The mechanism of low serum T3 in the fetus: hepatic T4-5'-monodeiodinase versus tissue sulfhydryl content--a clarification.

Using optimal tissue concentration, supplemented with a thiol-protecting agent, the fetal liver has lower T4 5'-monodeiodinating activity than maternal or neonatal tissue. This is a true deficiency and is not due to deficiency of sulfhydryl groups in fetal tissue as previously suggested. Evidence indicates a dissociation of the neonatal surge of serum T3 and the increase in hepatic T4 to T3 conversion activity, suggesting that the neonatal T3 surge is related more to thyroid gland stimulation than to T4 to T3 conversion in non-thyroid tissues.