Inhibition of Local and Metastatic Hepatoma Growth and Prolongation of Survival after Induction of Hypothyroidism¹

The local growth rate of Morris Hepatoma 44 (generation time, 6 months) was inhibited by 66 to 87%, and host survival was prolonged by 36 to 78% after the induction of hypothyroidism within 2 weeks of tumor implantation by propylthiouracil (0.1% in Purina chow), ¹³¹I (1 mCi/100 g body weight i.p.), or surgical thyroidectomy. In additional experiments, we studied the effects of inducing hypothyroidism (¹³¹I) at different stages in the natural history of Morris Hepatoma 44 on local and metastatic growth as well as on host survival. Induction of hypothyroidism within 2 weeks of tumor implantation (Group I) reduced local tumor growth as well as the number and size of pulmonary metastases, and prolonged survival by 70 to 80%. Induction of hypothyroidism at 6 weeks postimplantation when tumors were palpable (Group II) inhibited local growth by 39%, reduced the number and size of pulmonary metastases by approximately 80%, and prolonged host survival by 35%. Initiation of ¹³¹I treatment at 11 weeks when microscopic pulmonary emboli were present in most animals (Group III) reduced local growth by 19% and the number and size of pulmonary metastases by 72 and 50%, respectively. In this case, survival was prolonged by 17%. We conclude from these results that the local and metastatic growth of Morris Hepatoma 44 as well as host survival are thyroid hormone-dependent processes. The mechanisms responsible for these observations remain to be explained.