Effect of Thyroactive Materials Upon Mammary Gland Growth and Lactation in Rats

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Abstract
The feeding of thyroxine and triiodothyronine to rats at levels approximately 13-14 times above the normal thyroxine secretion rate during pregnancy resulted in a marked reduction in the survival rate of the litters, even though a control feed was fed to the dams during lactation. These results did not take place if the thyroactive feed was withdrawn on the 19th day of pregnancy. These levels of thyroactive materials did not affect the litter size or average weight of the young at parturition. The rats receiving the thyroactive materials throughout pregnancy had higher amounts of dry, fat-free tissue and nitrogen contents of the abdominal-inguinal mammary glands at parturition than the control animals. In a second experiment it was found that the feeding of these compounds after parturition at a level approximately 10 times above the normal thyroxine secretion rate resulted in a marked decrease in the number of litters surviving through 21 days of lactation and in the growth rate of the surviving young. These results indicate that thyroactive materials fed at these levels cause a marked inhibition of milk secretion but have a stimulatory effect on mammary gland growth.