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Pharmacology · Antithyroid agents

Carbimazole !

A thionamide prodrug: the body turns it into methimazole. Widely used for an overactive thyroid in the UK and other countries.

Updated 2026-11-01

  • Class: Antithyroid agents
  • Enzyme: Thyroid peroxidase (TPO), through methimazole
  • ATC code: H03BB01

The figure shows methimazole, the active form of carbimazole, bound to lactoperoxidase (goat), a heme peroxidase closely related to thyroid peroxidase.

Target in 3D: Thyroid peroxidase (TPO), through methimazole. PDB structure 5FF1 (RCSB PDB, public domain). View the structure at RCSB PDB

Mechanism of action

Carbimazole is converted to methimazole after absorption. Methimazole inhibits thyroid peroxidase, blocking oxidation and organification of iodide on thyroglobulin and the coupling of iodotyrosines, so no new T4 or T3 is made. Stored hormone is not affected, so thyroid levels fall over weeks.

  1. Carbimazole is converted to methimazole in the body
  2. Methimazole inhibits thyroid peroxidase
  3. New T4 and T3 synthesis stops; levels fall over weeks

Pharmacokinetics

Absorption
Given by mouth and converted to methimazole
Onset and duration
Patients may feel better in weeks; the full effect takes longer

Indications

Adverse effects

Common

  • Nausea
  • Headache
  • Rash and itching
  • Joint pain

Contraindications

  • Allergy to carbimazole or other thionamides
  • Previous acute pancreatitis on carbimazole
  • Early pregnancy, when propylthiouracil is usually used instead

Key interactions

Warfarin
Its effect can change; INR is checked
Theophylline
Levels change as thyroid function normalises
Beta-blockers
Dose may need changing as thyroid function normalises
Prednisolone and erythromycin
Their effect may change; review is needed

What is monitored

  • Blood counts and liver tests before and during treatment
  • Free T4, T3 and TSH

Administration and safety

  • Given by mouth; doses are adjusted to blood tests
  • Stopped at once, pending a blood count, if fever, sore throat or mouth ulcers appear

Nursing care

  • Teach the patient to report fever, sore throat, mouth ulcers, bruising or flu-like illness at once
  • Ask about yellow eyes or skin and dark urine
  • Ask about pregnancy plans; a switch to propylthiouracil is usual in early pregnancy

Pregnancy and breastfeeding

Not usually recommended in pregnancy because, like methimazole, it can rarely affect early fetal development; propylthiouracil is often used instead.

Test yourself

Think of the answer, then tap the question to see it.

What is the relationship between carbimazole and methimazole?

Carbimazole is a prodrug converted to methimazole in the body; they share the mechanism and the risks.

Which symptoms on carbimazole need an urgent blood count?

Fever, sore throat, mouth ulcers or flu-like illness, which can signal agranulocytosis.

Which antithyroid drug usually replaces carbimazole in early pregnancy?

Propylthiouracil, because carbimazole and methimazole are linked to rare malformations in early pregnancy.

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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: Side effects of carbimazole - NHS (NHS (National Health Service, UK), 2025); Who can and cannot take carbimazole - NHS (NHS (National Health Service, UK), 2026); Taking carbimazole with other medicines and herbal supplements - NHS (NHS (National Health Service, UK), 2025); Farmacología básica y clínica (Katzung), 16.ª ed., cap. 38 (McGraw Hill, 2025).

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Representative model, for educational purposes only.