Fetal plasma thyroxine levels as well as thyroxine turnover and placental permeability to this hormone were investigated in the conscious pregnant ewe with fetal and maternal intravascular catheters. Fetal plasma thyroxine levels ranged from 4.6 to 6.2 μg/100 ml between 103 days of gestation and the day of birth. Maternal plasma thyroxine levels varied between 2.3 and 4.1 μg/100 ml over the same period. The maternal to fetal ratio across the placenta for thyroxine varied from 0.52 to 0.65. Distribution of radioactive thyroxine injected into the fetal or maternal circulation demonstrated the impermeability of the placenta to thyroxine. Maternal to fetal ratios for labelled thyroxine were 6.2 to 11.9 when injected into the maternal ciruclation and 0.013 to 0.003 when injected into the fetal circulation. The sheep placenta appears to be capable of actively transporting iodide to maintain a fetal to maternal iodide ratio of up to 8:1. Fetal thyroxine utilization was of the same order at 111 days of gestation as immediately before parturition when expressed per unit body weight. Utilization of thyroxine per kg by the fetus was about five times that of the mother. Various factors which influence thyroid function are discussed and the activity of the fetal pituitary thyroid system is compared with other fetal endocrine systems.
ASJC Scopus subject areas
- Endocrinology, Diabetes and Metabolism