Iodine-129 uptake and effects of lifetime feeding in rats.

Irradiation of thyroid glands in rats at a dose rate of approximately 1 rad/day from continually ingested 129I resulted in no significant increase in thyroid tumors compared to controls, nor in other thyroidal effects, nor was there any difference in longevity between the two groups. Marked decreases in 131I uptake by thyroid glands of 129I-fed rats were observed, but these decreases reflected "blocking" of the thyroid by the large quantities of iodine required because of the low specific activity of 129I. In view of the lack of effects on the thyroid gland invoked by lifetime exposures to 129I, and considering the low specific activity of the radionuclide and the limited capacity of the thyroid gland for iodine, the hazard from 129I appears to be extremely limited.