Thyroid hormones and experimental cancer cachexia

The relationship between circulating thyroid hormones and nutritional status was studied in sarcoma-bearing inbred C57BL/6J mice and control mice. Supplementation with exogenous thyroxine (T4) was also evaluated. Tumor-bearing animals had depressed levels of circulating thyroid hormones. This was also found in food-restricted (pair-fed and pair-weighed) controls. Plasma levels of thyroid hormones decreased with increased tumor burden. Thyrotropin-releasing hormone caused an increased response of thyroid-stimulating hormone in tumor-bearing animals. Low levels of thyroid hormones in sarcoma-bearing mice were due to depressed hormone production by the thyroid gland rather than to increased clearance rate of hormones. Plasma levels of triiodothyronine (T3) correlated to the amount of whole-body nitrogen among sarcoma-bearing mice and food-restricted controls. Exogenous T4 increased food intake by 20% in sarcoma-bearing mice. The benefit of this was probably counteracted by an increased metabolic rate, since reversal of plasma levels of T3 and free T4 had no net effect on body composition of freely eating sarcoma-bearing mice, although it had a negative effect on body and muscle composition in food-restricted controls. Exogenous T4 did not stimulate tumor growth. The results indicate that low circulating levels of thyroid hormones in experimental cancer cachexia are probably caused by the reduced food intake (anorexia), which is in agreement with findings in clinical cancer. Depression of thyroid hormones is probably a physiological means to reduce energy expenditure and to preserve substrates in progressive cancer disease.