Methimazole
methimazole — Tapazole, Northyx
Indications for Methimazole
| Indication | Approved Population | Therapy Type | Status |
|---|---|---|---|
| Hyperthyroidism (Graves disease, toxic multinodular goitre, toxic adenoma) | Adults and paediatric patients | Antithyroid medical therapy | FDA Approved |
| Pre-operative preparation for thyroidectomy | Adults | Adjunctive (achieve euthyroidism before surgery) | FDA Approved |
| Pre-radioactive iodine (RAI) preparation | Adults | Adjunctive (may be used to control hyperthyroidism before RAI) | FDA Approved |
Methimazole is the preferred first-line antithyroid drug (ATD) for the medical management of hyperthyroidism in the United States and is recommended by the ATA 2016 guideline. It works by inhibiting thyroid peroxidase (TPO), the enzyme responsible for iodination and coupling of tyrosine residues within thyroglobulin, thereby reducing the synthesis of T4 and T3. Unlike propylthiouracil (PTU), methimazole does not inhibit peripheral T4-to-T3 conversion. Methimazole offers important advantages over PTU: longer duration of action (allowing once-daily dosing in many patients), lower hepatotoxicity risk, and a more favourable side-effect profile overall. For Graves disease, remission rates of approximately 50% are achieved after 12–18 months of ATD therapy.
Thyroid storm (as part of multimodal therapy): Methimazole 60–80 mg/day in divided doses is used; however, PTU is generally preferred in the acute setting because it additionally blocks peripheral T4-to-T3 conversion (ATA 2016). (Evidence quality: Moderate)
Amiodarone-induced thyrotoxicosis (type 1): Methimazole is recommended to reduce thyroid hormone production in iodine-excess thyrotoxicosis caused by amiodarone (ATA 2016). (Evidence quality: Moderate)
Iodine-induced thyrotoxicosis: Methimazole may be used in iodinated contrast-induced thyrotoxicosis to suppress hormone synthesis. (Evidence quality: Low)
Methimazole Dosing
Adult Dosing by Clinical Scenario
| Clinical Scenario | Starting Dose | Maintenance Dose | Maximum Dose | Notes |
|---|---|---|---|---|
| Graves disease — mild hyperthyroidism (FT4 1–1.5x ULN) | 10–15 mg once daily | 5–10 mg/day once euthyroid | 15 mg/day (initial) | Once-daily dosing adequate for most mild cases (ATA 2016) Titrate based on FT4 and T3 at 4–6 weeks; lower maintenance dose when TSH normalises |
| Graves disease — moderate hyperthyroidism (FT4 1.5–3x ULN) | 20–30 mg once daily | 5–15 mg/day | 30 mg/day (initial) | Doses >30 mg/day associated with increased adverse effects (ATA 2016) Can split into 2 divided doses for better efficacy in severe cases |
| Graves disease — severe hyperthyroidism (FT4 >3x ULN) | 30–40 mg/day in 2–3 divided doses | 5–15 mg/day | 60 mg/day (FDA PI) | Divided dosing (BID or TID) increases efficacy; duration of action may be <24 hours in severe disease Duration of ATD therapy: typically 12–18 months for Graves disease |
| Toxic multinodular goitre or toxic adenoma — pre-definitive therapy | 10–20 mg/day | Lowest effective dose | Individualised | Used to achieve euthyroidism before RAI or surgery; remission unlikely without definitive therapy (ATA 2016) Chronic low-dose methimazole may be appropriate if patient is not a candidate for definitive therapy |
| Pre-operative preparation for thyroidectomy | Dose sufficient to normalise FT4 and T3 | Continue until euthyroid | Per clinical response | Discontinue on day of surgery (ATA 2016); potassium iodide (SSKI) may be added 10 days before surgery to reduce thyroid vascularity |
| Thyroid storm — acute management (off-label; PTU preferred) | 60–80 mg/day in divided doses q8h | Reduce as thyroid storm resolves | 80 mg/day | PTU is preferred for acute thyroid storm as it additionally blocks T4→T3 conversion Can be given rectally (suppository) if oral route unavailable |
| Pregnancy — 2nd / 3rd trimester (if ATD required) | Lowest dose to maintain maternal FT4 at or just above the upper limit of normal | Titrate to maternal FT4; aim for mildly hyperthyroid range | Minimise dose | AVOID methimazole in first trimester (use PTU instead); may switch from PTU to MMI after week 16 (ATA 2016) Crosses placenta — excessive dosing causes fetal hypothyroidism and goitre |
