Carbimazole is used in the treatment of hyperthyroidism, including thyrotoxicosis (such as thyroid storm), and for preparing patients prior to thyroid surgery. It may also be used alongside radioactive iodine therapy.

Carbimazole is an antithyroid agent that reduces the production of thyroid hormones. It decreases the uptake and concentration of inorganic iodine within the thyroid gland, and primarily inhibits the formation of di-iodotyrosine and thyroxine. The drug is rapidly absorbed from the gastrointestinal tract and distributed throughout the body. It is completely converted into methimazole, which is responsible for its therapeutic action. Carbimazole readily crosses the placenta and can also reach significant levels in breast milk.

The initial dose: 20–60 mg, in 2–3 divided doses until the patient is euthyroid. Daily dosage should be divided.

Maintenance regimen: The dose is gradually reduced to maintain a euthyroid state. The final dosage is usually in the range of 5–15 mg/day which may be taken as a single daily dose.

Neonates & Children below 12 years: The usual initial dose is 250 mcg/Kg/day in divided doses.

Duration of treatment: 18 to 24 months.

Carbimazole may interact with other medications. Excess iodine intake can reduce its effectiveness, possibly requiring higher doses or longer treatment duration. As thyroid function returns toward normal, the response to oral anticoagulants may decrease; however, if hypoprothrombinemia occurs due to thioamide therapy, anticoagulant effects may increase, requiring dose adjustment based on prothrombin time. Serum levels of digoxin and digitoxin may rise as thyroid function normalizes, so dose reduction of digitalis glycosides may be needed once the patient becomes euthyroid.

Carbimazole is contraindicated in patients with known hypersensitivity to carbimazole or other thiourea antithyroid medications.

Possible adverse effects include rash, itching, changes in skin pigmentation, paresthesia, urticaria, headache, joint pain, and gastrointestinal disturbances such as nausea, vomiting, and abdominal discomfort. Other reported effects include hair loss, drug-induced fever, lupus-like syndrome, vasculitis, nephritis, liver disorders (commonly jaundice), and alterations in taste.

Carbimazole may be used during pregnancy in thyrotoxic patients, but the lowest effective dose should be maintained, as excessive doses can harm the fetus. The drug crosses the placenta and is excreted into breast milk, which may lead to fetal or neonatal hypothyroidism and goitre.

Carbimazole should be used with extreme caution, or avoided, in patients with any degree of tracheal obstruction, as high doses may cause enlargement of the thyroid gland and worsen obstructive symptoms.

Overdose or accidental ingestion may lead to hypothyroidism and goitre. If blood disorders develop, the drug should be stopped immediately. Management should be symptomatic and supportive.

Anti-thyroid drugs

Store below 30°C, protected from light and moisture. Keep out of reach of children.