Effect of hypophysectomy on the induction of thyroid tumours in rats

Hypophysectomized and sham-hypophysectomized rats were injected with 3 µc 131I or not and placed on a low iodine diet with or without an iodide supplement. The thyroid glands were examined under the light microscope and tumours counted and measured by a histometric technique developed in this department. The incidence of animals with tumours in the sham-hypophysectomized group given 3 µc 131I and a low iodine diet was 96% with a mean tumour number of 5.5 and a mean tumour volume of 0.57 x 10^8 cu µ. Sham-hypophysectomized animals given 131I alone did not develop tumours and hypophysectomized animals given a low iodine diet alone, 131I alone, or a combination of both, also did not develop thyroid tumours. It is concluded that 3 µc 131I alone is not tumourigenic within six months; and that in the absence of TSH stimulation, thyroid tumourigenesis is unable to occur following a low iodine diet, alone or in combination with 3 µc 131I