Paediatric Dosing
| Clinical Scenario | Starting Dose | Maintenance Dose | Maximum Dose | Notes |
|---|---|---|---|---|
| Hyperthyroidism — children and adolescents | 0.4 mg/kg/day (FDA PI) or 0.2–0.5 mg/kg/day (ATA 2016) divided q8h | ~0.2 mg/kg/day (approximately half the initial dose) | 30 mg/day; up to 1 mg/kg/day in severe cases (off-label) | Reduce dose by ≥50% once euthyroid; adjust based on TSH Monitor growth; prolonged high-dose use may cause growth retardation. Safety not established in infants <1 year |
Two prescribing strategies exist. In the titration regimen (preferred by ATA 2016), the initial high dose is gradually reduced every 4–8 weeks based on thyroid function tests, aiming for the lowest maintenance dose that keeps the patient euthyroid (typically 5–15 mg/day). In the block-replace regimen, a high dose of methimazole is maintained alongside levothyroxine to prevent iatrogenic hypothyroidism. The block-replace approach requires fewer thyroid function tests but carries slightly higher rates of adverse effects due to the sustained higher ATD dose.
Pharmacology of Methimazole
Mechanism of Action
Methimazole (1-methyl-2-mercaptoimidazole) is a thionamide antithyroid agent that inhibits thyroid peroxidase (TPO), the enzyme responsible for the initial steps of thyroid hormone biosynthesis. By acting as an alternative substrate for TPO, methimazole prevents the iodination of tyrosine residues on thyroglobulin and the coupling of iodotyrosines to form T4 and T3. This effectively reduces new thyroid hormone production within the thyroid gland. Importantly, methimazole does not block the release of preformed thyroid hormone from the gland, which is why several weeks of therapy are typically required before clinical improvement is apparent. Unlike propylthiouracil, methimazole does not inhibit the peripheral conversion of T4 to T3 by type I deiodinase. Methimazole may also have immunomodulatory effects in Graves disease, potentially contributing to the remission seen after 12–18 months of therapy, though the exact mechanism remains unclear.
ADME Profile
| Parameter | Value | Clinical Implication |
|---|---|---|
| Absorption | Rapidly and nearly completely absorbed. Bioavailability 80–95% (mean ~93%). Tmax 1–2 hours. Not significantly affected by food. | Consistent absorption regardless of meal timing; convenient for patient compliance. No need for fasting requirements, unlike levothyroxine. |
| Distribution | Vd ~40 L. <10% protein bound (virtually unbound). Concentrated in the thyroid gland, where intrathyroidal effects persist beyond plasma half-life. Crosses the placenta and enters breast milk. | Minimal protein binding means drug-drug displacement interactions are uncommon. Placental transfer underlies teratogenicity risk in the first trimester. |
| Metabolism | Hepatic metabolism; detailed pathways not fully characterised. Concentrated in the thyroid, where it is iodinated and degraded. <10% excreted unchanged in urine. | Elimination half-life is prolonged in hepatic insufficiency (proportional to degree of impairment). No dosage adjustment needed in renal failure. |
| Elimination | Plasma t½ 4–6 hours (IV study: 4.9–5.7 hours). Intrathyroidal duration of action exceeds plasma half-life. Total clearance ~200 mL/min. | Despite the short plasma half-life, once-daily dosing is effective for mild-to-moderate hyperthyroidism because of sustained intrathyroidal drug concentrations. Divided dosing improves efficacy only in severe disease. |
Side Effects of Methimazole
Methimazole is generally well tolerated. Minor adverse reactions occur in approximately 5–15% of patients and are often manageable. However, serious adverse effects including agranulocytosis, hepatotoxicity, and ANCA-positive vasculitis are rare but potentially life-threatening and require immediate recognition. The majority of adverse reactions occur within the first 90 days of therapy (ATA 2016).
