The MHRA advises prescribers to ensure use of effective contraception in women of childbearing potential during treatment with carbimazole. Use of the drug must only be considered in pregnancy after a thorough individual benefit-risk assessment, and only at the lowest effective dose without additional administration of thyroid hormones.
The advice follows an EU review of evidence from epidemiological studies and case reports, which concluded that carbimazole is associated with an increased risk of congenital malformations, especially when administered in the first trimester of pregnancy and at high doses (≥15mg daily).
The malformations reported include aplasia cutis congenita (absence of a portion of skin, often localised on the head), craniofacial malformations (choanal atresia; facial dysmorphism), defects of the abdominal wall and gastrointestinal tract (exomphalos, oesophageal atresia, omphalomesenteric duct anomaly), and ventricular septal defect.
The use of carbimazole during pregnancy should be reserved for patients in whom prior thyroidectomy or radioiodine treatment were not possible or with hyperthyroidism newly occurring or recurring during pregnancy. Close maternal, foetal and neonatal monitoring is recommended.
The agency also warns of infrequent reports of acute pancreatitis during treatment with carbimazole. Treatment should be stopped immediately and permanently if this condition occurs as re-exposure may result in life-threatening acute pancreatitis with a decreased time to onset.