Activity of
Graves' disease (GD) is known to improve during gestation, as values of
thyrotropin (
TSH) receptor antibody (TRAb) also improve. However, the risk of neonatal
hyperthyroidism increases when maternal TRAb values are high in the second to third trimester. A 29-year-old woman who had undergone radioactive
iodine (RAI)
therapy for GD 10 years earlier visited our hospital
at 17 weeks of gestation, showing subclinical
hypothyroidism and a positive TRAb value of 2.6 IU/L (reference range, <2.0 IU/L).
Thyroid hormone replacement
therapy was commenced and thyroid function normalized within 4 weeks, although TRAb was elevated at the time (3.8 IU/L). Prenatal check-up showed normal growth development and no irregularities. At 29 weeks of gestation, serum TRAb was extremely elevated, up to 16.8 IU/L. Since the risk of neonatal
hyperthyroidism was of great concern, delivery was planned at an advanced-care medical center. At 38 weeks 5 days of gestation, she delivered a female neonate without any complications, although blood testing of the neonate showed subclinical
hyperthyroidism with positive TRAb and
TSH receptor stimulating antibody (TSAb). According to the American Thyroid Association guidelines, the TRAb value should be checked in the third trimester if mothers show a TRAb elevation between the initial visit after pregnancy and 18-22 weeks of gestation. However, if the mother has a history of RAI
therapy for GD, regardless of thyroid function during gestation, the possibility of TRAb values elevating over time even years after the definitive
therapy must be considered.