methimazole
Methimazole is used to treat hyperthyroidism, which happens when your thyroid makes more thyroid hormones than what the body needs. This medication is a tablet that’s taken by mouth. Common side effects include rash, drowsiness, and headache. Methimazole isn’t recommended for use in the first trimester of pregnancy, but it’s the preferred option during the second and third trimesters. While it doesn’t have a brand name on the market currently, there’s a lower-cost generic version available.
What is Methimazole?
What is Methimazole used for?
High thyroid hormone levels (hyperthyroidism) in people:
With Graves’ disease or other medical conditions who can’t receive thyroid surgery or radioactive iodine treatment
Who are preparing to undergo thyroid surgery or radioactive iodine treatment
How Methimazole works
Methimazole belongs to a class of medications called antithyroids. It works by blocking the thyroid gland from making thyroid hormones.
Thyroid hormones help regulate your body temperature, control how your body uses energy, and keep your organ systems working well. But having too much thyroid hormone in your body can cause unpleasant symptoms, including weight loss, irregular heartbeat, frequent bowel movements, an enlarged thyroid gland, and feeling hot when others don’t.
By blocking your thyroid from making these hormones, methimazole helps to bring your thyroid hormone levels down to a healthy level. This will help relieve your symptoms and give you more energy throughout the day.
Drug Facts
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What are the side effects of Methimazole?
Common Side Effects
- Raised, itchy rash
- Nausea
- Vomiting
- Stomach upset
Other Side Effects
- Loss of taste
- Unusual hair loss
- Drowsiness
- Vertigo (feeling like your surroundings are spinning)
- Headache
- Muscle pain
- Joint pain
Serious Side Effects
- Low white blood cells (agranulocytosis): fever, fatigue, chills, sore throat
- Inflamed blood vessels (vasculitis): rash, nerve pain, chest pain, stomach pain
- Unusual bleeding or bruising: black stools, blood in urine, bleeding or bruising that doesn’t go away
- Liver problems: upper right stomach pain, loss of appetite, yellowing of skin or whites of eyes (jaundice)
Source: DailyMed
The following side effects have also been reported
Along with its needed effects, a medicine may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.
Check with your doctor immediately if any of the following side effects occur:
Incidence not known
Black, tarry stools
bleeding gums
bleeding under the skin
bloody or cloudy urine
burning, crawling, itching, numbness, prickling, "pins and needles", or tingling feelings
chills
dark urine
difficulty in breathing
dizziness or lightheadedness
drowsiness
feeling of constant movement of self or surroundings
general feeling of discomfort, illness, or weakness
headache
hoarseness
itching, skin rash
light-colored stools
loss of appetite and weight
loss of consciousness
lower back or side pain
painful or difficult urination
pinpoint red spots on the skin
redness, soreness, or itching skin
sensation of spinning
soreness of the muscles
sores, ulcers, or white spots on the lips or in the mouth
sores, welts, or blisters
stomach pain
swelling of the face, feet, or lower legs
swollen or painful glands
swollen salivary glands
swollen, painful, or tender lymph glands in the neck, armpit, or groin
tightness in the chest
unusual bleeding or bruising
unusual tiredness or weakness
unusual weight gain
upper right abdominal pain
yellow eyes or skin
Some side effects may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. Also, your health care professional may be able to tell you about ways to prevent or reduce some of these side effects. Check with your health care professional if any of the following side effects continue or are bothersome or if you have any questions about them:
Incidence not known
Abnormal loss of hair
changes in skin coloring
difficulty in moving
joint pain or swelling
loss of taste
muscle pain, cramps, or stiffness
pain or discomfort in the chest, upper stomach, or throat
Other side effects not listed may also occur in some patients. If you notice any other effects, check with your healthcare professional.
Call your doctor for medical advice about side effects. You may report side effects to the FDA at 1-800-FDA-1088.
