To evaluate the diagnostic value of the ratio of serum tyroxine$(T_4)$/thyroxine-binding globulin (TBG) for the thyroid status in pregnancy and newborn serum thyroxine, TBG, triiodothyronine, and free thyroxine levels were radioimmunoassayed in normal pregnant women at each of the trimesters, and the calculated serum $T_4/TBG$ ratios were compared with other parameters such as $T_3/TBG$ ratio and free $T_4/TBG$ ratio and free $T_4/TBG$ ratio. Serum $T_4$ levels were elevated with the propotionate increase in TBG levels during pregnancy, leading to the nearly constant value of serum $T_4/TBG$ ratios as in normal non-pregnant controls. In contrast, serum $T_3/TBG$ and free $T_4/TBG$ ratios varied considerably during pregnancy. In newborn, $T_4$ levels were nearly not changed with compared non-pregnant control value and TBG levels were elevated. The results indicate that serum $T_4/TBG$ ratio is a better parameter than others in evaluating the thyroid status during pregnancy and but newborn is a no better.