Studies on the Pituitary Body-Thyroid Gland System of Fetus and Response to Labor as Viewed from TSH, T_4, T_3, rT_3 and Thyopac-3

The TSH, T_4, T_3, rT_3 and Thyopac-3 levels in maternal blood and umbilical arterial blood were determined for a better management of labor, delivery and of the newborn immediately after delivery.
The TSH level in both maternal and umbilical arterial blood immediately after delivery was high, while the T_3 level in umbilical arterial blood was low. Thus it was surmised that the feed-back mechanism of the pituitary body-thyroid gland system of the fetus had been completed.
From the results of T_4, Thyopac-3 and FT_4I, it was deduced that TBC would increase in excess of maternal blood T_4 and that fetal blood T_4 would be secreted to about the same extent in women with a normal thyroid function.
The T_3 level and the T_3/T_4 ratio in maternal blood were high, while the T_3 level and T_3/T_4 ratio in umbilical arterial blood were extremely low. Conversely, the rT_3 level and the rT_3/T_4 ratio in maternal blood was low, while the rT_3 level and the rT_3/T_4 ratio in umbilical arterial blood were high.
Thus, there appears to be a reciprocal relationship between T_3 and rT_3 and that these are probably converted from T_4 to T_3 with strong biological activity in the mother and from T_4 to biologically inactive rT_3 in the fetus.