Abstract
Extract: Kinetic studies of thyroxine (T4) and triiodothyronine (T3) and measurements of hormone turnover were conducted in chronically catheterized maternal and fetal sheep during the last trimester of gestation. Mean serum total T4 and free T4 (FT4) concentrations were higher in fetal than in maternal serum (7.5 versus 5.4 $mUg/100 ml and 5.1 versus 2.4 ng/100 ml, respectively), whereas total T3 and free T3 (FT3) concentrations were higher in maternal than in fetal serum (74 versus <18 ng/100 ml and 176 versus <90 pg/100 ml, respectively). Mean maximal thy-roxine-binding globulin (TBG) binding capacity was higher in maternal than in fetal blood (16.5 versus 8.1 $mUg T4/100 ml, respectively). Fetal serum was observed to contain a T4-binding α-1-globulin, which probably is identical with fetuin, a glycoprotein unique to sera of fetuses and neonates of several species including the calf, foal, sheep, pig, and chick. Mean T4 clearance values were 0.614 and 0.107 liters/kg/24 hr, respectively, in fetal and maternal sheep and mean T3 clearance values were 8.52 versus 1.66 liters/kg/24 hr, respectively. Mean T4 turnover values were 46.1 and 5.59 $mUg/kg/24 hr, and mean T3 turnover values were < 1.50 and 1.22 $mUg/kg/24 hr in fetal and maternal sheep, respectively. No placental transfer of T4 occurred and net maternal to fetal transfer of T3 amounted to only about 1% of total turnover of fetal thyronine.
These data confirm the autonomy of the fetal hypothalamic-pituitary-thyroid axis and they indicate that hormone utilization is much higher in fetal sheep (per kilogram body weight) than in maternal sheep. In addition, the higher T4/T3 serum concentration ratio and T4/T3 turnover ratios in the fetus (>471/1 versus 74/1 and >31/1 versus 4.6/1) indicate that the relative rates of T3 secretion and/or T3 conversion from T4 in peripheral tissues are less in fetal than in maternal sheep. Finally, the data indicate a poor correlation between free hormone concentrations and either hormone clearance or hormone turnover rates, which suggests tissue binding and/or metabolism are important determinants of thyroid hormone turnover in the fetus.
Speculation: The present data regarding thyroid hormone metabolism in fetal sheep are consistent with present evidence about thyroid hormone metabolism in the human fetus and suggest that the sheep can serve as a useful model for the human system. Furthermore, data in both species support the concept of autonomy of fetal thyroid function. The ob-
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Fisher, D., Dussault, J., Erenberg, A. et al. Thyroxine and Triiodothyronine Metabolism in Maternal and Fetal Sheep. Pediatr Res 6, 894–899 (1972). https://doi.org/10.1203/00006450-197212000-00007
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DOI: https://doi.org/10.1203/00006450-197212000-00007