Alterations in Tissue Thyroxine-5′-Monodeiodinating Activity in Perinatal Period*

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Abstract
In vitro conversion of T4 [thyroxine] to T3 [triiodothyronine] by liver and kidney homogenates was studied in maternal and fetal (or neonatal) sheep sacrificed before the onset of labor (control, group 1), during labor (group 2), at 2-12 h after birth (group 3), and after cortisol administration (20 mg every 8 h for 3 days) to the fetus (group 4). T3 produced from T4 by liver and kidney averaged (mean .+-. SE; ng T3 produced/.mu.g T4 per geq tissue per h at 37.degree. C) 99 .+-. 19 and 32 .+-. 6.2, respectively, for mother and 27 .+-. 4.0 and 1.9 .+-. 0.5, respectively, for fetal group 1. The values of T3 produced from T4 by liver homogenates of fetal groups 2-4 were 84 .+-. 20, 266 .+-. 23 and 107 .+-. 30, respectively, which were all significantly higher than that in the control fetal group. T3 produced by liver homogenates of fetal group 3 was also significantly higher than that of fetal group 2 or 4. The T4 to T3 converting activities in kidney homogenates of fetal groups 1, 2 and 4 were comparable to one another but were all clearly lower than that in fetal group 3 as well as the maternal group. Kinetic studies using liver homogenates suggested that the various above-mentioned changes in fetal tissues are associated with a similar change in the Vmax but no demonstrable change in the apparent Km of conversion of T4 to T3. T4 and T3 converting activity in fetal tissues is lower than that in maternal tissues and there is a progressive increase in the T4 to T3 converting activity of the extrathyroidal tissues of the fetus from before-labor through during-labor to neonatal period. Cortisol enhances T4 to T3 conversion by some extrathyroidal tissues of the fetus, and the various changes in T4 to T3 conversion in fetal tissues are due mainly to the changes in capacity rather than the affinity of the putative enzyme activity.