Abstract: Disclosed is an assay for fetal thyroid function involving determining the amount of fetal thyroid function indicator that is present in the maternal blood and maternal urine and then comparing the determined value to a known standard for the gestation age of the fetus. It is defined as below one standard deviation of the normal values. Also disclosed is a method of treating a fetus having hypothyroidism involving determining that the fetus is hypothyroid using the assay of the present invention and thereafter, administering supplemental thyroid hormone (T.sub.4, 500 ug/wk) into the amniotic sac. Also disclosed is a method of identifying euthyroid state using the assay of the present invention in hypothyroid fetuses receiving T.sub.4 therapy.