Although the upper limit of the reference range for TT4 concentration in our laboratory is 65 nmol, TT4 concentration at discharge of ≥40 nmol/L was considered inappropriately high in this study sample. The modal TT4 concentration measured 19 days after RAI treatment is reported as 10.0 nmol/L (IQR 10.0‐13.6 nmol/L) 18 . Furthermore, older cats with comorbidities are expected to have serum TT4 concentration lower than 40 nmol/L 23 …”

A dosing algorithm for individualized radioiodine treatment of cats with hyperthyroidism

“…Almost all cats with mild to moderate hyperthyroidism responded to very low doses of radioiodine (<2 mCi [<75 MBq]). These 131 I doses are lower than the lowest dose given with most variable scoring protocols (2‐3 mCi [75‐110 MBq]) 3,6,8,27,31 and much lower than doses administered with traditional fixed‐dose methods (4‐5 mCi [148‐185 MBq]) 1,2,9 . In contrast, cats with severe hyperthyroidism and large thyroid tumor volumes (but without scintigraphic evidence of malignancy) sometimes required up to 10 mCi (370 MBq) of 131 I to restore euthyroidism.…”