Background: Historical and recent literature disagree on whether a higher PO 131 I dosage, compared to IV or SC routes, is required for successful resolution of spontaneous hyperthyroidism in cats, necessitating investigation into the effect of PO and injectable radioactive iodine administration on % thyroidal radioactive iodine uptake (RAIU).
Hypothesis/objectives: To investigate the effect of PO and SC routes of 123 I administration on paired % thyroidal RAIU in euthyroid cats. Specifically, a 1.5-fold difference (50% relative change) was hypothesized, which in absolute terms can be expressed as a 3.25% increase in the mean %RAIU of 7.04% after PO 123 I administration to 10.56% after SC dosing.
Animals: Seven healthy euthyroid teaching-research colony cats.
Methods: A randomized, radiologist-blinded crossover study comparing %RAIU after PO and SC 123 I administration.
Results: Percentage thyroidal RAIU values (mean ± SD; 95% confidence interval) after PO (4.81% ± 1.63%; 3.30%-6.23%) and SC (5.26% ± 2.43%; 3.01%-7.51%) 123 I administration were associated with a median within-pair absolute difference of 0.2% (range: min, 0.1%-max, 4.9%). Statistical significance was not achieved (P = .45). Six of 7 cats had a within-pair absolute difference of 0.1% to 0.9% (relative change of 4%-20%), but a single outlier cat had a within-pair absolute difference of 4.9% (relative change of 108%).
Conclusions and clinical importance: This study did not detect an effect of 123 I administration route on paired % thyroidal RAIU in euthyroid cats. However, a type 2 statistical error due to small sample size is possible.
Keywords: 123I thyroid scintigraphy; euthyroid cats; percentage thyroidal radioactive iodine uptake; route of iodine administration.
© 2021 The Authors. Journal of Veterinary Internal Medicine published by Wiley Periodicals LLC on behalf of American College of Veterinary Internal Medicine.