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Stimulation of placental, fetal and neonatal growth by thyroxine administration to pregnant rats.

The effects of daily thyroxine (T4) administration to pregnant rats on the growth of their fetuses and placentae were examined. Additional trials examined the effect of such treatment on the postnatal growth of the offspring. Injection of 10 micrograms T4 into the mother caused a tenfold elevation in maternal plasma T4 concentration, which remained elevated for more than 12 h, but returned to basal levels by 24 h after injection. Despite the increase in maternal T4 concentrations, no increase in fetal plasma T4 was found. A greater weight of both the fetuses (20.1%; P < 0.001) and placentae (14.6%; P < 0.05) was found in the young of T4-treated mothers compared with those from control mothers at delivery by hysterotomy on day 20 of gestation. In trials where mothers delivered naturally, birth weights of the young from mothers treated with T4 (10, 20 or 50 micrograms daily) were also significantly heavier than controls, and continued to grow faster postnatally. These data suggest that maternal thyroid hormone status is important for fetal growth, and further indicate that stimulation of the thyroid hormone axis during pregnancy could provide a method for improving postnatal growth.