Determination of the Kinetic Parameters for 123I Uptake by the Thyroid, Thyroid Weights, and Thyroid Volumes in Present-day Healthy Japanese Volunteers.

The purpose of this study was to evaluate the kinetic parameters that determine the uptake rate of radioiodide in the thyroid over 24 h after administration and to estimate thyroid volumes/masses of present-day Japanese. Methods: We determined the thyroid uptake rate of I in healthy male Japanese after oral administration (4.5-8.0 MBq) without iodine restriction. Masses of thyroid glands were collected in 2012-2016 during autopsies of 7,651 male and 3,331 female subjects. Volumes of thyroid glands were estimated by ultrasonography and magnetic resonance imaging in 52 male subjects. Results: The thyroid uptake rate of I for 24 h was 16.1 ± 5.4%. Kinetic model analysis was conducted to obtain the clearances (L h) for thyroid uptake and urinary excretion of I (0.499 ± 0.258 and 2.10 ± 0.39 L h, respectively). The masses of thyroid glands were on average 19.8 g (95% confidence interval of 18.3-19.5 g) and 15.5 g (95% confidence interval of 14.7-16.2 g) in male and female subjects aged 19-52 y, respectively. Volumes of thyroid glands estimated by ultrasonography and magnetic resonance imaging were 17.5 ± 5.2 and 14.2 ± 5.3 mL, respectively. In healthy Japanese, there has been no significant change for at least 50 y in the thyroid uptake of radioiodide over 24 h or in its kinetic parameters. These Japanese-specific kinetic parameters will allow quantitative estimation of the radiation exposure from the Fukushima accident and its variance during the individual's evacuation from or stay in Fukushima.