[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Pharmacology](https://durgical.com/en/pharmacology/) › [Antithyroid agents](https://durgical.com/en/pharmacology/classes/antithyroid-agents/) › Methimazole

[Pharmacology](https://durgical.com/en/pharmacology/) · [Antithyroid agents](https://durgical.com/en/pharmacology/classes/antithyroid-agents/)

# Methimazole

The first-choice tablet for an overactive thyroid. It blocks the enzyme that builds thyroid hormone; rarely it wipes out white blood cells.

Updated 2026-11-01

- Class: [Antithyroid agents](https://durgical.com/en/pharmacology/classes/antithyroid-agents/)
- Enzyme: Thyroid peroxidase (TPO)
- ATC code: H03BB02
- WHO Model List of Essential Medicines

Also called: Thiamazole

✂**Thyroid peroxidase (TPO)**Methimazole

ⓘ No human thyroid peroxidase structure with the drug exists; the figure shows methimazole bound to lactoperoxidase (goat), a closely related heme peroxidase, in the same type of heme site.

**Target in 3D:** Thyroid peroxidase (TPO). PDB structure 5FF1 (RCSB PDB, public domain). [View the structure at RCSB PDB](https://www.rcsb.org/structure/5FF1) · Drag to rotate · pinch to zoom

## Mechanism of action

Methimazole is a thionamide that inhibits thyroid peroxidase. It stops the oxidation and organification of iodide on thyroglobulin and the coupling of iodotyrosines, so new T4 and T3 are not made. It does not inactivate hormone already stored or circulating, so levels fall over weeks. It is more potent and longer-acting than propylthiouracil and has a lower risk of severe liver injury.

- Methimazole is concentrated in the thyroid gland
- It inhibits thyroid peroxidase
- Iodide is not attached to thyroglobulin; no new T4 or T3
- As stored hormone runs out, hyperthyroidism settles

## Pharmacokinetics

**Absorption**
Well absorbed by mouth

**Distribution**
Concentrated in the thyroid; crosses the placenta and reaches breast milk

**Onset and duration**
Slow: symptoms improve over weeks as stored hormone is used up

## Indications

- Graves disease [Graves disease (hyperthyroidism) →](https://durgical.com/en/conditions/graves-disease/)
- Toxic multinodular goitre [Goitre →](https://durgical.com/en/conditions/goitre/)
- Preparation for thyroid surgery or radioactive iodine

## Adverse effects

### Common

- Skin rash and itching
- Joint pain
- Nausea and altered taste
- Hypothyroidism if overtreated

### Serious (rare, important)

- Agranulocytosis
- Aplastic anaemia and thrombocytopenia
- Liver injury, including acute liver failure
- Vasculitis

## Contraindications

- Hypersensitivity to methimazole
- First trimester of pregnancy, when propylthiouracil is preferred

## Key interactions

**Warfarin**
Its effect may increase; INR is checked more often

**Beta-blockers**
Clearance falls as the patient becomes euthyroid; the dose may need lowering

**Digoxin**
Levels may rise as the patient becomes euthyroid

**Theophylline**
Clearance falls as the patient becomes euthyroid

## What is monitored

- Free T4 and T3, then TSH
- Complete blood count if fever or sore throat appear
- Liver tests if symptoms suggest liver injury

## Administration and safety

- Given by mouth; doses are adjusted to thyroid tests over weeks
- Stopped at once, pending a blood count, if fever or sore throat appear

## Nursing care

- Teach the patient to report fever, sore throat or mouth ulcers at once: possible agranulocytosis
- Ask about yellow skin, dark urine, itching or right upper abdominal pain (liver injury)
- Check heart rate, weight and symptoms of hyper- or hypothyroidism at each visit
- Ask about pregnancy plans: the choice of drug changes by trimester

## Pregnancy and breastfeeding

Avoided in the first trimester because it is linked to congenital malformations; it may be used later in pregnancy at the lowest effective dose.

## Memory aid

MethiMAZole is the MAin choice; PTU is for the first trimester and the storm.

## Test yourself

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

## Why is methimazole preferred over propylthiouracil for most adults with Graves disease?

It is more potent and longer-acting, and propylthiouracil carries a boxed warning for severe liver injury.

## Why is methimazole avoided in the first trimester of pregnancy?

It crosses the placenta and is linked to congenital malformations (such as aplasia cutis and oesophageal atresia); propylthiouracil is used then.

## Why can a patient's warfarin effect rise while on methimazole?

Methimazole may inhibit vitamin K activity, so the anticoagulant effect can increase; INR is checked more often.

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## Related drugs

- [Propylthiouracil](https://durgical.com/en/pharmacology/propylthiouracil/)
- [Levothyroxine](https://durgical.com/en/pharmacology/levothyroxine/)

## Related conditions

- [Graves disease (hyperthyroidism)](https://durgical.com/en/conditions/graves-disease/)
- [Goitre](https://durgical.com/en/conditions/goitre/)

[← Back to the class: Antithyroid agents](https://durgical.com/en/pharmacology/classes/antithyroid-agents/)

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

Sources: [Methimazole: MedlinePlus Drug Information](https://medlineplus.gov/druginfo/meds/a682464.html) (MedlinePlus (U.S. National Library of Medicine), 2017); [DailyMed - METHIMAZOLE tablet](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=06bb4bcc-5e47-48b6-a76c-648f846704e8) (DailyMed (U.S. National Library of Medicine), 2026); [ATCDDD - ATC/DDD Index](https://atcddd.fhi.no/atc_ddd_index/?code=H03BB02) (WHO Collaborating Centre for Drug Statistics Methodology, 2026); Farmacología básica y clínica (Katzung), 16.ª ed., cap. 38 (McGraw Hill, 2025).

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Representative model, for educational purposes only.

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