TRANSFER of 131I-tagged thyroxine (T4) from the human mother to the fetus at term occurs slowly and is possibly incomplete. Even with infusion seven days before parturition, equilibrium is not reached between the maternal and umbilical-cord blood at delivery.1 The same investigators infused tri-iodothyronine (T3) tagged with 131I and suggested from this study that T3 might cross the placenta more readily than T4. In a similar study Kearns and Hutson2 injected 131I-tagged T4 and T3 into pregnant women two to eight hours before inducing delivery. Approximately 5 per cent of levo-T4 . . .
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Raiti et al. (1967) studied this question.
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