| Adverse Effect | Incidence | Clinical Note |
|---|---|---|
| Gastrointestinal upset (nausea, epigastric distress) | ~13% | Usually mild and self-limiting; taking with food may help; dose-related |
| Adverse Effect | Incidence | Clinical Note |
|---|---|---|
| Skin rash / urticaria / pruritus | 3–5% | Most common minor reaction; may be managed with antihistamines or formulation switch; if persistent, consider switching to PTU |
| Arthralgia / myalgia | 1–5% | Joint pain may mimic early vasculitis; distinguish from ANCA-positive arthritis |
| Headache | 1–3% | Usually transient; dose-related |
| Transient mild leukopenia | ~3–12% | Mild WBC decrease (not agranulocytosis) is common in hyperthyroidism itself; monitor but do not discontinue unless ANC <1,000 |
| Taste disturbance / loss of taste | 1–2% | Reversible upon dose reduction or discontinuation |
| Adverse Effect | Estimated Frequency | Typical Onset | Required Action |
|---|---|---|---|
| Agranulocytosis (ANC <500/µL) | 0.2–0.5% (dose-dependent: 0.13% at 10 mg, 0.47% at 30 mg/day) | Usually within first 90 days; rarely up to 1 year | Discontinue immediately; obtain CBC with differential; start broad-spectrum antibiotics if febrile; G-CSF (filgrastim) may accelerate recovery; never rechallenge with any thionamide |
| Hepatotoxicity (cholestatic pattern) | 0.1–0.2% | Weeks to months; often within first month | Discontinue if clinically significant LFT elevation; typically cholestatic (vs hepatocellular with PTU). Risk lower than with PTU, especially in paediatric patients (FDA PI) |
| ANCA-positive vasculitis | Rare; risk increases with prolonged therapy | Months to years (unlike other ADRs) | Discontinue; may require corticosteroids, immunosuppressants, or plasmapheresis. Can involve skin, kidneys (GN), lungs (haemorrhage), or CNS |
| Aplastic anaemia / pancytopenia | Very rare | Variable | Discontinue immediately; haematology referral; bone marrow biopsy; supportive care |
| Congenital malformations (first trimester exposure) | Increased risk with first-trimester use | Organogenesis (weeks 6–10 of gestation) | Avoid methimazole in first trimester; use PTU instead. Reported defects: aplasia cutis, choanal atresia, oesophageal atresia with TE fistula, omphalocele (FDA PI) |
All patients starting methimazole must be educated to immediately report fever, sore throat, mouth ulcers, or any signs of infection. The onset of agranulocytosis is typically abrupt and unpredictable. Routine WBC monitoring does not reliably prevent it, but a baseline CBC before initiation is recommended. Higher doses (≥30 mg/day) carry a dose-dependent increased risk. If agranulocytosis occurs, the patient must never be rechallenged with any thionamide (methimazole or PTU).
Drug Interactions with Methimazole
Methimazole’s drug interactions are primarily pharmacodynamic, arising from the changing thyroid status as the patient transitions from hyperthyroid to euthyroid. As hyperthyroidism is corrected, the metabolism of many co-administered drugs changes, necessitating dose adjustments. Methimazole also has direct anti-vitamin K activity that affects oral anticoagulants.
