Thyroid function was studied in fetal blood samples obtained by funipuncture from 49 small for gestational age (SGA) fetuses at 21-38 weeks' gestation. Levels of TSH, thyroxine-binding globulin, thyroxine (T4), free T4, triiodothyronine (T3), and free T3 were compared with those from 62 appropriate for gestational age (AGA) fetuses. Levels of TSH were significantly higher and T4 and free T4 significantly lower in the SGA than in AGA fetuses. Furthermore, there were significant associations between the increase in TSH and decrease in free T4 and the degrees of fetal hypoxemia and acidemia, respectively.