What is Congenital Thyrotoxicosis?

In pregnant patients with florid thyrotoxicosis, the foetus may develop congenital thyrotoxicosis. This may also occur in a foetus whose mother is euthyroid but has a history of previously treated thyrotoxicosis. Congenital thyrotoxicosis is rare, occurring in 1 in 70 pregnancies of patients with established thyrotoxicosis. The disease is transient, lasting up to three or four months.

Congenital thyrotoxicosis is due to the passage from the mother to the foetus of maternal immunoglobulins that stimulate the foetal thyroid. The condition is self-limiting because the infant can clear the maternal immunoglobulin from its circulation. This process takes 3-4 months. In severe disease there may be foetal death. Affected newborns may have a rapid pulse, heart failure, and severe weight loss with failure to thrive. Rarely the infant will have a goitre with eye signs.

The infant is treated with beta-blockers and antithyroid drugs. Treatment is very effective and can usually be stopped at four months. Stimulatory thyroid immunoglobulins may be present in mothers who have had thyrotoxicosis in the past and are at the time of pregnancy euthyroid. These immunoglobulins can cause congenital thyrotoxicosis. It is therefore important that infants born of mothers with a known history of thyrotoxicosis are assessed for thyroid disease.

What is Maternal Thyrotoxicosis?

Careful management of hyperthyroidism in pregnancy is needed to reduce the risk of foetal loss, low birth weight, pre-eclampsiaand premature labour, while at the same timecontrolling the symptoms of maternal hyperthyroidism. In the UK it occurs in 2/1000 pregnancies and in 85% of cases is caused byGraves' disease.

Graves' disease must not be confused with the very much more common entity oftransient gestational hyperthyroidism which is seen particularly in the first trimester and is self-limiting. It occurs in 2-3% of Europeans but occurs more frequently in South Asian women.

During pregnancy

The drug of choice for treatment is propylthiouracil in a starting dose of 300-450mg per day, in two or three divided doses.Once the thyrotoxicosis is controlled (which may take 4-6 weeks) the dose is reduced to as low as possible to control the clinical symptoms and maintain the thyroid function test at the upper range of normal. This greatly reduces the risk of foetal hypothyroidism. Regular foetalultrasound scans should be performed.

If foetal hyperthyroidism is suspected then the dose of propylthiouracil should be increased, but if foetal growth rate suggests foetal hypothyroidism then the antithyroid drugs should be stopped and intra-amniotic thyroxine considered."Block and replace regimes" must not be used in pregnancy, as the transfer of thyroid hormone across the placenta is far less than the anti-thyroid drug and the foetus may become significantly hypothyroid.

Postpartum period

One to five per cent of neonates of mothers with Graves' disease have thyrotoxicosis due to the passage across the placenta of maternal thyrotrophin receptor stimulating antibodies (TRAb).

If at 30 weeks of pregnancy a high TRAb concentration is detected then the neonate should be tested for hyperthyroidism at 6 hours post delivery and if necessary treated with carbimazole.One catch is that if the mother is taking antithyroid drugs at term then the baby should be screened at 4 to 14 days as the neonate may be euthyroid at birth but several days later become hyperthyroid. This hyperthyroidism is self limiting however, as with the passage of time the thyrotrophin receptor stimulating antibodies vanish from the infant.

Breast feeding is not contraindicated when the mother is taking propylthiouracil. The mother should feed the child first and then when possible take her medication.The drug is concentrated in limited amounts in breast milk, but it is prudent to keep the dosage moderate (not greater than 300mg a day). It is alsowise to check the infant's thyroid function at regular intervals.

During the post-partum period the mother is at risk ofrecurrence or exacerbation of the thyrotoxicosis.Thyroid function should be checked at 6 weeks and 3 months post delivery.