Glucocorticoids have been shown to accelerate the development of blood coagulation in the fetal lamb (Kisker, Robillard & Bohlken, 1983). The current studies examine the influence of high levels of triiodothyronine (T3) on the development of blood coagulation in the fetal lamb. Eight twin fetal lambs were studied during the last trimester of pregnancy (109-138 days gestation) using chronically placed arterial and venous catheters for infusion of T3 and withdrawal of blood samples. One fetus of each twin was infused intravenously with T3 at a constant rate of 0.6 mug T3/0.4 ml/h for 48 h. The other twin was infused with 5% dextrose at 0.4 ml/h for 48 h. Blood samples for measuring PT, PTT, TT, fibrinogen, factors II, V, VII, VIII, IX, X, XI and XII were obtained prior to and at the completion of the infusions. The results were analyzed for differences between the samples from paired control and T3 treated animals. Factor V significantly decreased in the T3 infused animals from 51.2% +/- 12.5% to 44.8% +/- 13.8% (P=0.038). Factor VII also decreased in the treated animals 57.8% +/- 20% to 43.6% +/- 10.8%, but the changes were not statistically significant (P=0.06). Factor XII significantly increased in the treated animals from 37% +/- 10.5% to 45% +/- 13.7% (P=0.004). High levels of T3 in third trimester fetal lambs are accompanied by a moderate decrease in factor V and an increase in factor XII activity.

Prothrombin levels in the fetal and newborn lamb follow a gestational age-related pattern of development. The changes in activity are reflected by similar changes in prothrombin antigen concentration. Our investigations indicate that these changes in prothrombin antigen and activity levels are paralleled by changes in the levels of prothrombin messenger RNA present in liver samples from fetuses of various gestational ages.