Fetal hypothyroidism complicating medical treatment of thyrotoxicosis in pregnancy

Two women with thyrotoxicosis were treated with antithyroid drugs during pregnancy. One women had inadvertently received a therapeutic dose of radioiodine at 21 weeks gestation and the other suffered from severe thyrotoxicosis with a serum LATS level of 1850%. In both patients, the serum triiodothyronine was maintained above 500 ng/dl by the concurrent oral administration of this hormone. Despite this precaution, cord serum thyrotrophin levels were markedly elevated and both infants showed clinical signs of hypothyroidism at birth. This experience indicates that triiodothyronine does not prevent fetal hypothyroidism when given to the mother in pharmacological amounts.