Methimazole (Methimazole (Thiamazole))
Also sold as: Neomercazole, Thiamazole, Tapazole, Carbimazole (prodrug, converted to methimazole)
Pregnancy
Cat D
Lactation
Caution
Schedule
H
Forms
Tablet 5mg +2
Indications
- Hyperthyroidism (Graves disease) — first-line antithyroid drug (except first trimester of pregnancy)
- Toxic multinodular goitre
- Toxic thyroid adenoma
- Preoperative preparation for thyroidectomy
- Hyperthyroidism in 2nd and 3rd trimester of pregnancy
- Thyroid storm (acute control)
Adult Dosing
Mild–moderate hyperthyroidism
10–20mg once daily (OD dosing possible due to longer half-life)
Once to twice daily; doses ≤30mg can often be given once daily
Longer half-life (6–8 hours) than PTU allows OD or BD dosing, improving compliance over PTU TDS. Potency approximately 10× greater than PTU.
Severe hyperthyroidism
30–60mg per day in 3 divided doses
TDS until euthyroid
Higher doses for thyroid storm: up to 120–180mg/day (specialist setting)
Maintenance after achieving euthyroidism
5–15mg per day
Once or twice daily for 12–18 months
Titration based on free T4/TSH. Remission in Graves disease ~40–50% after 12–18 months.
Pediatric Dosing
Methimazole generally preferred over PTU in children due to lower hepatotoxicity risk. Avoid methimazole in first trimester — aplasia cutis risk.
Renal Dose Adjustment
| CrCl / eGFR | Dose Adjustment |
|---|---|
| CrCl <10 mL/min | Reduce dose by 25–50%; monitor TFT closely |
| Hemodialysis | Standard dose post-dialysis; methimazole is partially dialysable |
Hepatic Adjustment
Use with caution in hepatic impairment. While hepatotoxicity risk is lower than PTU, cholestatic jaundice can occur. Monitor LFTs. Reduce dose in severe impairment.
Pregnancy & Lactation
Pregnancy: Category D
Category D. Methimazole crosses the placenta and causes fetal hypothyroidism/goitre if overdosed. CRITICALLY: avoid methimazole in FIRST TRIMESTER — associated with aplasia cutis congenita (scalp skin defects), choanal atresia, tracheo-esophageal fistula, and methimazole embryopathy. Switch to PTU in first trimester; switch back to methimazole after 12 weeks (PTU hepatotoxicity risk). Target maternal free T4 upper limit of normal using lowest effective dose.
Lactation: Caution
Methimazole transfers more extensively into breast milk than PTU. At doses ≤20mg/day, limited studies show no thyroid dysfunction in breastfed infants. Monitor infant TSH and free T4 monthly. PTU preferred during lactation by many experts. The benefit of breastfeeding generally outweighs risk at low doses.
Top Drug Interactions
| Interacting Drug | Effect | Severity |
|---|---|---|
| Warfarin / Coumarins | As hyperthyroidism corrects on methimazole, warfarin clearance decreases and anticoagulant effect increases. Monitor INR closely with dose adjustments. | Major |
| Beta-blockers (propranolol) | Clearance of propranolol increases in hyperthyroidism; as methimazole restores euthyroidism, beta-blocker levels rise. Reduce beta-blocker dose with dose adjustments. | Moderate |
| Amiodarone | Amiodarone can cause both hypothyroidism and hyperthyroidism. Methimazole used for amiodarone-induced thyrotoxicosis type 1 (iodine-induced). Complex interaction — specialist endocrinology review required. | Moderate |
| Lithium carbonate | Additive antithyroid effect via lithium inhibition of thyroid hormone secretion. Risk of additive hypothyroidism. Monitor TFT. | Moderate |
| Potassium iodide / Lugol iodine | Wolff-Chaikoff escape: high iodine initially inhibits thyroid hormone synthesis but gland escapes. Iodine used pre-thyroidectomy after methimazole renders patient euthyroid to reduce gland vascularity. | Minor |
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Side Effects
Common
- Skin rash and pruritus (3–5%)
- Urticaria
- Arthralgia
- Nausea and vomiting
- Headache
- Loss of taste (ageusia)
- Mild leucopenia
- Alopecia (scalp hair thinning)
Serious / Discontinue If
- AGRANULOCYTOSIS (0.2–0.5%) — fever and sore throat; stop drug immediately and check CBC
- Aplasia cutis congenita (in first trimester pregnancy — embryopathy)
- Cholestatic jaundice (rare, less common than PTU hepatotoxicity)
- ANCA-associated vasculitis (rare)
- Thrombocytopenia
- Aplastic anaemia (rare)
- Lupus-like syndrome
Contraindications
- First trimester of pregnancy (aplasia cutis and embryopathy risk — use PTU instead)
- Previous agranulocytosis with any thionamide drug
- Severe hypersensitivity to methimazole or carbimazole
Available Indian Brands
| Brand | Manufacturer | Price (approx) |
|---|---|---|
| Neomercazole 5mg | GlaxoSmithKline India | 30 tab ₹78 |
| Neomercazole 10mg | GlaxoSmithKline India | 30 tab ₹110 |
| Thiamazole 10mg | Sun Pharmaceutical | 30 tab ₹85 |
| Methimazole 5mg | Cipla Ltd | 30 tab ₹72 |
| Carbimazole 5mg (prodrug) | Nicolas Piramal India | 30 tab ₹68 |
Monitoring Required
- Free T4 and TSH — every 4–6 weeks initially; monthly until stable; then every 3–4 months
- CBC with differential — at baseline and IMMEDIATELY if fever or sore throat develops (agranulocytosis alert)
- Liver function tests (LFTs) — baseline; repeat if symptoms of hepatic dysfunction (jaundice, abdominal pain)
- ANCA levels if vasculitis symptoms develop (rash, haematuria, sinusitis)
- INR in anticoagulated patients
- Infant thyroid function if used in lactation
- Fetal thyroid monitoring by ultrasound in pregnancy (goitre)
Patient Counseling Points
- URGENT: If you develop fever or sore throat while taking this medicine — stop it immediately and go to hospital for a blood count test. Agranulocytosis is a medical emergency.
- Methimazole MUST NOT be taken in the first 3 months of pregnancy — inform your doctor immediately if you are pregnant.
- Unlike PTU, methimazole can often be taken once daily — take at the same time each day.
- Regular blood tests (thyroid function and blood count) are essential — do not miss appointments.
- Report: jaundice (yellow eyes/skin), dark urine, severe joint pain, or skin rash.
- Do not stop without consulting your doctor — rebound hyperthyroidism can be severe.
- Avoid iodine supplements and high-iodine foods (kelp, excess seafood).
- This is a Schedule H medicine — prescription required.
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Clinically reviewed by: Dr. Sanjay Kalra, DM (Endocrinology), Bharti Hospital Karnal
Last reviewed: 2026-03-20
References
- American Thyroid Association Guidelines for the Diagnosis and Management of Hyperthyroidism. Thyroid. 2016.
- FDA Prescribing Information: Tapazole (methimazole). Covis Pharmaceuticals.
- Andersen SL et al. Antithyroid drug side effects in the population and in pregnancy. J Clin Endocrinol Metab. 2016;101(4):1606–1614.
- Indian Pharmacopoeia Commission. Methimazole monograph. IP 2022.
- CIMS India Drug Database 2025 — Methimazole