Enhancing effect of thyroxine on tumor growth and metastases in syngeneic mouse tumor systems

We studied the effect of thyroxine treatment on tumor growth and metastases resulting from tumor implants on the hind feet of mice in two syngeneic systems. In control, untreated A/Jax mice, tumor Sarcoma 1 at Day 14 after implantation had average tumor weight of 582 +/- 60 (S.D.) mg and showed an incidence of 57% metastases to regional popliteal nodes and 5% metastases to thymus. In contrast, the thyroxine-treated group (40 microgram/mouse s.c., 5 times/week for 1 month) had an average tumor weight of 808 +/- 56 mg (p less than 0.001), and metastases to popliteal nodes and thymus were 90 and 35%, respectively. In another syngeneic tumor system, Lewis fibrosarcoma was implanted in C57BL/6J mice, and the tumor weight and metastatic index (derived from the number and size of the pulmonary tumor foci) were determined at Day 28. Again, the synthetic L-thyroxine treated group showed a significant enhancement tumor growth and metastatic index. The mean tumor weight in the treated group was 385 +/- 26 mg (control, 694 +/- 25 mg; p less than 0.005) and metastatic index was 84 +/- 29 (control, 30 +/- 25; p less than 0.001). Induced hypothyroidism (treatment with 131I, 100 microCi/mouse i.p.) showed the reverse effect on both tumor systems. These results suggest that both tumor systems are dependent on thyroid hormones for their growth and spread.