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Changes in the concentration of thyroxine in the plasma of rat fetuses during late gestation: influence of ligation of the maternal uterine vein and artery.

Plasma thyroxine levels were detectable in rat fetuses from day 17 of pregnancy and increased exponentially from 3.48 +/- 0.13 (S.E.M.) nmol/l to 11.33 +/- 0.90 nmol/l at birth. Ligation of the maternal uterine vein and artery, performed on day 17 of pregnancy, significantly reduced both the fetal growth rate, observed initially on day 19 of pregnancy (controls: 2.24 +/- 0.02 g; fetuses from ligated horns: 1.71 +/- 0.03 g, P less than 0.001) and plasma thyroxine levels, observed initially on day 20 of pregnancy (controls: 6.56 +/- 0.13 nmol/l; fetuses from ligated horns: 5.41 +/- 0.13 nmol/l, P less than 0.001). In addition we observed positive relationships between plasma thyroxine levels and body weights of the two groups on days 20 and 21 of pregnancy and at birth. These results suggest that relationships observed between plasma thyroxine levels and body weight in fetuses or newborn animals may be explained by the common effect of the placental blood flow mediated by fetal nutrition.