Serial in utero ultrasonographic measurements of the fetal thyroid: A new complementary tool in the management of maternal hyperthyroidism in pregnancy

Objective: Treatment of maternal hyperthyroidism during pregnancy is complicated by the lack of readily available measures of the thyroid status of the fetus. The aim of this study is to describe the use of serial in utero ultrasound measurements of fetal thyroid in patients being treated for Graves' disease in pregnancy. Methods: Over a 24-month period, all pregnant women with Graves' disease attending our special Fetal Thyroid Unit were followed. Maternal thyroid status was assessed by thyroid function tests. Fetal thyroid size was measured serially by transvaginal ultrasonography between 14 and 17 weeks of gestation and by abdominal ultrasonography between 18 and 37 weeks of gestation in 20 women with Grave's disease. Results: In 15 fetuses, thyroid width and circumference were within the 95% confidence interval of the normal population. In five fetuses, thyroid size was above the 95th percentile for gestational age. In three of them, thyroid size decreased concurrently with a decrease in maternal thionamide dosage, reaching normal range. These three fetuses were born euthyroid. In two fetuses, thyroid size was unaffected by a decrement in maternal drug dosage. Both had neonatal thyrotoxicosis at birth. Conclusions: Serial in utero ultrasonography measuring fetal thyroid size in mothers with Graves' disease can serve as an effective noninvasive tool for the early detection of enlarged fetal thyroid. These findings can be used to monitor the maternal antithyroid drug dosage, thereby preventing intrauterine hypothyroidism in some cases. When a dosage reduction does not cause a decrease in fetal thyroid size, transplacental passage of thyroid-stimulating antibodies causing fetal thyrotoxicosis should be suspected.