[Iodine deficiency induces thyroid cancer in rats and mice]

Wistar rats and Km mice were fed with a normal diet or a diet deficient in iodine (0.12 micrograms/l iodide in drinking water and 195.3 micrograms/kg iodide in the grain, collected from an iodine deficient area). The animals were sacrificed at 4, 7 and 12 months, one-third of the animals for each, respectively, after initiation of the experiment. It was found that, under iodine deficiency, the thyroid gland increased in weight with marked epithelial hyperplasia of the follicles, and nodular, diffuse goiter, epithelial atypia which eventually progressed to carcinoma in 19 (8 rats and 11 mice) of the 122 animals studied. Histologically, papillary carcinoma was observed in 14 and follicular carcinoma in 5. No malignant tumor developed in animals on normal iodine intake. Moreover, there was marked hyperplasia of basophilic cells in the anterior pituitary with an increase in serum thyroid stimulating hormone (TSH) level. The results suggest that, the development of thyroid carcinoma is the result of feedback hypersecretion of TSH in response to hypofunction of the thyroid gland due to iodine deficiency.