[Transient neonatal hyperthyroidism caused by transplacental transport of pituitary TSH receptor antibodies].

Background: Neonatal hyperthyroidism is a relatively rare condition. It can be severe and difficult to treat when the maternal hyperthyroidism has recently been recognized.

Case report: A baby was born at 37 weeks of gestational age to a mother whose hyperthyroidism was only suspected at the 34th week. The mother was not given propylthiouracil until the day before delivery. At birth, the newborn presented with meconial fluid; it weighed 2,380 grams, was 46 cm long and had a head circumference of 32 cm. Clinical examination showed exophthalmos, tachycardia, hepatosplenomegaly, restlessness, moderate goiter and premature craniosynostosis. Laboratory data showed elevated serum free T4 (76 pmol/l), unmeasurable TSH, and the presence of thyroid-stimulating immunoglobulins. The infant was given propranolol (10 mg/day), and carbimazol (3.75 mg/day) from day 13. Clinical and biological improvement allowed the carbimazol to be stopped at 1 month. A relapse was treated with carbimazol plus L-thyroxine. Both drugs were discontinued at the age of 4 months. At 12 months, the baby is perfectly well and has developed normally.

Conclusion: This transient thyrotoxicosis was due to the transplacental passage of TSH receptor antibodies. Its management was difficult because the mother was treated late during her pregnancy.