Methimazole

About Methimazole

Methimazole blocks the thyroid’s production of new thyroid hormone; it does not inactivate hormone already stored in the gland or circulating in the blood. The label approves it for people with Graves’ disease with hyperthyroidism or toxic multinodular goiter for whom surgery or radioactive iodine is not an appropriate option, and to ease symptoms of hyperthyroidism before thyroid surgery or radioactive iodine treatment. Adults usually start at 15 mg a day for mild disease, 30 to 40 mg for moderately severe disease and 60 mg for severe disease, divided into three doses about 8 hours apart, and later take 5 to 15 mg a day. Children start at 0.4 mg per kg of body weight a day in three doses, with a maintenance dose about half that. Thyroid blood tests (TSH and free T4) guide each dose change, because too much methimazole causes an underactive thyroid.

There is no boxed warning. The label calls agranulocytosis potentially life-threatening: a sore throat, fever, skin eruptions, headache or feeling generally unwell should be reported right away, and the drug is stopped if agranulocytosis or aplastic anemia develops. Liver injury, including acute liver failure, has been reported, though the risk appears lower than with propylthiouracil, especially in children; loss of appetite, itching or pain in the upper right abdomen calls for liver tests, and treatment stops promptly with a significant liver abnormality, such as liver enzymes above three times the upper limit of normal. Vasculitis with severe complications has occurred, so a new rash, blood in the urine or less urine, shortness of breath or coughing up blood needs prompt attention. Methimazole can also lower prothrombin and cause bleeding, so prothrombin time is checked, especially before surgery. Minor reactions, such as rash, hives, nausea, joint pain, loss of taste and hair loss, are more common. It must not be taken by anyone allergic to it.

Methimazole crosses the placenta. First-trimester use has been linked to rare birth defects, among them aplasia cutis, choanal atresia, esophageal atresia and omphalocele, so the label says an alternative antithyroid medicine may be appropriate, particularly in the first trimester, and that switching from propylthiouracil to methimazole may be preferable for the second and third trimesters. Propylthiouracil has its own boxed warning for severe liver injury and acute liver failure, though its label notes it may be the treatment of choice during or just before the first trimester. Untreated or inadequately treated Graves’ disease also carries risks in pregnancy, including maternal heart failure, miscarriage, preterm birth and stillbirth; if methimazole is used, the lowest dose that controls the disease is given, since too much can cause goiter and cretinism (severe thyroid deficiency) in the fetus. On breastfeeding the sources differ: the label says methimazole is present in breast milk, that studies found no effect on nursing infants and that thyroid function is checked at frequent intervals, while the MedlinePlus patient page says it should not be used while breastfeeding, so nursing is a decision to make with the prescriber. For children who need an antithyroid drug, the label names methimazole as the preferred choice.

Taking Methimazole

How to take it

Usually three times a day, about every 8 hours; MedlinePlus says to take it with food. Keep the thyroid blood tests your prescriber orders.

If you miss a dose

Take the missed dose as soon as you remember, unless it is almost time for the next dose; then skip it and continue your regular schedule. Do not take a double dose.

Storage

At room temperature in a tightly closed, light-resistant container, away from excess heat and moisture (not in the bathroom), out of reach of children.

Interactions to know about

It may strengthen warfarin and other oral anticoagulants, so extra prothrombin time or INR checks may be needed, especially before surgery. As thyroid levels return to normal, blood levels of beta blockers, digoxin and theophylline may rise, so their doses may need lowering.

This summarizes the official prescribing information. Your prescriber’s and pharmacist’s instructions, and the leaflet that comes with your medicine, take priority.

Why the cash price for Methimazole differs from pharmacy to pharmacy

Generic methimazole tablets come from several companies, and each pharmacy sets its own cash price, including its dispensing fee, so quotes for the same tablets can differ. Doses are split through the day and adjusted against blood tests, so the strength and tablet count written shape each quote, and a dose change can mean a new fill.

Tablets come in 5 and 10 mg, so a daily dose may be written as tablets of one strength or a mix of both, and each strength is quoted on its own.

Ways to pay less for Methimazole without insurance

• Ask whether methimazole is on the pharmacy’s flat-price generic list, and compare more than one pharmacy.
• Ask for quotes on your daily dose as 5 mg tablets and as 10 mg tablets, and ask the prescriber to write the prescription the way that suits you.
• While your dose is still being adjusted, ask for a quantity that lasts until your next thyroid blood test.
• Once your thyroid tests and dose are stable, ask the prescriber for a ninety-day supply.
• Upload the prescription to Script Unlock so licensed pharmacies can quote their cash price. You pay the pharmacy directly.

Questions to ask your pharmacist about Methimazole

1. 1.Which symptoms of infection, liver trouble or vasculitis should I report right away?
2. 2.How far apart should I take my doses, and should I take them with food?
3. 3.Will my warfarin, beta blocker, digoxin or theophylline dose need to change as my thyroid settles?
4. 4.What should I do if I am planning a pregnancy or become pregnant?

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