Malignant Thyroid Tumors Occurring in the Mouse After Prolonged Hormonal Imbalance Duringthe Ingestion of Thiouracil*

IT IS generally recognized that the surgical removal of an endocrine gland will affect the secretion of other endocrine glands. It would be expected, too, that a chemical agent capable of specifically blocking the secretion of a single endocrine gland would also alter the normal hormonal balance. Experiments on mice have been in progress for over seven years in our laboratories on the effects of long continued chemically produced hormonal imbalance (1). The goitrogenic drug, thiouracil, in these experiments has been the chemical agent used to interfere with the secretions of the thyroid gland. The expected hypertrophy and hyperplasia of the thyroid gland of mice treated for prolonged periods with thiouracil were accompanied by structural changes in other endocrine glands notably the adrenal, pituitary, testes and ovary (2, 3). Young mice ingesting thiouracil continuously also ceased increasing in body weight at an early age. Estrus was inhibited or completely suppressed. Mammary gland development was arrested, and mammary-tumor formation in susceptible mice was reduced to a very low incidence or to zero (3, 4).