The short answer: A usual first course of methimazole for Graves' disease lasts 12 to 18 months. Your doctor then checks TSH and TRAb (the Graves' antibody) before the drug is stopped. Methimazole does not cure the disease, and about half of people relapse after they stop. A longer course on a low dose is an option for some people.

How long is a usual methimazole course for Graves' disease?

The American Thyroid Association (ATA) says methimazole "can be continued for 12-18 months and then discontinued if TSH and TRAb levels are normal at that time." The 2018 European Thyroid Association guideline gives the same range. It says people with newly diagnosed Graves' hyperthyroidism are usually treated for 12 to 18 months, with methimazole as the preferred drug.

The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) puts the average total treatment time at about 1 to 2 years. It adds that treatment can continue for many years.

The 12 to 18 month range comes from trials. A Cochrane review pooled 26 randomized trials with 3,388 participants. Four of those trials tested the length of treatment. With a dose that is adjusted over time, 12 months was better than 6 months. The review found no benefit from treatment beyond 18 months. Its authors rated the overall quality of the trials as poor.

Two timelines are easy to confuse. The NIDDK says thyroid hormone levels may take several weeks or months to reach the normal range. Normal levels on the drug show that the dose works. They do not show remission.

The FDA label does not set a length of treatment. It gives a dose range only. The methimazole label says "the maintenance dosage is 5 to 15 mg daily" for adults. It also says a rising TSH, after symptoms have resolved, indicates that a lower maintenance dose should be used.

What is remission, and how often does Graves' disease come back?

Remission means your thyroid levels stay normal after you stop the drug. Relapse means hyperthyroidism returns. Trials measure relapse over a set follow-up time, so a longer follow-up finds more relapses.

Methimazole does not remove the cause of the disease. The NIDDK says antithyroid medicines "are not a permanent cure for Graves' disease." The ATA says these medicines "do not cure Graves' hyperthyroidism" but control it when the dose is adequate.

The numbers from pooled studies are close to one in two:

  • A 2017 meta-analysis of 54 trials counted 3,696 relapses in 7,595 participants after antithyroid drugs were stopped. That is 48.7%.
  • The Cochrane review reported relapse rates of 51% and 54% for the two dosing methods it compared.
  • In a randomized trial published in 2019, 53% of people who took methimazole for about 19 months relapsed within 48 months of stopping.

Remission can also end in a different way. The ATA says hypothyroidism can still occur in people treated with antithyroid drugs alone.

Who is more likely to relapse after stopping?

The 2017 meta-analysis looked for risk factors measured before treatment. These were significantly associated with relapse:

  • Graves' eye disease (orbitopathy)
  • Smoking
  • A larger thyroid, measured by ultrasound volume or goiter size
  • Higher free T4 and total T3
  • Higher TRAb

Sex, age, and the first total T4 level did not show a significant association in that analysis. The authors noted that most of the trials were small, with a moderate to high risk of bias.

Sources do not fully agree. The ATA patient page lists women, mild disease, small goiters, and negative or low antibody levels as signs of a high likelihood of remission. In the 2019 trial, older age, higher T3 or TRAb, and lower TSH were independently associated with relapse after a standard course.

TRAb at the end of the course matters most for the decision to stop. The European guideline recommends a TRAb measurement before antithyroid treatment is stopped. The ATA says that if TRAb stays high, "the chances of remission are much lower." Eye disease has its own risks, which we cover in thyroid eye disease after Graves' treatment.

Does a longer, low-dose course improve the odds?

Newer trial evidence says it can. This differs from the older Cochrane finding.

The 2019 trial enrolled 302 people with a first episode of Graves' hyperthyroidism. After 18 to 24 months of methimazole, 258 were randomized to stop or to continue on a low dose. The long-term group took the drug for about 95 months in total. Within 48 months of stopping, hyperthyroidism returned in 15% of the long-term group (18 of 119) and 53% of the standard group (65 of 123).

A later report from the same trial followed people for 84 months after they stopped. Relapse occurred in 17% of the long-term group (20 of 118) and 56% of the standard group (67 of 120).

A 2017 meta-analysis of six studies examined antithyroid treatment longer than 24 months. It reported a remission rate of 57%. Complications occurred in 19.1% of patients, and 1.5% were major. Smoking lowered the remission rate.

There are limits. One research group in Tehran ran the trial and wrote the meta-analysis. Guidelines treat a longer course as an option. The ATA says prolonged treatment "is safe and may increase chances of remission" when TRAb stays high. The European guideline says such patients can continue methimazole and repeat the TRAb test after another 12 months.

What safety problems should you watch for while you take it?

The methimazole label has no boxed warning. Its Warnings section covers these risks:

  • Agranulocytosis. The label calls this severe drop in white blood cells "potentially a life-threatening adverse reaction." It tells patients to report fever or sore throat to their physician immediately.
  • Liver toxicity. The label cites reports that include acute liver failure. It says the risk appears lower than with propylthiouracil. Loss of appetite, itching, or pain in the upper right abdomen should prompt liver tests.
  • Pregnancy. Methimazole crosses the placenta and can cause fetal harm in the first trimester. The label says another antithyroid drug may be appropriate at that stage. Tell your doctor right away if you are pregnant or plan a pregnancy.
  • Hypothyroidism and vasculitis. The label calls for routine TSH and free T4 tests. It also says to report a new rash, blood in the urine, or trouble breathing promptly.

In the 2019 trial, 14 people had skin reactions and 2 had liver enzyme rises in the first 18 months. The authors reported no further methimazole reactions during the longer course.

What happens if Graves' disease comes back?

A relapse can start with nervousness, shakiness, and a rapid heart rate, the symptoms the ATA hyperthyroidism page lists for an overactive thyroid. These overlap with an anxiety disorder, a topic our sister site anxiety.md covers. A blood test separates the two.

The European guideline gives two paths after a relapse. Definitive treatment is recommended, but continued long-term low-dose methimazole can be considered.

Definitive treatment means radioactive iodine or surgery. The NIDDK says almost everyone who has radioiodine therapy later develops hypothyroidism. It also says that people who have the whole thyroid removed need thyroid hormone medicine for life. The European guideline says radioactive iodine is contraindicated in people with active or severe Graves' eye disease. Our article on thyroidectomy recovery time covers the surgical path, and our hyperthyroidism treatment overview compares all three options.

The bottom line

Plan for 12 to 18 months of methimazole, then a TSH and TRAb check before the drug is stopped. About half of people relapse after a standard course. Smoking, eye disease, a large goiter, and high TRAb raise that risk. One randomized trial found far fewer relapses after 5 to 10 years of low-dose treatment. If the disease returns, the choices are radioactive iodine, surgery, or more methimazole.

Last updated: October 2026. This article is for informational purposes only and does not constitute medical advice. Do not stop or change your methimazole dose without your endocrinologist, and report fever or sore throat right away.