Suppression of Thyroid Function by Pituitary Tumor MtTW5

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Abstract
A transplantable pituitary tumor, MtTW5, which secretes prolactin and growth hormone but no TSH (thyroid stimulating hormone) or ACTH (adrenocorticotropic hormone) causes a marked increase in total body weight and in the weight of most internal organs. However, the pituitary glands of tumor-bearing rats weighed significantly less in proportion to body weight than did pituitaries in control rats. Incorporation of I131 into the trichloroacetic acid-soluble and the protein fractions of the thyroid and serum was measured. Less I131 was incorporated into these fractions of the thyroid and into the protein fraction of serum of tumor-bearing rats than in control rats. These results were obtained in intact female, spayed female, neph-rectomized female and male rats. After acute treatment with propyl-thiouracil, tumor-bearing rats incorporated less I131 into their thyroids than did controls similarly treated with propylthiouracil. Thus, the primary defect in the thyroid of tumor-bearing rats is in its inability to concentrate iodine from the blood. Histological examination of the thyroid revealed a significant reduction in the height of the follicular epithelium of tumor-bearing rats. Chromatographic separation of thyroid hydrolysates revealed that radioactivity in the inorganic iodide fraction was decreased 76% in tumor-bearing rats as compared with controls. The organification of I131 was decreased to the same extent. The serum iodide concentration was the same in both groups. Injection of TSH into tumor-bearing rats caused a return of thyroid function toward normal. The TSH content of the pituitary gland of tumor-bearing rats was 70% of that found in controls. Thyrotropic cells in pituitary glands of tumor-bearing rats were fewer in number than those in controls. These results show that hormones secreted by pituitary tumor MtTW5 cause atrophy of the pituitary gland, suppression of thyroid function and a decrease in pituitary gland TSH content.

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