The production of TBG in liver increases during pregnancy. The thyroid stimulating substance is also suggested to exist in molar pregnancy. We have studied the thyroid functions with T4, T3, T3 resin uptake, TBG, free T4, TSH, HCG, TBG binding capacity and T4 binding by TBG in non-pregnant women, pregnant women and molar pregnant women. T4 and T3 levels were high during pregnancy and remarkably elevated in mole. TBG levels were 49.0 +/- 5.1/micrograms/ml in 3rd. trimester, which were 2.6 times higher than non-pregnant women and 38.4 +/- 8.4 micrograms/ml in mole. Res-O-Mat T3 values distributed within normal levels in 1st trimester and mole and higher in 3rd. trimester. Free T4 indices were almost normal during pregnancy and 2.4 times higher in mole. Free T4 levels, measured with dialysis and RIA, were slightly low during pregnancy but remarkably high in mole. Basal TSH levels were within normal range for these subjects. Beta-HCG levels in mole were greatly in excess of peak concentrations that were seen at 9-13 weeks of normal pregnancy. The significant correlations of beta-HCG with T4, T3 and free T4 index were found in mole, with coefficient of r = 0.691, 0.687, 0.644 respectively. The correlations of beta-HCG with free T4 and TBG were also suggested in mole. The levels of T4 binding by TBG were remarkably elevated for incremental TBG in mole, which pattern was particular for thyroid hyperfunction. From the above results, we considered that euthyroidism is sustained during pregnancy and thyroid stimulating substance existed in mole. That substance is probably HCG, which is remarkable elevated in mole.