Pros and cons of Methimazole
Pros
Maintenance dosing is once a day
Lower-cost therapy since it’s only available as generic on the market
May work better than propylthiouracil (PTU) to improve thyroid hormone levels
Less likely to cause serious liver damage compared to other antithyroid medications
Cons
Need to get blood tests often to check thyroid hormone levels for dosage adjustments
May lower your body’s ability to fight infections
May increase your risk of bleeding
Unsafe to use during the first trimester of pregnancy
Pharmacist tips for Methimazole
Methimazole may cause stomach upset. If this happens, don’t stop taking the medication. Staying hydrated, eating smaller portion sizes, and taking methimazole with food are all helpful ways to relieve stomach issues you might have with the medication.
When you first start taking methimazole, it’s usually taken 3 times a day. But eventually, you may just take it once a day.
It can sometimes take up to a few weeks to months to start seeing your thyroid hormone levels improve after starting methimazole. And the time it takes for your symptoms to improve depends on how severe your condition is. So, don’t be discouraged if you don’t see any improvement right away. Continue taking the medication as prescribed. Discuss with your provider about whether methimazole is working for you.
Follow up with your provider periodically to get blood work done. This helps your provider check your thyroid hormone levels and make sure they’re not too high or too low. Based on your lab work results, your dose of methimazole may change to keep your hormone levels within a normal range.
Methimazole can potentially cause low thyroid hormone levels (hypothyroidism). Contact your provider if you notice symptoms of hypothyroidism, such as depression, tiredness, dry skin, constipation, and feeling cold when other people don’t. Your provider may want to do a physical exam to take a look at your thyroid gland and ask about these symptoms. You might need to do additional blood work to check your thyroid hormone levels.
Methimazole can increase your risk of bleeding. Please notify your provider if you notice bloody stool or urine, black, tarry stools, or any unusual bleeding or bruising.
If you get pregnant or are trying to get pregnant while you’re taking methimazole, let your provider know immediately. Methimazole isn’t recommended during the first trimester of pregnancy as it could harm your unborn baby. But it can be used carefully in the second and third trimesters.
Frequently asked questions about Methimazole
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What are the risks and warnings for Methimazole?
Methimazole can cause some serious health issues. This risk may be even higher for certain groups. If this worries you, talk to your doctor or pharmacist about other options.
Harm to unborn baby during the first trimester of pregnancy
Risk factors: Currently pregnant | Women looking to become pregnant
Methimazole can cause birth defects in the unborn baby if it’s taken during the first trimester of pregnancy. For this reason, methimazole isn’t recommended for use in early pregnancy. If you’re pregnant or looking to become pregnant, contact your healthcare provider for alternative options that are safer for you during your first trimester.
Low white blood cell count (agranulocytosis)
Risk factors: Taking other medications that lower your immune system’s ability to fight infections (e.g., steroids) | Medical conditions that weaken the immune system (e.g., cancer, rheumatoid arthritis)
Methimazole can lower your white blood cell counts, which can reduce your body’s ability to fight off infections. This can sometimes be life-threatening. If you notice a sore throat or fever while you’re taking methimazole, you should notify your healthcare provider immediately.
Liver damage
Risk factors: Alcohol use | History of liver problems
Methimazole can sometimes harm the liver. Liver damage with methimazole appears to be less likely than with other antithyroid medications, especially for children. But it’s still important to watch out for symptoms of liver injury. If you notice any yellowing of the skin or whites of the eyes, upper right stomach pain, dark urine, nausea, or vomiting, tell your healthcare provider right away. Your provider may want you to get blood work done to check your liver function and possibly stop the medication if needed.
Low levels of thyroid hormone (hypothyroidism)
Methimazole works by stopping your thyroid gland from making thyroid hormones. Because of how the medication works, taking methimazole can sometimes lead to having thyroid hormone levels that are below the normal range. This can cause symptoms of hypothyroidism, such as fatigue, dry skin, or constipation. If you’re experiencing these symptoms, contact your provider to see if your thyroid hormone levels need to be checked. Your provider may adjust your dose of methimazole to keep your hormone levels in the normal range.
