Influence of Age and of Iodine Intake on the Production of Thyroid Tumors in the Rat2

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Abstract
The influence of age and of iodine intake on the production of neoplastic thyroid nodules was investigated in 1-, 4-, and 14-month-old rats fed for 12 months on a low iodine diet with or without iodine supplement. After autopsy, the number and size of thyroid nodules was estimated in semiserial sections of the entire gland. While the thyroid glands of the animals given iodine supplement were grossly and histologically normal, those of iodine-deficient rats showed gross enlargement, hyperplasia, decreased colloid, and hyperemia. Nodules were observed and classified on the basis of histological appearance into two broad categories referred to as type-β and type-γ. Type-β nodules were found in 84 of the 135 iodine.deficient rats but in none of the 38 iodine-supplemented rats. Hence the formation of these nodules required the iodine deficiency and, presumably, the high level of circulating thyrotrophic hormone induced by this condition. The proportion of rats with type-β nodules, 74 percent, 52 percent, and 64 percent in the 1 -, 4-, and 14-month groups, respectively, was not increased by aging. Type-γ nodules were found in 3 of the iodine-supplemented rats and in 30 of the iodine-deficient rats. Hence iodine deficiency and presumably the resulting high level of circulating thyrotrophic hormone were not essential to, but facilitated the formation of, type-γ nodules. The incidence, 9 percent, 20 percent, and 42 percent in the 3 age groups, respectively, and the size of type-γ nodules increased with age.