Changes in the fetal thyroid axis after induction of premature parturition by low dose continuous intravascular cortisol infusion to the fetal sheep at 130 days of gestation

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.