Changes in fetal thyroid hormone levels in adrenalectomized fetal sheep following continuous cortisol infusion 72 h...

Continuous infusion of cortisol into adrenalectomized fetal sheep during the last 72 h of gestation (term = 145 +/- 2 days) produced a significant (P < 0.05) rise in fetal serum tri-iodothyronine (T3) mean +/- S.E.M. concentrations from 398 +/- 65 to 1340 +/- 238 ng/l. A concurrent decrease in plasma thyroxine (T4) levels was observed in three out of four animals. No significant changes in the concentrations of either hormone were noted prior to the start of cortisol infusion. The plasma concentrations of cortisol, T3 and T4 at term were similar to those in untreated full-term lambs. Adrenalectomized fetuses not given cortisol infusions still had low levels of T3 at term, with no increase being observed. The results suggest that cortisol plays an important role in the increase of fetal plasma T3 observed towards the end of gestation. This is probably achieved by the stimulation of the monodeiodination of T4 to T3 in the peripheral tissues.