Effect of Age on the Morphologic Response of the Rat Thyroid to Irradiation by Iodine-131

Prenatal, newborn, weanling, and adult rats were exposed to graded doses of131I up to and including 3.0 μCi/g. Animals from each age-dose group were killed at 1, 3, and 7 days and at 4 months postexposure. The gross and histopathologic effects were determined and correlated with the previously established radiation doses to the thyroid. The primary effect seen in the younger animals was an inhibition of growth and development of the thyroid; the magnitude of this inhibition was generally proportional to radiation dose. In contrast, degeneration followed by fibrosis predominated in the older animals. The magnitude of these effects was sufficient to account for the previously reported decrease in thyroid function. In the younger animals exposed to 1.0 or 3.0 μCi/g, the trachea underlying the thyroid gland received sufficient radiation dose to inhibit its growth.