Endocrine studies in the untreated F1 and F2 progeny of rats treated neonatally with thyroxine.

When neonatal rats are made hyperthyroid with large doses of thyroxine during the first 5 days of life, they develop hypothalamopituitary, thyroidal and gonadal abnormalities that persist through life. When female rats, which have been treated neonatally, are subsequently mated to normal males, their offspring show unexpected gonadal and thyroidal defects, such as reduced weaning weight and delayed vaginal opening and first estrus in females; males show a significant increase in relative thyroid weight and a significant decrease in ventral prostate weight. Cross-fostering was done to separate prenatal from postnatal influences. Even more surprising was the finding that when the untreated female F1 progeny were mated with normal males, the untreated F2 progeny showed more defects and to a greater degree than those present in the maternal parent.