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