In pregnancy, there is an increase in the levels of thyroxine-binding globulin (TBG). This causes an increase in the levels of total thyroxine but does not affect the free thyroxine level.

Thyrotoxicosis

Untreated thyrotoxicosis increases the risk of fetal loss, maternal heart failure and premature labour

Graves' disease is the most common cause of thyrotoxicosis in pregnancy. It is also recognised that activation of the TSH receptor by HCG may also occur - often termed transient gestational hyperthyroidism. HCG levels will fall in the second and third trimester

Management

• propylthiouracil has traditionally been the antithyroid drug of choice
• however, propylthiouracil is associated with an increased risk of severe hepatic injury
• propylthiouracil is generally used in the first trimester of pregnancy in place of carbimazole, as carbimazole may be associated with an increased risk of congenital abnormalities
• maternal free thyroxine levels should be kept in the upper third of the normal reference range to avoid fetal hypothyroidism
• thyrotrophin receptor stimulating antibodies should be checked at 30-36 weeks gestation - helps to determine the risk of neonatal thyroid problems
• block-and-replace regimes should not be used in pregnancy
• radioiodine therapy is contraindicated

Hypothyroidism

Key points

• thyroxine is safe during pregnancy
• serum thyroid-stimulating hormone measured in each trimester and 6-8 weeks post-partum
• women require an increased dose of thyroxine during pregnancy
  • by up to 50% as early as 4-6 weeks of pregnancy
• breastfeeding is safe whilst on thyroxine