Fetal/Neonatal Thyrotoxicosis in a Newborn From a Hypothyroid Woman With Hashimoto Thyroiditis.

Context: Fetal/neonatal thyrotoxicosis is a rare but potentially life-threatening condition. It is most commonly observed in poorly controlled Graves’ disease during pregnancy. Case Description: Here we describe the first reported case of thyrotoxicosis in a fetus/newborn from a woman with Hashimoto’s thyroiditis and levothyroxine-treated hypothyroidism. Transplacental passage of stimulating TSH-receptor antibodies, measured by a cell-based bioassay, was the underlying mechanism of fetal/neonatal thyrotoxicosis even though the mother had no history of hyperthyroidism. Conclusion: Diagnosis and management of fetal hyperthyroidism can be challenging. TSH receptor antibody testing should be co…