Fetal Hyperthyroidism: Intrauterine Treatment with Carbimazole in Two Siblings.

Hyperthyroidism can manifest very early in fetal life (fetal thyrotoxicosis) or immediately after birth (neonatal thyrotoxicosis). The authors describe outcome of pregnancies in a woman with Graves' disease who received medical management and underwent subtotal thyroidectomy. The first pregnancy resulted in macerated stillbirth at 32 wk. Fetal tachycardia was followed by intrauterine death at 30 wk in the second pregnancy and macerated stillbirth at 26 wk in the third pregnancy. Fetal tachycardia was detected at 17 wk in the fourth pregnancy. Treatment with carbimazole along with thyroxine was followed by a live birth at 35 wk; but the baby developed severe fatal neonatal thyrotoxicosis with crisis on day 9 and died on day 12. Fetal tachycardia was noted in the fifth pregnancy as well and she was treated with carbimazole and thyroxine. She delivered a male baby at 37 wk. He developed neonatal hypothyroidism on day 8 which was controlled with thyroxine.

Fetal and neonatal thyrotoxicosis.

Fetal thyrotoxicosis is a rare disease occurring in 1 out of 70 pregnancies with Grave's disease or in 1 out of 4000-50,000 deliveries. The mortality is 12-20%, usually from heart failure, but other complications are tracheal compression, infections and thrombocytopenia. It results from transfer of thyroid stimulating immunoglobulins from mother to fetus through the placenta. This transplacental transfer begins around 20(th) week of pregnancy and reaches its maximum by 30(th) week. These autoantibodies bind to the fetal thyroid stimulating hormone (TSH) receptors and increase the secretion of the thyroid hormones. The mother has an active autoimmune thyroid disease or has been treated for it in the past. She may be absolutely euthyroid due to past treatment by drugs, surgery or radioiodine ablation, but still have active TSH receptor stimulating autoantibodies, which can cause fetal thyrotoxicosis. The other features of this disease are fetal tachycardia, fetal goiter and history of spontaneous abortions and findings of goiter, ascites, craniosyntosis, fetal growth retardation, maceration and hydrops at fetal autopsy. If untreated, this disease can result in intrauterine death. The treatment for this disease consists of giving carbimazole to the mother, which is transferred through the placenta to the fetus. The dose of carbimazole is titrated with the fetal heart rate. If the mother becomes hypothyroid due to carbimazole, thyroxine is added taking advantage of the fact that very little of thyroxine is transferred across the placenta. Neonatal thyrotoxicosis patients are very sick and require emergency treatment. The goal of the treatment is to normalize thyroid functions as quickly as possible, to avoid iatrogenic hypothyroidism while providing management and supportive therapy for the infant's specific signs and symptoms.