Placental Transmission and Mammary Gland Secretion of Thyroxin in the Rat

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Abstract
When large doses of thyroxin are administered to pregnant rats, propylthiouracil goitrogenesis in the fetuses as well as in the mother is prevented. One hour following the intravenous injection of thyroxin-I131 into pregnant rats a small fraction of TCA-precipitable, butanol-extractable radioactivity was recovered from homogenates of their fetuses. Part of this was shown by means of Kieselguhr column chromatography to represent thyroxin. Large amounts of thyroxin administered to lactating rats over a 10-day period failed to prevent propylthiouracil-induced goiter in their suckling offspring. It is concluded that the rat placenta is slightly permeable to thyroxin but the rat mammary gland does not secrete thyroxin in a physiologically active form.