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 a fetus/newborn patient with thyrotoxicosis who was born of a woman with Hashimoto thyroiditis and levothyroxine-treated hypothyroidism. Transplacental passage of stimulating thyrotropin (TSH) receptor antibodies, which were measured by a cell-based bioassay, was the underlying mechanism of fetal/neonatal thyrotoxicosis, although the mother had no history of hyperthyroidism. Conclusion: Diagnosis and management of fetal hyperthyroidism can be challenging. TSH receptor antibody testing should be considered in pregnant women with any history of autoimmune thyroid disease and symptoms of fetal hyperthyroidism.