Circulating concentrations of triiodothyronine (T3) were measured in human serum by the method of Sterling et al. (1). Values in control nonpregnant subjects were compared with women in all three trimesters and at term. Concentrations of T3 in blood increased from a mean of 196 ng/100 ml during the first trimester to 299 ng/100 ml in the third trimester. Values during labor averaged 421 ng/100 ml as compared with 193 ng/100 ml in corresponding cord blood. These studies indicated a progressive rise in total T3 concentration during pregnancy that paralleled the increase in circulating thyroxine (T4) and thyroxine binding globulin (TBG) normally observed. They provide further information on the binding of T3 to TBG in vivo and on the interrelationships of T3, T4, and TBG in serum.
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Hotelling et al. (1971) studied this question.