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
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

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) →
Toxic multinodular goitre Goitre →
Preparation for thyroid surgery or radioactive iodine

Adverse effects

Common

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

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.

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.

Add to my cards (3)

← Back to the class: 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 (MedlinePlus (U.S. National Library of Medicine), 2017); DailyMed - METHIMAZOLE tablet (DailyMed (U.S. National Library of Medicine), 2026); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026); Farmacología básica y clínica (Katzung), 16.ª ed., cap. 38 (McGraw Hill, 2025).

Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake

Representative model, for educational purposes only.