Thirty-four hypothyroid women on thyroid hormone substitution were followed through 37 pregnancies, and 16 women having previous surgery for thyroid carcinoma and thereafter placed on suppressive thyroxine treatment were followed through 19 pregnancies. The thyroxine treatment needed readjustment in 13 pregnancies (23%) to maintain euthyroidism. At delivery, the maternal free thyroxine index was 126 nmol/L in the group of patients treated for hypothyroidism and 146 nmol/L in the patients with treated thyroid carcinoma. The amniotic fluid thyroxine level in normal pregnancies was 6.7 nmol/L, in hypothyroid patients 6.7 nmol/L, and in patients with thyroid carcinoma 5.6 nmol/L. The amniotic fluid reverse triidothyronine level in normal pregnancies was 0.51 nmol/L, in hypothyroid patients 0.66 nmol/L, and in patients with thyroid carcinoma 0.70 nmol/L. All infants were euthyroid.