What are the potential side effects of antithyroid medications that I

Guideline-aligned answer with reasoning, red flags and references. Clinically reviewed by Dr Kola Tytler MBBS CertHE MBA MRCGP.

When discussing antithyroid medications with patients, it is important to cover potential side effects and risks:

Severe Liver Injury: Propylthiouracil carries a small risk of severe liver injury, which can rarely present as acute liver failure . It can cause fulminant hepatic necrosis that may be fatal . Carbimazole can also cause hepatotoxicity, typically cholestatic .
Acute Pancreatitis: Carbimazole use is associated with a risk of acute pancreatitis, and the medication should be stopped if this is suspected ,.
Agranulocytosis: If a patient develops agranulocytosis, all antithyroid drugs should be stopped and not restarted .
Risks in Pregnancy: For women of childbearing potential, it is crucial to discuss the need for effective contraception during carbimazole treatment due to an increased risk of congenital malformations ,. Both propylthiouracil and carbimazole, if used in high doses during pregnancy, can cross the placenta and may cause fetal goitre and hypothyroidism, which is associated with an increased risk of abnormal brain development, spontaneous miscarriage, and pre-term delivery . Propylthiouracil is generally recommended in the first trimester if antithyroid drugs are needed, as carbimazole may be associated with a higher risk of congenital malformations .
Other Adverse Reactions: Propylthiouracil may be considered for adults who experience adverse reactions to carbimazole .
Monitoring: Before starting antithyroid drugs, full blood count and liver function tests should be checked .

What are the potential side effects of antithyroid medications that I should discuss with my patients?