Monitoring for Methimazole
- FT4 and Total T34–6 weeks after initiation; then q4–8 weeks during titration
RoutinePrimary monitoring parameters during initial treatment. TSH may remain suppressed for weeks after FT4 normalises in severe hyperthyroidism; use FT4 and T3 to guide early dose adjustments. - TSHAfter FT4 normalises; then q2–3 months on maintenance; q6–12 months when stable
RoutineA rising TSH indicates overtreatment (excessive antithyroid effect); reduce methimazole dose. Target TSH within normal range. Continue monitoring for at least 12 months after discontinuation to detect relapse. - CBC with DifferentialBaseline before initiation; then if fever, sore throat, or infection symptoms
Trigger-basedAgranulocytosis (ANC <500) typically presents abruptly with fever and sore throat. Routine periodic WBC monitoring does not reliably predict or prevent agranulocytosis. Educate all patients to report symptoms immediately and obtain urgent CBC. - Liver Function TestsBaseline; then if jaundice, dark urine, abdominal pain, or pruritus develop
Trigger-basedMethimazole-related hepatotoxicity is typically cholestatic (elevated bilirubin, ALP). Discontinue if clinically significant hepatic dysfunction occurs. Risk is lower than with PTU. - PT/INRBefore surgical procedures; if on anticoagulants
Trigger-basedMethimazole has intrinsic anti-vitamin K activity causing hypoprothrombinemia. Monitor PT/INR before any surgery and regularly in patients on warfarin (FDA PI). - TRAb (TSH Receptor Antibodies)Baseline and before planned discontinuation in Graves disease
RoutineElevated TRAb at discontinuation predicts higher relapse risk. Consider extending therapy if TRAb remains elevated after 12–18 months (ATA 2016). - Pregnancy TestBefore initiation in women of childbearing potential
Trigger-basedMethimazole is teratogenic in the first trimester. Use effective contraception during therapy. If pregnancy occurs, switch to PTU immediately and consult endocrinology.
Contraindications & Cautions
Absolute Contraindications
- Known hypersensitivity to methimazole or any excipient
- Prior agranulocytosis or severe hepatotoxicity with any thionamide — never rechallenge
Relative Contraindications (Specialist Input Recommended)
- First trimester of pregnancy — methimazole is associated with congenital malformations (aplasia cutis, choanal atresia, oesophageal atresia, omphalocele); use PTU instead in the first trimester (ATA 2016, FDA PI)
- Concurrent use with clozapine or other drugs known to cause agranulocytosis — additive risk
Use with Caution
- Hepatic impairment — elimination half-life is prolonged; no specific dose adjustment guidelines exist, but use lowest effective dose and monitor LFTs
- Patients >40 years or receiving high doses (≥30 mg/day) — increased risk of agranulocytosis
- Pre-existing leukopenia — obtain baseline CBC; distinguish from hyperthyroidism-associated leukopenia
Methimazole can cause congenital malformations when administered in the first trimester of pregnancy, including aplasia cutis, craniofacial malformations, gastrointestinal defects (oesophageal atresia), and omphalocele. Because of this risk, other agents (PTU) are preferred in the first trimester. Agranulocytosis is a potentially life-threatening adverse reaction. Patients must be instructed to immediately report fever, sore throat, or any symptoms suggestive of infection. The drug should be discontinued in the presence of agranulocytosis, aplastic anaemia, hepatitis, or exfoliative dermatitis.
Patient Counselling
Purpose of Therapy
Methimazole reduces the amount of thyroid hormone your thyroid gland produces. It is used to treat an overactive thyroid (hyperthyroidism), most commonly caused by Graves disease. It does not remove or destroy the thyroid gland; rather, it controls hormone production while you take it. Your prescriber will determine how long you need to take this medication, typically for 12 to 18 months.
How to Take
Take methimazole exactly as prescribed, usually once daily or in divided doses throughout the day. It can be taken with or without food. Do not stop taking methimazole suddenly without medical advice, as your hyperthyroidism may return. Take all doses on time for the best effect.
Sources
- Pfizer Inc. Tapazole (methimazole) tablets — Full Prescribing Information. Drugs.com: Tapazole PIPrimary US prescribing reference for oral methimazole; source for dosing, PK, contraindications, and drug interactions.