Thickening of blood vessels (vasculitis)
Although extremely rare, some people taking methimazole have had inflamed blood vessels. This problem may get better once you’ve stopped taking this medication. However, in some cases, you may need to take other medications to help treat the problem. Symptoms depend on which blood vessels are affected, but they may include rashes, chest pain, shortness of breath, stomach pain, and discolored urine. Speak to your healthcare provider if you notice any of these symptoms.
Methimazole dosage forms
Typical dosing for Methimazole
Your dose depends on whether you’re starting or continuing therapy. Your provider will adjust your continuing dose throughout treatment based on your thyroid hormone levels.
Adults: The typical starting dose ranges from 15 mg to 60 mg per day, depending on how severe your hyperthyroidism is. The total daily dose is split into 3 smaller doses that are taken by mouth every 8 hours. Once your thyroid hormone levels become normal, you’ll continue treatment at a maintenance dose that typically ranges from 5 mg to 15 mg taken once a day.
Children: The dose will depend on your child’s body weight. The typical starting dose is 0.4 mg/kg of body weight per day. This total dose is split into 3 smaller doses given by mouth every 8 hours. The typical maintenance dose is about half of the starting dose.
Interactions between Methimazole and other drugs
Using this medicine with any of the following medicines is usually not recommended, but may be required in some cases. If both medicines are prescribed together, your doctor may change the dose or how often you use one or both of the medicines.
- Bupropion
- Tegafur
Using this medicine with any of the following medicines may cause an increased risk of certain side effects, but using both drugs may be the best treatment for you. If both medicines are prescribed together, your doctor may change the dose or how often you use one or both of the medicines.
- Acenocoumarol
- Anisindione
- Dicumarol
- Phenindione
- Phenprocoumon
- Warfarin
How much does Methimazole cost?
What are alternatives to Methimazole?
High thyroid hormone levels (hyperthyroidism) in people:
With Graves’ disease or other medical conditions who can’t receive thyroid surgery or radioactive iodine treatment
Who are preparing to undergo thyroid surgery or radioactive iodine treatment
High thyroid hormone levels (hyperthyroidism) in people:
What is the latest news about Methimazole?
Methimazole images
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References
Best studies we foundAmerican Thyroid Association. (n.d.). Hypothyroidism (underactive).
American Thyroid Association. (n.d.). Radioactive iodine.
American Thyroid Association. (n.d.). Thyroid and weight.
Benker, G., et al. (1995). Response to methimazole in Graves' disease. The European Multicenter Study Group. Clinical Endocrinology.
Chartwell RX, LLC. (2022). METHIMAZOLE tablet [package insert]. DailyMed.
Contreras-Jurado, C., et al. (2015). Thyroid hormone signaling controls hair follicle stem cell function. Molecular Biology of the Cell.
Cooper, D. S., et al. (2020). Methimazole drug allergy: A possible solution using a “methimazole solution”. Endocrine Practice.
Ebner, S. (2019). Risk of birth defects in babies from mothers with hyperthyroidism treated with methimazole or propylthiouracil. Clinical Thyroidology for the Public.
MedlinePlus. (2022). Metabolism.
National Institute of Diabetes and Digestive and Kidney Diseases. (2021). Hyperthyroidism (overactive thyroid).
Office of Dietary Supplements. (2023). Iodine. National Institutes of Health.
Ross, D. S., et al. (2016). 2016 American Thyroid Association guidelines for diagnosis and management of hyperthyroidism and other causes of thyrotoxicosis. Thyroid.
Sedhai, Y. R., et al. (2023). Agranulocytosis. StatPearls.
Sharma, V. (2019). Long-term methimazole is an effective treatment for toxic nodular goiters. Clinical Thyroidology for the Public.
Sulanc, E. (2020). Treatment for hyperthyroidism increases the risk of obesity in some patients. Clinical Thyroidology for the Public.
Tan, S., et al. (2021). The efficiency and safety of methimazole and propylthiouracil in hyperthyroidism: A meta-analysis of randomized controlled trials. Medicine.
You and Your Hormones. (2020). Thyroid gland. Society for Endocrinology.
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