Comparison of radioactive iodide uptake in the rat thyroid between oral and intravenous bolus administration

Abstract

Radioiodide is commonly used to diagnose and treat hyperthyroidism and thyroid carcinoma. However, we could not find any experimental data that strictly compared the biodistribution and thyroid uptake of radioactive iodide between the oral and intravenous (iv) routes with time. This prompted us to compare (123)I biodistribution and thyroid uptake to clarify the differences between oral and iv bolus administration in rats. The rats were divided into two groups, A and B (n=5, each). In the first imaging experiment, Na(123)I solution (35MBq/200μL) was administered as a bolus to the rats orally in group A and intravenously in group B. Two weeks later, the second imaging experiment was performed as a crossover experiment. (123)I biodistribution was evaluated visually and quantitatively with a gamma camera at 10min, 3, 6, 12, 24, and 48h after (123)I administration. Thyroid uptake was compared between oral and iv groups. Correlation of (123)I thyroid uptake and whole-body excretion was evaluated. The area under the curve (AUC) of thyroid uptake was also calculated. (123)I biodistribution differed visually during 6h between the two groups. (123)I thyroid uptake was significantly higher in the iv group at 10min (P<0.05) and in the oral group at 6 or more hour time points (P<0.005-P<0.0001) and peaked at 12h in both groups (oral: 24.4±2.8%ID, iv: 15.2±2.8%ID). (123)I thyroid uptake showed significant inverse correlations with whole-body excretion from 6h (r=-0.799, P<0.0001), and thereafter [12h (r=-0.957, P<0.0001), 24h (r=-0.905, P<0.0001) and 48h (r=-0.893, P<0.0001)], respectively. (123)I whole-body excretion was significantly higher in the iv group at each time point (P<0.0001). The AUC of (123)I thyroid uptake was 1.6 times higher in the oral group than the iv group. These results suggest that radioiodide accumulates in the rat thyroid more effectively by oral than iv administration probably due to slower and lower (123)I clearance from the body in the oral administration when administered in a bolus fashion.