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JEAN DUSSAULT, VAS V. ROW, GORDON LICKRISH, ROBERT VOLPÉ, Studies of Serum Triiodothyronine Concentration in Maternal and Cord Blood: Transfer of Triiodothyronine Across the Human Placenta, The Journal of Clinical Endocrinology & Metabolism, Volume 29, Issue 4, 1 April 1969, Pages 595–603, https://doi.org/10.1210/jcem-29-4-595
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Abstract
Serum total triiodothyronine (T3) concentrations have been determined in pregnant women and newborn infants. The total serum T3 in pregnant women was similar to that found in nonpregnant control persons, in contradistinction to the marked elevation of serum thyroxine (T4) in pregnancy (due to increased thyroxine binding globulin). Cord blood samples showed the newborn serum T3 values to be similar to the maternal and normal values. The administration of varying doses of T3 (50–300 μg/day) to pregnant subjects for as long as 3 weeks before parturition caused a rise in both maternal and newborn T3 values, indicating the placental transfer of T3. However, newborn T3 values were significantly lower than maternal values, indicating a partial placental block to T3 transfer. The use of T4/T3 ratios and free T3 concentrations also helped to demonstrate placental T3 passage. The observation that the increased TBG of pregnancy does not cause an increase in total serum T3 again confirms the negligible role that TBG plays in the in vivo transport of T3, and indicates that it is not a factor in the partial placental retention of T3 on the maternal side.