Thyroid function in the perinatal period

In the sheep fetus before 130 days gestation, plasma thyroxine and reverse triiodothyronine are elevated compared with the concentrations in the ewe. In contrast plasma triiodothyronine concentrations are low. In the 10 days or so before delivery, fetal plasma triiodothyronine concentration rises gradually and plasma thyroxine may fall slightly. These changes can be reproduced by the infusion of physiological doses of cortisol to the fetus at 130 days gestation. Very similar changes have been observed in the calf before and after term and cortisol induced premature delivery. After delivery at term there is a further rise in neonatal plasma triiodothyronine concentration. This rise is due to a further increase in peripheral deiodination probably resulting upon the increase in pO₂ and the change in the external environment.