carbimazole

Introduction

carbimazole is the first line thionamide in almost all patients with hyperthyroidism needing Rx with an antithyroid medication as it results in a more rapid improvement in thyroid hormone levels, has less hepatotoxicity, and can be given once daily due to its longer half life

starting dose of carbimazole is 10–30 mg/day in 2–3 divided doses depending on severity of thyrotoxicosis, although larger doses may be used in severe disease
Four weeks following initiation of therapy, clinical review with repeat thyroid function tests should be undertaken to avoid hypothyroidism
Antithyroid drug therapy is tapered to a maintenance dose (usually carbimazole 2.5–10 mg) and ceased after 12–18 months of therapy

patients should be warned to see a doctor if either:¹⁾
- fever, mouth ulcers, sore throat or other symptoms suggestive of infection (suspend drug and urgently report to obtain neutrophil count)
- severe fatigue, nausea, abdominal pain, jaundice, dark urine or pale stools (suspend drug and urgently report for investigation)
- rash - could it be vasculitis?
- arthralgia

relatively common effects:
- Gastrointestinal effects (nausea, vomiting, gastric discomfort)
- Rash (urticarial or macular)
- arthralgia or fever - may be a sign of more serious effects so generally cease Rx
- transient mild neutropenia
agranulocytosis (neutrophil count <0.5×109/L) is a rare but life-threatening complication of both antithyroid drugs with an incidence of 0.2–0.5% and due to cross reactivity between the two antithyroid medications, agranulocytosis with one drug is an absolute contraindication to trialling the other
cholestatic hepatitis
Polyarthritis
ANCA-positive vasculitis
use in pregnancy
- carbimazole during pregnancy has been associated with birth defects, including aplasia cutis and 'carbimazole embryopathy', characterised by choanal atresia or oesophageal atresia.
- it is usually recommended that propylthiouracil be used in the first trimester and then changed to carbimazole in the second trimester ²⁾
other effects see documentation

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