Kinetics of accumulation of radioiodine by thyroid gland: longer time intervals

¹ Laboratory of Physiology, National Cancer Institute, Bethesda, Maryland

The radioiodide and the protein-bound radioiodine (PBI¹³¹) concentrations in the thyroid gland and the serum radioiodide concentrations in mice and rats were measured at various time intervals (up to 25 hr) after injection of radioiodide. The ratio of the radioiodide concentrations in the thyroid gland and serum (T/S) increased progressively with the time interval after injection. If it is assumed that a small but constant fraction of the thyroid PBI¹³¹ decomposed during the fractionation of the thyroid I¹³¹, the corrected thyroid radiodiode concentration yielded a constant T/S up to 4 hr after injection and the rate of increase of the thyroid PBI¹³¹ was proportional to the thyroid radioiodide concentration. The corrected data were therefore consistent with a previously published simple model. Analysis of the source of thyroid radioiodide indicated that at 4 hr after injection almost all was derived from blood serum, but at 25 hr most could have been derived from thyroidal PBI¹³¹ by physiological degradation.