The occurrence of thyroid tumors induced by N-methyl-N-nitrosourea (MNU) and low iodine diet in Long-Evans (LE) rats was studied with special reference to sex difference, effect of gonadectomy, and estradiol administration. Rats of experimental groups 1–6 were given i. v. injections of 40 mg of MNU/kg of body weight at 50 days of age and fed on low iodine diet from 28 days of age to the end of the experiment (30 weeks after MNU administration). They consisted of male, female, castrated male, ovariectomized female, and gonadectomized male and female rats given 2.5 mg estradiol pellets s. c. Rats of groups 7–10 served as the respective controls without MNU or low iodine diet. Levels of serum thyroid stimulating hormone and estrogen receptor of the thyroid lesions were also examined. It was noted that the incidence of thyroid carcinoma was higher in females than in males (P < 0.01) and did not change by castration in males but decreased in ovariectomized rats (P < 0.01). Administration of estradiol after gonadectomy significantly increased the incidence of thyroid carcinomas in castrated and ovariectomized rats. Increase of mean serum thyroid stimulating hormone levels and thyroid and pituitary weights was also predominant in females. Mean thyroid stimulating hormone levels of both sexes were decreased by gonadectomy. Mean thyroid and pituitary weights were inhibited from increasing not by castration but by ovariectomy. Estradiol supplemented after gonadectomy significantly increased all of these factors. Estrogen receptors were detected in transplanted thyroid tumors but not in euthyroid tissues.

The results suggest that estradiol promoted the thyroid tumorigenesis through activation of thyrotrophs in pituitary or direct interaction of estradiol and estrogen receptors in the thyroid.

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This work was supported in part by Grant-in-Aid for Cancer Research from the Ministry of Education, Science and Culture of Japan.

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