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Drug Reference

carbimazole

antithyroid drug· Brands: Neo-Mercazole, CarbimazolePO

Mechanism of action
Carbimazole is a prodrug that is rapidly converted to its active metabolite, methimazole. It inhibits thyroid peroxidase, the enzyme that oxidises iodide and incorporates it into thyroglobulin, thereby blocking the synthesis of thyroxine (T4) and triiodothyronine (T3). It also reduces the generation of reactive oxygen species and may have immunomodulatory effects that lower thyroid-stimulating hormone receptor antibody levels.
Pharmacokinetics
Carbimazole is well absorbed after oral administration. It is rapidly and completely metabolised to methimazole, which is the active form. Peak plasma concentration of methimazole occurs about 1 hour after ingestion. The half-life of methimazole is approximately 3 to 5 hours, but its intrathyroidal concentration persists for much longer. It is widely distributed and crosses the placental barrier and enters breast milk. Metabolites are excreted in the urine, with a small amount as unchanged drug. Carbimazole itself is usually not detected in plasma due to rapid metabolism.
Indications & dosing

Hyperthyroidism (initial treatment)

Typically given in two or three divided doses. Titrate to response.

  • adult · PO: 20–60 mg daily · Typically given in two or three divided doses. Titrate to response.

Hyperthyroidism (maintenance)

After initial control, reduce to lowest effective dose. Monitor thyroid function.

  • adult · PO: 5–20 mg daily · After initial control, reduce to lowest effective dose. Monitor thyroid function.

Thyroid storm

Use higher initial doses in divided doses. In severe cases, may be given per local protocol.

  • adult · PO: 60–120 mg daily divided · Use higher initial doses in divided doses. In severe cases, may be given per local protocol.

Preparation for thyroid surgery

Given to achieve euthyroid state before surgery. May be combined with iodine.

  • adult · PO: 20–60 mg daily · Given to achieve euthyroid state before surgery. May be combined with iodine.

Adjunct to radioiodine therapy

Used to control symptoms before or after radioiodine. Individualise dose.

  • adult · PO: 20–40 mg daily · Used to control symptoms before or after radioiodine. Individualise dose.

Hyperthyroidism (pediatric)

Dose is per kilogram body weight daily, usually divided into two or three doses.

  • pediatric · PO: 0.4–0.6 mg/kg/day daily divided · Dose is per kilogram body weight daily, usually divided into two or three doses.
Contraindications & precautions
  • Previous agranulocytosis due to antithyroid drugs · Do not rechallenge in patients with a history of agranulocytosis caused by carbimazole or methimazole.
  • Hypersensitivity to carbimazole or methimazole · Cross-reactivity may occur; avoid in patients with known sensitivity to these agents.
  • Severe hepatic impairment · Use with caution and monitor liver function, as hepatotoxicity can occur.
  • Pregnancy (especially first trimester) · Risk of fetal harm; use lowest effective dose and monitor carefully. Alternative antithyroid drugs may be preferred in early pregnancy.
  • Monitor blood counts; advise patients to report sore throat, fever, or mouth ulcers immediately, as these may indicate agranulocytosis.
  • Use the lowest effective dose and monitor thyroid function regularly to avoid hypothyroidism.
  • Caution in pregnancy, especially during the first trimester; use the lowest effective dose and monitor maternal and fetal thyroid function closely.
  • Monitor liver function; advise patients to report symptoms such as nausea, jaundice, or dark urine, which may indicate hepatotoxicity.
  • Caution in patients with pre-existing hepatic or renal impairment; consider dose adjustment and closer monitoring.
Adverse effects
  • Agranulocytosisserious
  • Hepatitis (including cholestatic jaundice)serious
  • Vasculitis (e.g., ANCA-associated)serious
  • Aplastic anaemiaserious
  • Rash
  • Gastrointestinal upset (nausea, dyspepsia)
  • Hypothyroidism (due to excessive dose)
  • Arthralgia
Monitoring, pregnancy & toxicity
  • Thyroid function tests (TSH, free T4) · Baseline, then every 4-6 weeks until euthyroid, then every 3-6 months.
  • Full blood count (FBC) · If fever, sore throat, or mouth ulcers occur; urgent test and stop if neutropenia confirmed.
  • Liver function tests (LFTs) · At baseline and if suspected liver injury (jaundice, dark urine).
  • Signs of hypothyroidism (e.g. weight gain, cold intolerance) · Monitor clinically; dose may need reduction.
  • Pregnancy: Carbimazole should be avoided in the first trimester if possible, as it may increase the risk of congenital malformations. Propylthiouracil is the preferred antithyroid drug in the first trimester; carbimazole may be considered in the second and third trimesters. Use the lowest effective dose and monitor maternal and fetal thyroid function closely.
  • Toxicity: Agranulocytosis: fever, sore throat, mouth ulcers, malaise. · Stop carbimazole immediately, perform emergency FBC, and manage neutropenic sepsis with appropriate antibiotics and supportive care.
  • Toxicity: Hepatotoxicity: jaundice, dark urine, abdominal pain, nausea. · Stop carbimazole, monitor LFTs, provide supportive care, and refer for specialist liver assessment.
  • Toxicity: Hypothyroidism (from excessive dose): weight gain, cold intolerance, fatigue, constipation. · Reduce carbimazole dose and monitor TFTs; if severe, temporarily withhold drug.
  • Toxicity: Acute overdose: risk of nausea, vomiting, and subsequent hypothyroidism. · Administer activated charcoal if recent large ingestion; monitor TFTs and manage symptoms; treatment is supportive.
Clinical notes
Dose adjustment and monitoring are essential. Carbimazole is used for hyperthyroidism, especially Graves' disease, and for preparation before thyroidectomy or radioiodine therapy. Dosing typically starts at 20 to 60 mg daily, divided or once daily, and is titrated to maintain normal thyroid function. Maintenance doses are usually 5 to 20 mg daily. Because of the risk of agranulocytosis, patients should be counselled to report sore throat, fever, or mouth ulcers immediately, and a white cell count should be measured if symptoms occur. Other adverse effects include rash, arthralgia, hepatotoxicity, and cholestatic jaundice. It is teratogenic and should be avoided in the first trimester of pregnancy if possible; if required, the lowest effective dose should be used. Monitoring includes thyroid function tests every 4 to 6 weeks. Local guidelines may vary, so these are teaching examples and not prescribing advice.

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BNF; local formulary