Safety Update for Anti-thyroid Drugs

The Therapeutic Goods Administration (TGA) has published a Medicines Safety Update advising of a change to the pregnancy category for carbimazole and propylthiouracil. These two anti-thyroid medications will move from pregnancy category C to category D. This update has occurred due to reports of congenital abnormalities in the post-market setting.

The Australian categorisation system for prescribing medicines in pregnancy provides the following definitions for category C and category D:

• Category C – Drugs which, owing to their pharmacological effects, have caused or may be suspected of causing, harmful effects on the human foetus or neonate without causing malformations. These effects may be reversible.
• Category D – Drugs which have caused, are suspected to have caused or may be expected to cause, an increased incidence of human foetal malformations or irreversible damage. These drugs may also have adverse pharmacological effects.

The TGA advises that carbimazole and propylthiouracil should not be prescribed for women of childbearing potential unless the potential benefits outweigh the possible risks. The Therapeutic Guidelines: Bone and Metabolism recommend propylthiouracil as the preferred option in the first trimester as it is associated with less severe congenital abnormalities than carbimazole. If anti-thyroid therapy is required later in the pregnancy, carbimazole is preferred as it has a lower risk of liver injury. It is important to note that untreated hyperthyroidism can also have a detrimental effect on the mother and the developing foetus.

Obstetric drug information services can provide further information on the use of medicines in pregnancy. Details of services available in each State and Territory can be found on the TGA website.