[Studies on the relationship between reproductive phenomena and thyroid function--dynamics of the concentrations of...

Thyroid function during pregnancy, parturition and puerperium was studied by measuring the concentration of free thyroxine (f-T4), free triiodothyronine (f-T3) and reverse triiodothyronine (r-T3) in maternal and cord sera, and simultaneously metabolism of thyroid hormones in chorionic tissues was examined. Results obtained were as follows: In cases of normal pregnancy, maternal f-T4 and f-T3 levels tend to decrease during the course of pregnancy toward parturition. Pathological pregnancies such as intra uterine growth retardation (IUGR) and toxemia (H, h) showed a tendency for maternal f-T3 to decrease and for r-T3 to increase, compared to normal cases at the same gestational age. Maternal f-T3 levels were significantly higher and r-T3 were lower than those of cord sera. Inner monodeiodination activity was found in the placenta, and T4 incubated with chorionic tissue was mainly converted to r-T3. From these findings, it was suggested that the metabolic state of pregnant women would become hypometabolic during the course of pregnancy toward parturition. Higher r-T3 and lower f-T3 levels in pathological pregnant women indicated that T4 to r-T3 conversion might be more dominant than T4 to T3 because of the maternal intensified hypometabolism. In the meanwhile, placental inner monodeiodination seems to prevent active movement of thyroid hormones from the dam to fetus.