Hyperthyroidism.
Carbimazole is metabolised to methimazole. This acts by inhibiting thyroid peroxidise and thereby the production of thyroid hormones. Propylthiouracil acts in a similar way through the same enzyme pathway. It has an additional effect of blocking T4 conversion to T3 in the liver and other tissues.
Onset of action is dependent on depletion of thyroid hormone stores and therefore these drugs take several weeks to show an effect.
Given orally.
Carbimazole
- GI disturbance
- Rash and itch
- Bone marrow suppression
- Myopathy
- Jaundice
Propylthiouracil has been implicated in serious liver toxicity and is therefore no longer first line therapy. It has similar side effects to carbimazole and additionally can cause nephritis and lupus-like syndromes.
Patients can develop hypothyroidism very rapidly with either regime.
Several commonplace drugs such as azathioprine and olanzapine can cause increased risks of myelosuppression with carbimazole/propylthiouracil
Regular monitoring of thyroid status is needed both in initiation and maintenance. Patients should be warned about the risk of haematological problems and to seek medical assistance if they experience sore throat, mouth ulcers, bruising, fever, etc. Patients on propylthiouracil should be informed on the signs of hepatotoxicity and to seek medical help should these develop.
Nil of note.
