Peterson M.E. and Broome M.R.
Conference Proceedings, (2014). American College of Veterinary Internal Medicine, Nashville:
Radioactive iodine (131I) provides a simple, effective, and safe treatment for cats with hyperthyroidism. Ideally, treatment with a single dose of 131I restores euthyroidism without inducing hypothyroidism (i.e., goal is to irradiate and destroy all adenomatous thyroid tissue but to leave enough normal thyroid tissue to prevent hypothyroidism). The protocol used to determine the 131I dose differs widely among treatment facilities, which may influence the prevalence of persistent hyperthyroidism (131I dose too low) and iatrogenic hypothyroidism (131I dose too high). The methods for 131I dose determination can be generally divided into either fixed 131I dosing (usually administration of 4 mCi to all cats) or an individualized or “patient-specific” 131I dose scoring system based primarily on the pretreatment serum T4 concentration (131I doses generally range from 3–5 mCi; Peterson et al. J Am Vet Med Assoc. 1995;207:1422). In our clinics, we have developed a graded dose scoring system that also takes into account the serum T4 concentration; however, with our method, thyroid volume is measured using data derived from thyroid scintigraphy and the 131I dose calculated primarily on the basis of each cat’s thyroid tumor volume.
During a 1-year period (Sept 2012 to Sept 2013) in which we treated approximately 900 cats withhyperthyroidism, an ultra-low 131I dose (≤ 2 mCi; ≤ 75 mBq) was calculated for 225 of these cats (25%). The purpose of this report is to review the prevalence of persistent hyperthyroidism and iatrogenic hypothyroidism in 131 cats in which 1- and 3-month follow-up data were available. In the 131 cats, the low doses of 131I administered ranged from 1.0–2.0 mCi (median, 1.85 mCi; 68 mBq). Before treatment, the serum T4 concentrations ranged from 3.5–20.8 μg/dl (median, 6.2 μg/dl, reference range, 0.8–4.0 μg/dl); the 5 cats with high-normal T4 values all had high free T4 concentrations. After treatment, median T4 values fell significantly (p < 0.001) to 1.7 μg/dl at both 1- and 3-month recheck periods. At 1 month, T4 values were normal in 119 cats, low in 8, and persistently high in 4. At 3 months, T4 values were within normal limits in 125 cats (95%), low in 4 (3%), and persistently high in only 2 (1.5%). In 3 of the 4 cats with low T4 values, hypothyroidism was confirmed by measuring a high cTSH concentration (0.7–7.5 ng/mL; normal < 0.3 ng/ml).
Results of this study indicate that very low doses of radioiodine (e.g., ≤ 2 mCi; ≤ 75 mBq) result in the cure of most cats with milder forms of hyperthyroidism that have small thyroid tumors on scintigraphy. These radioiodine doses are much lower than the lowest dose given with our original scoring system (3 mCi) or with the fixed-dose methods (4–5 mCi). Despite our ultra-low doses, 3% still developed hypothyroidism, suggesting that many more would become hypothyroid after larger doses commonly used. Overall, we believe that determination of thyroid tumor volume plays a key role in calculating the best 131I dose needed to ablate all tumor tissue (resulting in cure) while preserving enough normal thyroid tissue to prevent iatrogenic hypothyroidism.