Pituitary /umors in Rodents Following Changes in Thyroid Function" A Review*

It is the primary purpose of this review to examine the problem of the genesis of tumors of the pituitary gland following thyroid removal or destruction by radioiodine. Since this problem is now under active investigation, it is hoped that such a summation of the pertinent data may be helpful in evaluating what has been done and in orienting research that may follow. Radioiodine-induced hypophyseal growths are especially interesting, because they occur in response to a single experimental manipulation in such high incidence (almost 100 per cent) that they can be distinguished readily from normal or "spontaneous" tumorous growths in old animals. Furthermore, the high incidence of the tumors in response to the Ira-treatment suggests that a definite analyzable mechanism for atypical growth is triggered by the treatment. The frequency of the pituitary tumor permits the use of relatively small numbers of animals and thus facilitates analysis of the factors which precipitate tumorous growth. The fact that the tumorous organ is endocrine creates an especially valuable and interesting situation, since the physiologic properties and responsiveness of the atypical tissue can be subjected to study at any stage. Before concentrating on the best studied situation, the experimental elicitation of hypophyseal tumors by radiothyroidectomy in C57BL mice, it would be well to consider, even briefly, the general problem of hypophyseal tumorigenesis in rodents. Within this context the more specific problem can be better evaluated. At least five different factors now have been reported as effective in rats and mice in bringing about tumorous hypophyseal enlargement: (a) old age, (b) hypothyroidism (with or without radiation), (c) gonadectomy, (d) chron-