The effect of cortisol infusion on plasma T4 and T3 concentrations has been investigated in the fetal lamb at 130–134 days of gestation. Infusion of cortisol at rates equivalent to one-third the quantity produced by the fetus at the time of parturition results in a significant fall in fetal plasma T4 concentration and a significant rise in fetal plasma T3 concentration. Delay in the initiation of labor as a result of progesterone administration to the ewe did not prevent the changes in the fetal thyroid axis. These modifications of thyroid function therefore appear to be caused by the infusion of cortisol rather than to be secondary to the induction of labor.
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