Fetal Heart Rate as an Indirect Indicator of Treatment Response in Fetal Hyperthyroidism Secondary to Transplacental Passage of Maternal Thyrotropin Receptor Antibodies

Maternal hyperthyroidism is prevalent in 0.2-0.4% of pregnancies. Graves’ disease accounts for 85% of these cases. Approximately 1-5% of neonates born to these mothers develop hyperthyroidism. Transplacental passage of thyrotropin receptor antibodies (TRAbs) are considered to be the likely cause of transient fetal and neonatal hyperthyroidism. An 18-year-old G1P0 with a history of Graves’ disease treated by radioactive ablation presented with persistent fetal tachycardia at 23 weeks gestational age. TRAb was elevated and fetal hyperthyroidism secondary to transplacental crossing of maternal TRAb was suspected. There was no evidence of overt fetal hyperthyroidism or goiter on ultrasound examination. Oral methimazole was initiated and the fetal tachycardia resolved. Upon decrease of the methimazole dosage, the fetal tachycardia returned. Methimazole dosing was again increased and stabilized at a higher dose and the fetal tachycardia remained resolved. At birth at 40 weeks gestation, the neonate was tachycardic with elevated TRAb. She was initiated on methimazole. At 1 month of life, the methimazole dose was halved. At 2 months of life, all medication was held. On further testing, the thyroid function test remained normal. In our patient who did not display evidence of fetal hyperthyroidism on ultrasound examination, as there were no abnormal fetal markers on ultrasound to evaluate treatment response, fetal heart rate on non-stress testing was successfully used as an indirect indicator of fetal thyroid status and guided appropriate titration of methimazole. J Clin Gynecol Obstet. 2019;8(3):91-96 doi: https://doi.org/10.14740/jcgo564