Altered Thyroxine Metabolism Resulting from the Chemical Carcinogen 3-Methylcholanthrene

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Abstract
Fifty-day-old virgin female Sprague-Dawley rats received daily sc [subcutaneous] injections of a fixed volume of 131I-labeled Na L-thyroxine for 17 days. When in iodine balance, untreated control rats and those receiving daily intragastric feedings of sesame oil excreted approximately 60% of the daily injected dose of 131I as iodide in the urine and the remainder as thyroxine in the feces. In contrast, rats receiving daily intragastric feedings of 10 mg 3-methylcholanthrene (MCA) excreted approximately 70% of the daily dose of radioactivity in the feces. As determined by paper chromatog-raphy, the administration of MCA did not change the chemical form of the 131I excreted in urine and feces. In addition, the plasma PBl31I concentration of MCA-treated rats was reduced to approximately half that of sesame oil-treated rats. The effects of MCA on thyroxine metabolism were apparently not mediated by increased fecal mass, since food intake was carefully matched and fecal mass was quite low in all groups.