Serum concentrations of T4,, T3, and free T4 were measured by radioimmunoassay in normal pregnant women at trimesters, in postpartum women, and cord blood of neonate. Total T4 were increased during pregnancy, remarkably high in the first trimester, and also somewhat increased in postpartum, and normal in neonate. Total T3 were in normal range during pregnancy, but increased in postpartum, whereas decreased in neonate. Free T4 were decreased in 2nd and 3rd trimesters of pregnancy, however normal in postpartum and neonate.