Inhibition of the growth of Morris hepatoma No. 44 in rats after induction of hypothyroidism: evidence that Morris hepatomas are thyroid dependent.

The growth rate of Morris Hepatoma No. 44 (generation time, 6 mo) was inhibited after the induction of hypothyroidism by Propylthiouracil (PTU) (0.1% in Purina Chow), I131 (1 mCi/100 g body wt i.p.), or surgical thyroidectomy. After 11 wk of treatment, hepatoma weight was 66%, 87%, and 75% (after correction for total body wt) relative to controls in PTU-fed, I131 injected, and thyroidectomized rats, respectively. In each case, exogenous thyroxine (T4) (8 microgram/kg body wt i.p.) reversed these inhibitory effects, while T4 administered to euthyroid rats stimulated hepatoma growth. The degree of growth-inhibition achieved with PTU was not observed in pair-fed rats. In addition, after correction for differences in body weight, the sex of the tumor-bearing rats did not influence the response to PTU. Pretreatment with PTU for 2 wk before implantation did not give any added advantage over the effects of PTU administered approximately 10 days after implantation. Serum levels of triiodothyronine (T3) and T4, as well as the concentration of various biochemic parameters, were determined at the time of death. These results suggest that the growth rate of Morris Hepatoma No. 44 is thyroid hormone dependent.