THYROID FUNCTION IN THE PRETERM FETUS

Abstract

Measurements of serum total thyroxine (T4), free thyroxine (FT4), and/or immunoreactive thyrotropin (TSH) were conducted on 14 pairs of maternal and cord blood specimens obtained at the time of elective therapeutic abortion of 11- to 18-week pregnancies and on 21 paired maternal and fetal cord blood specimens collected at the time of spontaneous, premature delivery of 22- to 34-week pregnancies. T4 concentrations were elevated in all maternal samples to levels characteristic of pregnancy; mean values were similar at 11 to 18 and at 22 to 34 weeks and did not differ from the mean level reported previously at term. Mean maternal TSH concentrations also were similar at 11 to 18 weeks, 22 to 34 weeks, and at term. The mean FT4 concentration in maternal serum between 11 and 18 weeks was significantly higher than the level reported previously at term. Fetal serum T4 and FT4 concentrations were low between 11 and 18 weeks and increased progressively between 22 weeks and term. Fetal serum TSH concentrations were low between 11 and 18 weeks of pregnancy but seemed to increase abruptly between 18 and 22 weeks to levels characteristic of term infants. These data suggest that the maternal concentration of FT4 is higher early in pregnancy than at term, perhaps because of the higher blood levels of human chorionic thyrotropin in early pregnancy. They also indicate autonomous function of the fetal hypothalamic-pituitary control system as early as 12 weeks' gestation. The abrupt increase in fetal serum TSH between 18 and 22 weeks suggests rapid maturation of the fetal hypothalamic-pituitary unit during this period. The subsequent progressive increase in fetal FT4 concentration indicates an increasing thyroidal response to the TSH stimulus.