CHANGES IN PLASMA TRIIODOTHYRONINE, THYROXINE,AND THYROID-STIMULATING HORMONE AFTER 131IIRRADIATION OF NEWBORN RATSFED WITH IODINE DEFICIENT DIET

Abstract

Background: Human thyroid gland is generally regarded as a relatively low-risk organ in terms of
developing radiation-induced tumorigenesis. However, a rapid increase in the incidents of thyroid cancer
after the Chernobyl nuclear reactor accident in 1986 provided additional insight into the risk of thyroid
cancer. Three key risk factors have been identified to be involved in this increase: (1) internal irradiation
from 131I fallout, (2) young age, and (3) a low-iodine diet. Our previous study demonstrated that the
thyroid radiation dose was highest in the newborn rats fed with low-iodine diet when rats of varying ages
were internally exposed to 131I at the same radioactivity per body weight.
Objective: To examine the short-term effects of a low dose internal irradiation of 131I on the status of
the thyroid hormone in rats of three different ages maintained on either standard diet or low-iodine diet.
Methods: 131I was injected intraperitoneally in F344 rats at the ages of 1, 4, and 9 weeks. Animals
were maintained with an iodine-deficient (IDD) or a standard (SD) diet. Changes in serum levels of
triiodothyronine (T3), thyroxine (T4), and thyroid-stimulating hormone (TSH) were examined.
Results: Dramatic changes in hormone levels were found only in the rats belonging to 1-week-old
IDD group, in which T3 levels rapidly dropped and TSH levels increased after 131I irradiation, whereas
they remained unchanged in the SD group. In 4- and 9-week-old rats, hormone levels were also steady
after irradiation, with no differences between the IDD and SD groups.
Conclusions: These data suggest that under low-iodine conditions, the status of thyroid hormone of
newborn rats is particularly sensitive to internal irradiation of 131I.