The effect of per os vs subcutaneous 123 iodine administration on percentage thyroidal radioactive iodine uptake in normal cats

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.