Effect of 3-methylcholanthrene on thyroid function in Sprague-Dawley rats.

Fifty-day-old Sprague-Dawley females received daily feedings of 10 mg of 3-methylcholanthrene (MCA) in sesame oil for 30 days. Controls received only sesame oil. Thyroid activity was estimated using the thyroid secretion rate (TSR) method. TSR of each rat was determined prior to, during, and after MCA treatment. Average TSR of sesame oil controls did not change either during or after feeding. Average TSR of MCAtreated rats decreased approximately 25% during the feeding period when compared to the average TSR obtained prior to the MCA treatment. Average TSR after MCA treatment was comparable to that obtained before administration of the carcinogen. The rate of thyroidal 131Irelease was also determined in MCA-treated rats. Beginning 24 hr after the administration of 131I,each rat received daily intragastric feedings of 1 ml of sesame oil for 3 days, followed by daily feedings of 10 mg of MCA in 1 ml of sesame oil for another 5-7 days. Average hourly release of thyroidal 131Iwas significantly less during the MCA feeding period than during the control period in which the rats received only sesame oil. Average hourly release of thyroidal 131I of diet-restricted animals was reduced to approximately the same degree as that of MCA-treated rats. However, the average plasma FBI concentration of MCA-treated rats was significantly less than that of diet-restricted rats. These data suggest that the effects of MCA administration on thyroid function may be partially, but not entirely, due to decreased food intake by the treated animals.