Fetal Goitrous Hyperthyroidism in a Pregnant Woman with Triiodothyronine-Predominant Graves' Disease

Abstract

Triiodothyronine (T3)-predominant Graves' disease is characterized by increased serum free T3 (FT3) levels after free thyroxine (FT4) levels become normal or even low during antithyroid drug treatment. We encountered a 34-year-old pregnant woman, gravida 5 para 4, who was complicated by T3-predominant Graves' disease. She was diagnosed with Graves' disease at 20 years old, and had received methimazole. Methimazole was changed to potassium iodide to reduce the risk of congenital anomalies during the first trimester. The dose of antithyroid drugs was adjusted based on maternal FT4 levels, so that maternal Graves' disease deteriorated and fetal goitrous hyperthyroidism appeared during the second trimester. Since the fetus presented goiter and tachycardia at 27-28 gestational weeks, doses of methimazole and potassium iodide were increased. A male newborn weighing 2604 g was delivered by a cesarean section at 35 gestational weeks. The newborn was diagnosed with neonatal hyperthyroidism, and received methimazole for six months. He developed normally with normal thyroid function at 1 year old. In pregnancies complicated by T3-predominant Graves' disease, the kinds and doses of antithyroid drugs have to be carefully selected to maintain maternal levels of FT4 as well as FT3 within the normal range, considering trimesters of pregnancy, teratogenicity of medication, and maternal levels of thyroid-stimulating hormone receptor antibody.

Keywords: Graves' disease; antithyroid drug; fetal goiter; fetal hyperthyroidism; tachycardia.

Figure 1

The course of the pregnancy. After switching from methimazole to potassium iodide at 4 gestational weeks (GW), maternal Graves’ disease got worse. MMI was added at 16 GW. We controlled the doses of maternal medications based on fetal heart rate and maternal FT3 serum levels. The woman underwent a repeated cesarean section at 35 GW.

Figure 2

Changes in fetal sonographic measurements. The fetus developed tachycardia and cardiomegaly at 27 GW. Fetal goiter had been noted since 28 GW. The fetal pericardial effusion was observed temporarily. The transplacental treatment improved fetal tachycardia.

Figure 3

Sonographic features of the fetal goiter (A) Transverse view of the fetal neck at 30 gestational weeks showing the enlarged thyroid gland. The estimated volume of the mass was 7.8cm³. The trachea is shown in the middle of the mass. (B) Color-flow Doppler imaging demonstrating the mass surrounded by abundant blood flow.

Figure 4

Fetal magnetic resonance imaging at 30 gestational weeks showing a hyperintensity neck mass on the T1-weighted image (arrow).

Figure 5

The clinical course of the newborn. Since the newborn developed hyperthyroidism, MMI and propranolol were initiated on day 3. He was discharged on day 50. LT4 replacement was initiated on day 100. We discontinued MMI and LT4 after we confirmed that TRAb was negative at 6 months old.