Effect of thyroid volume on radioiodine therapy outcome in hyperthyroid cats

Abstract

Introduction: Radioiodine therapy in hyperthyroid cats is commonly used and has a high success rate ranging from 85% to 95%. However given the drawbacks associated with a final hypothyroid or hyperthyroid outcome, a quest for causes of therapy failure is justified. As in human medicine, thyroid volume has been reported as a factor influencing radioiodine therapy outcome in hyperthyroid cats. Purpose: The goal of this study was to relate total thyroid volume, calculated by a newly constructed formula for feline patients, to the outcome of radioiodine therapy. We hypothesized that larger volumes would result in a higher percentage of persistent hyperthyroidism and smaller volumes would increase the risk for hypothyroidism. Materials & Methods: For the calculation of total thyroid volume a new formula was introduced (0.438*L*W²). One hundred and sixty-seven hyperthyroid cats were included in search of a correlation between total thyroid volume and therapy outcome. Additionally the relationship between pretreatment total T 4 and therapy outcome was investigated in 62 cats. Patients were categorized according the administered radioiodine dose and therapy outcome. The effect of volume and TT4 on the outcome was modeled by a logistic regression model with generalized logits, with the euthyroid cats as a reference category, and with dose as categorical, and volume and TT4 as continuous fixed effects. Results: All patients had a follow-up of at least 6 months after the radioiodine therapy, 66.5% of them were euthyroid, 24% were hypothyroid and 9.5% were hyperthyroid. 31.7% of our patients were unilaterally affected, 56.9% were bilaterally affected and 11.4% were either unilaterally or bilaterally affected with additional ectopic thyroid tissue. A borderline significant increase (P=0.0528) in the odds of hyperthyroidism relative to the odds of euthyroidism with increasing thyroid volume was found. Concerning the pretreatment total T 4 concentration no significant relationships were detected. Conclusion: The influence on therapy outcome is most likely multifactorial and although only a small effect was observed in this study, large thyroid volumes can increase the risk of a hyperthyroid outcome after radioiodine therapy. Surprisingly, in the two higher dose categories larger total thyroid volumes were also observed in final hypothyroid cats, compared to those with a euthyroid outcome, and the majority of them were bilaterally affected, possibly introducing another contributing factor on therapy outcome. No significant effect of pretreatment total T 4 on outcome was observed.

Evaluation of Renal Perfusion in Hyperthyroid Cats before and after Radioiodine Treatment

“…Only, 4.8% of the cats in our study developed post‐treatment azotemia, which is considerably lower than the 17–39% reported in literature . The percentage of cats having serum TT4 below the reference interval 1 month after radioiodine treatment (36%) was similar to values reported in literature (24–51%) . Iatrogenic hypothyroidism is well known to contribute to the development of renal dysfunction .…”

Investigation of a novel variable dosing protocol for radioiodine treatment of feline hyperthyroidism

“…In addition, the novel variable dose method used in our study is unique in its use of percent dose uptake and thyroid size based on scintigraphy as the sole parameters to determine radioiodine dose. Other studies investigating modified fixed dose methods have based dosing on criteria including serum T4 concentration, clinical signs, and thyroid gland size, but have not used well‐described and repeatable methods of dose calculation . Unfortunately, the dosing scheme used in our study failed to provide more favorable results.…”

“…Studies determining radioiodine dose based on severity of clinical signs, thyroid gland size, and serum T4 concentration resulted in 1.5%‐9.5% being persistently hyperthyroid and 2.1%‐30% becoming hypothyroid. Cats with bilateral disease and larger thyroid volumes were at increased risk for developing hypothyroidism . These studies had different criteria for classifying hypothyroidism, different means of measuring thyroid volume, various tiered dosing schemes, and inconsistent follow‐up periods that make comparisons problematic.…”

Evaluation of Quantitative Thyroid Scintigraphy for Diagnosis and Staging of Disease Severity in Cats With Hyperthyroidism: Comparison of the Percent Thyroidal Uptake of Pertechnetate to Thyroid‐to‐salivary Ratio and Thyroid‐to‐background Ratios

“…An ellipsoid formula has also been used to estimate thyroid volume using ultrasonography in hyperthyroid cats . Despite the widespread use of this ellipsoid formula, however, a multitude of other formulas have been recommended to improve the accuracy for calculation of thyroid volume or thyroid area in normal patients and patients with thyroid disease . Overall, it is clear that no single formula will be able to accurately measure thyroid volume in all cases, especially when applied to highly irregular, invasive, or multinodular thyroid masses…”

“…In cats, the most commonly used quantitative scintigraphic analysis involves calculation of the thyroid‐to‐salivary gland (T/S) ratio; however, a number of other forms of quantitative analysis have been reported, such as determination of thyroid‐to‐background (T/B) ratios, the percent thyroid uptake of the injected radionuclide (e.g., 99m Tc‐pertechnetate), or estimated thyroid size and volume . Unlike the T/S ratio, which can be influenced by either thyroid or salivary gland disease, none of the other quantitative scintigraphic analyses is dependent on intensity of salivary uptake.…”