Carcinogenic effect of 100, 250 and 500 rad X-rays on the rat thyroid gland.

Male rats were given 0, 100, 250 or 500 rad x-rays to the thyroid gland at 9-12 weeks of age and killed 18-20 months later. No thyroid tumours were found in the unirradiated animals, a few follicular adenomata developed after 100 and 250 rad and a follicular carcinoma after 500 rad. A similar group was set up and maintained on a diet with thyroxine added in a quantity to give a daily consumption of 5-6 μg/100 g body weight in order to suppress TSH secretion. Thyroid tumour production was considerably lowered. At the termination of the experiment the efficiency of the TSH suppression was tested by measurement in some rats of T/S (131)I concentration ratios and in others of the 24 h% (131)I thyroid uptake. In some rats TSH was totally suppressed, in others partially suppressed. A further group was set up and maintained on the goitrogen 0·1% aminotriazole in the drinking water to cause excessive TSH secretion. All its subgroups, including unirradiated animals, developed numerous follicular adenomata and carcinomata. Enhancement of carcinoma production was present in the 500 rad subgroup. It was concluded that the development of thyroid adenomata after the above doses of x-radiation may occur without an excessive rise in TSH secretion, that suppression of TSH lowers radiation tumour production and that therefore TSH may play a permissive role in the development of thyroid tumours following low dose x-radiation to the thyroid gland.