Thyroid hormone kinetics during late pregnancy in the ovine fetus.

The factors responsible for the changes in the plasma concentrations of thyroid hormones in the ovine fetus in late pregnancy were investigated by making serial measurements of the concentrations, metabolic clearance rates and production rates of T3 and T4 in 17 fetuses. The concentrations of T3 in fetuses of 135-145 days gestational age were four times higher than in those of 110-125 days but the concentrations of T4 were unchanged. The metabolic clearance rate of T3 halved over this period whereas that of T4 rose slightly. The production rate of T3 more than doubled and of T4 increased slightly but not significantly. We conclude that the concentration of T4 shows little change with increasing gestational age because the trends in metabolic clearance rates and production rates are weak and in the same direction. The sharp rise in the concentration of T3 is attributable to a fall in metabolic clearance rate coupled with a rise in production rate.