To evaluate the diagnostic value of the ratio of serum thyroxine(T4) /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 T4/TBG ratios were compared with other parameters such as T3/TBG ratio and free T4/TBG ratio and free T4/TBG ratio. Serum T4 levels were elevated with the propotionate increase in TBG levels during pregnancy, leading to the nearly constant value of serum T4/TBG ratios as in normal non-pregnant controls. In contrast, serum T3/TBG and free T4/TBG ratios varied considerably during pregnancy. In newborn, T4 levels were nearly not changed with compared non-pregnant control value and TBG levels were elevated. The results indicate that serum T4/TBG ratio is a better parameter than others in evaluating the thyroid status during pregnancy and but newborn is a no better.