Thyroid function in the term fetus.

Abstract

Measurements of serum total thyroxine, free thyroxine (FT 4 ) and/or immunoreactive thyrotropin (TSH) concentrations were conducted on 21 pairs of maternal and cord blood specimens obtained at the time of normal vaginal or cesarean section delivery. Thyroxine concentrations were similar in maternal and cord blood. FT 4 and TSH concentrations were significantly higher in cord blood samples obtained from infants delivered vaginally or by cesarean section. Serum TSH concentrations also were measured in 11 paired maternal and fetal scalp blood specimens obtained during labor 9 to 177 minutes prior to delivery; fetal scalp blood TSH concentrations exceeded maternal values in all instances. The higher TSH concentra- tions in cord blood were not due to cross reaction with HCG in the radioimmunoassay procedure; the gradient of HCG was opposite to that of TSH. It is also unlikely that the higher TSH concentrations were due to placental thyroid stimulating factor, since this material cross reacts poorly with the TSH antiserum used in the present studies. It is concluded that the gradient of free thyroxine in the human fetus at term is fetal-maternal and that this gradient is probably attributable to a higher fetal serum TSH concentration.