Surge of fetal plasma triiodothyronine before birth in sheep

Plasma thyroid hormone and cortisol concentrations were measured from early third trimester to delivery in seven pregnant ewes and their fetuses. A surge in fetal plasma triiodothyronine was found during the six days prior to delivery, the mean values rising from less than 30 to 125 ng. per deciliter. During this period the fetal plasma cortisol also increased from less than 1 to 8.1 microgram per deciliter. During this period there was no significant change in fetal plasma thyroxine (T4) or reverse T3 concentrations and no change in maternal plasma concentrations of any of these hormones. The abrupt increase in fetal plasma T3 concentrations with no rise in plasma T4 suggests that there was an alteration in extrathyroidal conversion of T4 to T3 rather than an increase in thyroid secretion, since the latter would be expected to cause an increase in both plasma T4 and plasma T3. The changes in plasma T3 and cortisol concentrations in a fetus that died in utero eight to 10 days before term resembled those occurring prior to normal parturition.