Effect of a Malignant Tumor on Thyroxine Metabolism and Thyroid Function in the Rat1

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Abstract
The effects of a malignant tumor (Walker 256) on thyroid hormone economy were assessed. PBI [protein-bound iodine] in serum was considerably reduced in tumor-bearing animals. Urinary excretion of 131I in tumorous rats isotopically equilibrated with 131i-labeled T4 (5 [mu]g/day) was significantly greater than in corresponding control animals, indicating that the deiodination of T4 was increased. This Increase was associated with a reduction in the concentration of 131l-T4 in serum; thus, the rate of clearance of T4 from plasma was greatly increased in the tumorous rats. This increase was due, not only to the enhanced rate of deiodination, but also to an increased rate of clearance by the gastro-intestinal pathway. The increased rate of clearance was apparently not the result of a primary stimulation of intracellular delodinating mechanisms; no change in the in vitro deiodination of T4 in tissues of tumorous animals was observed and deiodination by the tumorous tissue itself was minimal. In tumorous animals, increases were noted in the proportion and absolute concentration of free T4 and in the red blood cell uptake of 131I-T4 from serum. Studies of the thyroid gland Idicated hypofunction; the rate of uptake of iodide by the gland was greatly decreased in the tumorous animals, although glandular iodine greatly decreased in the tumorous animals, although glandular iodine content was unchanged. It is concluded that the changes in thyroid hormone economy In tumorous animals are secondary to decreased protein binding of T4, but it is not known whether this represents a nonspecific response to illness or a specific effect of the tumorous state.