Methimazole is the preferred antithyroid medication for the treatment of hyperthyroidism, including Graves' disease and toxic multinodular goiter. It inhibits thyroid hormone synthesis and is used to achieve euthyroidism prior to radioactive iodine therapy or surgery, or as long-term therapy for Graves' disease remission. It is generally preferred over propylthiouracil due to longer duration of action and fewer side effects.
Effectively lowers thyroid hormone levels in hyperthyroidism6,10
Once-daily dosing in most patients
Graves' disease remission in 30–50% after 12–18 months of therapy3,1
Rapid biochemical response (2–6 weeks)
Preferred over PTU due to better safety profile4,8
What to watch for
Skin rash and urticaria
Arthralgia and myalgia
Gastrointestinal upset
Known hypersensitivity to methimazole or thioamides1,2
First trimester of pregnancy (propylthiouracil preferred due to methimazole teratogenicity)4,9
The bottom line
Evidence rating strong. Most-documented uses: effectively lowers thyroid hormone levels in hyperthyroidism, once-daily dosing in most patients, graves' disease remission in 30–50% after 12–18 months of therapy. 10 sources indexed (2012–2025), with 4 interaction records on file.
The science
How it works, mechanistically.
Core mechanism
Inhibits thyroid peroxidase (TPO), the enzyme responsible for the iodination of tyrosine residues on thyroglobulin and the coupling of iodotyrosines to form T3 and T4. This blocks new thyroid hormone synthesis without affecting existing stored hormone. Methimazole may also have immunomodulatory effects that contribute to Graves' disease remission.6,10
Class
Antithyroid Agent
Dosing
Dosing & protocol.
Common range
5–30 mg daily initially; maintenance 5–15 mg daily (as prescribed by your physician)
Recommended form
Oral tablet
Can be taken with or without food. Once-daily dosing is usually adequate due to prolonged intrathyroidal activity.5
Safety
Full safety detail.
Side effects
Skin rash and urticaria
Arthralgia and myalgia
Gastrointestinal upset
Agranulocytosis (rare but serious; 0.1–0.5%)
Hepatotoxicity (cholestatic pattern)
Hair loss
Taste abnormalities
Contraindications
Known hypersensitivity to methimazole or thioamides1,2
First trimester of pregnancy (propylthiouracil preferred due to methimazole teratogenicity)4,9
Iodine intake can change how Graves' disease responds to methimazole. High or unstable iodine exposure can alter thyroid hormone synthesis and may delay control or change the methimazole dose needed, while pharmacologic iodide is sometimes used intentionally under specialist supervision. Unsupervised iodine or kelp-style supplementation can make thyroid labs unpredictable.
Recommendation: Do not add iodine supplements while taking methimazole unless your thyroid clinician specifically recommends it. Keep dietary iodine intake consistent and recheck thyroid labs after any meaningful iodine exposure change.
Selenium has been studied as an add-on to methimazole in Graves' disease. Some smaller trials suggest improved antibody or early biochemical measures, while larger and selenium-sufficient cohorts show little or no benefit for remission or quality of life. This is a possible adjunctive effect, not a replacement for antithyroid medication.
Recommendation: Do not use selenium instead of methimazole. If your clinician recommends selenium, keep the dose in a standard supplemental range and avoid high-dose long-term use unless specifically monitored.
Ashwagandha may increase thyroid hormone levels or trigger thyrotoxicosis in susceptible people. That can oppose the purpose of methimazole, which is used to bring excessive thyroid hormone production under control. The evidence is limited, but the clinical direction is concerning for people being treated for hyperthyroidism.
Recommendation: Avoid ashwagandha while taking methimazole unless your thyroid clinician explicitly approves it. If you have already started it, report palpitations, tremor, heat intolerance, anxiety, or worsening thyroid labs promptly.
Vitamin B7 can interfere with thyroid immunoassays used to monitor methimazole treatment. The pattern can mimic Graves' disease activity by making TSH look falsely low and T3 or T4 look falsely high on susceptible assay platforms. This can lead to unnecessary methimazole dose increases or delayed dose reduction.
Recommendation: Stop biotin for at least 48 to 72 hours before thyroid testing, and follow longer lab-specific instructions for high-dose biotin. Tell your prescriber and lab about all biotin-containing supplements before methimazole monitoring labs are interpreted.
Bolakale-Rufai IK, Abioro I, Ngene SO et al.. Efficacy of methimazole before the administration of radioactive iodine in the management of Graves' disease: a systematic review and meta-analysis. Sao Paulo medical journal = Revista paulista de medicina. 2023
Wu X, Qin X, Yao Y. Methimazole plus levothyroxine for treating hyperthyroidism in children: a systematic review and meta-analysis. Translational pediatrics. 2022
van Lieshout JM, Mooij CF, van Trotsenburg ASP et al.. Methimazole-induced remission rates in pediatric Graves' disease: a systematic review. European journal of endocrinology. 2021
Hackmon R, Blichowski M, Koren G. The safety of methimazole and propylthiouracil in pregnancy: a systematic review. Journal of obstetrics and gynaecology Canada : JOGC = Journal d'obstetrique et gynecologie du Canada : JOGC. 2012
Xie P, Shen L, Peng R et al.. Effects of Low-dose Methotrexate With Methimazole in Patients With Graves' Disease: Results of a Randomized Clinical Trial. The Journal of clinical endocrinology and metabolism. 2025
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