Plasma Triiodothyronines in Fetal Sheep: Effects of Illness and Thyroidectomy

Abstract

Plasma 3,5,3-triiodothyronine (T₃) and 3,3',5'-triiodothyronine concentrations were measured in fetal sheep prior to death in utero and after thyroidectomy. In six fetal sheep who subsequently died in utero, plasma rT₃ concentrations were elevated in all for 2 to 13 days prior to death. There were no consistent changes in plasma T₄ concentrations. In two thyroidectomized fetal sheep, plasma T₄ and rT₃ concentrations fell to low levels. Plasma T₃ concentrations remained low and there was no increase in plasma T₃ in the last week prior to parturition like that which occurs in normal fetal sheep. Parturition was preceded by the normal increase in fetal plasma Cortisol concentrations and occurred at the normal time. These data indicate that plasma rT₃ concentrations are increased as a result of illness in fetal sheep and that such measurements may be useful as an indicator of fetal distress. The normal increase in plasma T₃ late in gestation is not necessary for the late gestational Cortisol surge or for normal parturition.

Key words

Fetal Distress - Fetal Sheep - Reverse Triiodothyronine - Thyroxine - Triiodothyronine