Thyroid agents · Endocrine
Methimazole (thiamazole)
thiamazole · carbimazole (prodrug) · Tapazole®(brand names vary by market)
Blocks hormone synthesis, and can silently stop the bone marrow.
Mechanism
Inhibits thyroid peroxidase, blocking iodination and coupling. It stops new hormone being made and does nothing to the hormone already stored, which is why the effect takes weeks.
Indications
- Hyperthyroidism, especially Graves disease
- Preparation for thyroidectomy or radioiodine
Formulations
Dosing concepts
Lag
The gland holds weeks of stored hormone, so symptoms are covered with a beta blocker while the antithyroid drug takes effect.
Pregnancy
Propylthiouracil is preferred in the first trimester because of methimazole's embryopathy risk, then the switch is usually made back.
Concepts, not prescribing instructions. Practice numbers live in current references and local protocols.
Pharmacokinetics
- Bioavailability
- High
- Half-life
- 4 to 6 h, but the intrathyroidal effect lasts far longer, allowing once-daily dosing
- Elimination
- hepatic
- Protein binding
- Negligible
Adverse effects
- Rash and prurituscommon
- AgranulocytosisseriousRare, abrupt, and presents as fever and sore throat. Every patient must be told to stop the drug and get a white count that day.
- HepatotoxicityseriousCholestatic pattern with methimazole; propylthiouracil causes the more dangerous hepatocellular one.
- Arthralgiacommon
Contraindications & precautions
Contraindications
- Previous agranulocytosis or severe hepatotoxicity on an antithyroid drug
- First trimester of pregnancy, relative
Precautions
- Hepatic impairment
- Pregnancy and breastfeeding
- Concurrent immunosuppression
Interactions
No curated interaction entries yet for this agent.
Monitoring
Free T4 and T3 initially, then TSH
TSH stays suppressed for months and is useless early · Every 4 to 6 weeks until stable
White cell count if there is fever or sore throat
Agranulocytosis, which is time-critical
Liver enzymes if symptomatic
Hepatotoxicity
Counselling points
- If you develop a fever, a sore throat or mouth ulcers, stop the tablets and get a blood test the same day. This is the most important thing to remember about this drug.
- It takes several weeks to work; you may be given a second tablet to control symptoms meanwhile.
- Report yellowing of the eyes or skin, or dark urine.
- Tell us before trying to conceive.
Clinical pearls
- The agranulocytosis warning has to be given verbally and in writing every time. It is rare, it is sudden, and the patient's own action is what makes the difference.
- Following TSH early is a trap: it remains suppressed for months after the free hormones have normalised, and titrating against it causes over-treatment into hypothyroidism.
References & review
- Methimazole product monograph (consult the current version for your market) (monograph)
- Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed. (reference-work)
Last reviewed 2026-08-16 · jurisdiction: global · drug information changes; verify against current references before practice use.