Carbimazole is converted to methimazole, which inhibits thyroid peroxidase (TPO).

• TPO normally helps:
  • Iodinate tyrosine residues on thyroglobulin.
  • Couple iodinated residues to form T3 and T4.
• By blocking TPO, carbimazole reduces thyroid hormone synthesis.

Adverse effects

• agranulocytosis
• crosses the placenta, but may be used in low doses during pregnancy

Compared to Propylthiouracil (PTU)

• PTU also inhibits TPO.
• Additionally, PTU blocks 5'-deiodinase, reducing T4-to-T3 conversion in peripheral tissues.