The risks to the baby from Graves’ disease are due to one of three possible mechanisms:
1) UNCONTROLLED MATERNAL HYPERTHYROIDISM: Uncontrolled maternal hyperthyroidism has been associated with fetal tachycardia (fast heart rate), small for gestational age babies, prematurity, stillbirths and possibly congenital malformations.This is another reason why it is important to treat hyperthyroidism in the mother.
2) EXTREMELY HIGH LEVELS OF THYROID STIMULATING IMMUNOGLOBLULINS (TSI): Graves’ disease is an autoimmune disorder caused by the production of antibodies that stimulate thyroid gland referred to as thyroid stimulating immunoglobulins (TSI). These antibodies do cross the placenta and can interact with the baby’s thyroid. Although uncommon (2-5% of cases of Graves’ disease in pregnancy), high levels of maternal TSI’s, have been known to cause fetal or neonatal hyperthyroidism. Fortunately, this typically only occurs when the mother’s TSI levels are very high (many times above normal). Measuring TSI in the mother with Graves’ disease is often done in the third trimester. In the mother with Graves’ disease requiring antithyroid drug therapy, fetal hyperthyroidism due to the mother’s TSI is rare, since the antithyroid drugs also cross the placenta. Of potentially more concern to the baby is the mother with prior treatment for Graves’ disease (for example radioactive iodine or surgery) who no longer requires antithyroid drugs. It is very important to tell your doctor if you have been treated for Graves’ Disease in the past so proper monitioring can be done to ensure the baby remains healthy during the pregnancy.
3) ANTI-THYROID DRUG THERAPY (ATD). Methimazole (Tapazole) or propylthiouracil (PTU) are the ATDs available in the United States for the treatment of hyperthyroidism (see Hyperthyroidism brochure). Both of these drugs cross the placenta and can potentially impair the baby’s thyroid function and cause fetal goiter. Historically, PTU has been the drug of choice for treatment of maternal hyperthyroidism, possibly because transplacental passage may be less than with Tapazole. However, recent studies suggest that both drugs are safe to use during pregnancy. It is recommended that the lowest possible dose of ATD be used to control maternal hyperthyroidism to minimize the development of hypothyroidism in the baby or neonate. Neither drug appears to increase the general risk of birth defects. Overall, the benefits to the baby of treating a mother with hyperthyroidism during pregnancy outweigh the risks if therapy is carefully monitored.