- FDA. Methimazole tablets USP — Prescribing Information. Revised 2012. FDA Label: MethimazoleGeneric methimazole PI with updated warnings on teratogenicity, agranulocytosis, ANCA-positive vasculitis, and hepatotoxicity.
- Ross DS, Burch HB, Cooper DS, et al. 2016 American Thyroid Association guidelines for diagnosis and management of hyperthyroidism and other causes of thyrotoxicosis. Thyroid. 2016;26(10):1343–1421. https://doi.org/10.1089/thy.2016.0229Definitive ATA guideline for hyperthyroidism management; provides evidence-based recommendations on methimazole dosing, duration, pregnancy management, and thyroid storm treatment.
- Burch HB, Cooper DS. Management of Graves disease: a review. JAMA. 2015;314(23):2544–2554. https://doi.org/10.1001/jama.2015.16535Comprehensive JAMA review of Graves disease management; supports titration regimen and discusses once-daily vs divided dosing strategies.
- Alexander EK, Pearce EN, Brent GA, et al. 2017 Guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and the postpartum. Thyroid. 2017;27(3):315–389. https://doi.org/10.1089/thy.2016.0457ATA guideline for thyroid disease in pregnancy; source for PTU-first-trimester / methimazole-second-trimester switching recommendation.
- Nakamura H, Miyauchi A, Miyawaki N, Imagawa J. Analysis of 754 cases of antithyroid drug-induced agranulocytosis over 30 years in Japan. J Clin Endocrinol Metab. 2013;98(12):4776–4783. https://doi.org/10.1210/jc.2013-2569Largest case series of ATD-induced agranulocytosis; identifies dose-dependent risk, typical onset within 90 days, and risk factors including age >40, female sex, and higher daily doses.
- Azizi F, Malboosbaf R. Long-term antithyroid drug treatment: a systematic review and meta-analysis. Thyroid. 2017;27(10):1223–1231. https://doi.org/10.1089/thy.2016.0652Meta-analysis supporting long-term low-dose methimazole as a safe and effective option for patients with Graves disease who do not achieve remission or are not candidates for definitive therapy.
- Taurog A, Dorris ML, Guziec FS. Metabolism of 35S- and 14C-labeled 1-methyl-2-mercaptoimidazole in vitro and in vivo. Endocrinology. 1989;124(1):30–39. https://doi.org/10.1210/endo-124-1-30Key mechanistic study demonstrating that methimazole acts as an alternative substrate for thyroid peroxidase, diverting oxidised iodine from thyroglobulin.
- Vicente N, Cardoso L, Barros L, Carrilho F. Antithyroid drug-induced agranulocytosis: state of the art on diagnosis and management. Drugs R D. 2017;17(1):91–96. https://doi.org/10.1007/s40268-017-0172-1Review of agranulocytosis pathophysiology, clinical presentation, and management including G-CSF use; confirms typical onset within first 90 days.
- Jansson R, Dahlberg PA, Lindström B. Pharmacokinetic properties and bioavailability of methimazole. Clin Pharmacokinet. 1985;10(5):443–450. https://doi.org/10.2165/00003088-198510050-00006Definitive PK study: bioavailability 93%, t½ 4.9–5.7 h, no dose-dependent kinetics, prolonged t½ in hepatic failure; no adjustment needed in renal failure.
- Kampmann JP, Hansen JM. Clinical pharmacokinetics of antithyroid drugs. Clin Pharmacokinet. 1981;6(6):401–428. https://doi.org/10.2165/00003088-198106060-00001Comprehensive review of thionamide PK; establishes Vd ~40 L, <10% protein binding, intrathyroidal concentration, and t½ 3–5 hours for methimazole.
- Methimazole. StatPearls [Internet]. National Library of Medicine. Updated September 2023. StatPearls: MethimazoleContinuously updated clinical reference covering methimazole pharmacology, dosing by clinical scenario, adverse effects, and paediatric/pregnancy considerations.