Thyroid Function in the Foetal Lamb During the Last Third of Gestation

SUMMARY Foetal plasma thyroxine levels as well as thyroxine turnover and placental permeability to this hormone were investigated in the conscious pregnant ewe with foetal and maternal intravascular catheters. Foetal 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: foetal ratio across the placenta for thyroxine varied from 0·52 to 0·65. Distribution of radioactive thyroxine injected into the foetal or maternal circulation demonstrated the impermeability of the placenta to thyroxine. Maternal to foetal ratios for labelled thyroxine were 6·2 to 11·9 when injected into the maternal circulation and 0·013 to 0·003 when injected into the foetal circulation. The sheep placenta appears to be capable of actively transporting iodide to maintain a foetal to maternal iodide ratio of up to 8:1. Foetal 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 foetus was about five times that of the mother. Various factors which influence thyroid function are discussed and the activity of the foetal pituitary—thyroid system is compared with other foetal endocrine systems.