Pregnancy and thyrotoxicosis

Most common cause of hyperthyroidism during pregnancy is Graves' disease

• a large variation in the risk of hyperthyroidism during pregnancy:
  • high during the first trimester and very low in the third trimester

Pregnant women with hyperthyroidism require management by a hospital specialist.

Antithyroid drugs are the treatment of choice in pregnancy but the "block and replace" regime is contraindicated.

In North America propylthiouracil is preferred as it is excreted less in breast milk and carbimazole may be associated with aplasia cutis

• the suggested association of carbimazole with neonatal aplasia cutis has led some clinicians to prefer propylthiouracil for use during pregnancy

The smallest dose of antithyroid medication possible should be used because of the risk of fetal hypothyroidism - carbimazole 5-10 mg/day; propylthiouracil 50-100 mg/day. Generally treatment is withdrawn 4 weeks prior to delivery after which normal doses may be resumed.

Often hyperthyroidism due to Graves' disease improves spontaneously across pregnancy, so a smaller dose of antithyroid drug may be required.

Radio-iodine is contraindicated and pregnancy should be avoided for at least 4 months following receipt of I-131.

A systematic review has stated (1):

• when appropriately analysed the risk of birth defects associated with ATD (antithyroid drugs) use in pregnancy is attenuated. Although still elevated, the risk of birth defects is smallest with propylthiouracil (PTU) compared to methimazole/carbimazole (MMI/CMZ) and may be similar to that of untreated hyperthyroidism
  • the excess risk of any and major birth defects per 1000, respectively, was: 10.2 and 1.3 for PTU; 17.8 and 2.3 for MMI/CMZ; 32.5 and 4.1 for both MMI/CMZ and PTU; and 9.6 and 1.2 for untreated hyperthyroidism.

Reference:

• Morales DR et al. Antithyroid drug use during pregnancy and the risk of birth defects in offspring: systematic review and meta-analysis of observational studies with methodological considerations.Br J Pharmacol. 2021 Mar 30. doi: 10.1111/bcp.14805
• De Leo S, Lee SY, Braverman LE. Hyperthyroidism. Lancet. 2016 Aug 27;388(10047):906-918