Neonatal hypothyroidism despite maternal tri-iodothyronine toxicosis: a management problem?

A primigravida was referred with hyperthyroidism in early pregnancy from longstanding Graves' disease treated with propylthiouracil. She had selective elevation of free tri-iodothyronine (fT3) levels, low normal free tetra-iodothyronine (fT4) and suppressed thyroid-stimulating hormone (TSH). Given her symptoms of thyrotoxicosis and elevated TSH receptor antibodies, therapy was tailored towards maintaining clinical and biochemical euthyroidism. However the fetus developed a goitre secondary to hypothyroidism. This case highlights the dilemmas in managing maternal T3 toxicosis while aiming for a high normal fT4 to prevent fetal hypothyroidism including the role of fetal ultrasound monitoring and amniocentesis.