Some commonly used brand names are:

In the U.S.—

In Canada—

Another commonly used name for methimazole is thiamazole.

• Antihyperthyroid agent -- Methimazole; Propylthiouracil

Methimazole and propylthiouracil are used to treat conditions in which the thyroid gland produces too much thyroid hormone.

These medicines work by making it harder for the body to use iodine to make thyroid hormone. They do not block the effects of thyroid hormone that was made by the body before their use was begun.

Methimazole and propylthiouracil are available only with your doctor's prescription, in the following dosage forms:

Oral
• Methimazole
  • Tablets (U.S. and Canada)
• Propylthiouracil
  • Tablets (U.S. and Canada)

In deciding to use a medicine, the risks of taking the medicine must be weighed against the good it will do. This is a decision you and your doctor will make. For antithyroid agents, the following should be considered:

Allergies—Tell your doctor if you have ever had any unusual or allergic reaction to methimazole or propylthiouracil. Also tell your health care professional if you are allergic to any other substances, such as foods, preservatives, or dyes.

Pregnancy—Use of too large a dose during pregnancy may cause problems in the fetus. However, use of the proper dose, with careful monitoring by the doctor, is not likely to cause problems.

Breast-feeding—These medicines pass into breast milk. (Methimazole passes into breast milk more freely and in higher amounts than propylthiouracil.) However, your doctor may allow you to continue to breast-feed, if your dose is low and the infant gets frequent check-ups. If you are taking a large dose, it may be necessary for you to stop breast-feeding during treatment.

Children—This medicine has been used in children and, in effective doses, has not been shown to cause different side effects or problems in children than it does in adults.

Older adults—Elderly people may have an increased chance of certain side effects during treatment. Your doctor may need to take special precautions while you are taking this medicine.

Other medicines—Although certain medicines should not be used together at all, in other cases two different medicines may be used together even if an interaction might occur. In these cases, your doctor may want to change the dose, or other precautions may be necessary. When you are taking antithyroid agents, it is especially important that your health care professional know if you are taking any of the following:

• Amiodarone or
• Iodinated glycerol or
• Potassium iodide (e.g., Pima)—The use of these medicines may change the effect of antithyroid agents

• Anticoagulants (blood thinners)—The use of antithyroid agents may affect the way anticoagulants work in your body

• Beta-adrenergic blocking agents (e.g., Inderal, Metoprolol, Sotalol)—The use of antithyroid agents may change the amount of beta-blockers you need to take.
• Digitalis glycosides—The use of antithyroid agents may affect the amount of digitalis glycosides in the bloodstream

• Liver disease—The body may not get this medicine out of the bloodstream at the usual rate, which may increase the chance of side effects

Use this medicine only as directed by your doctor. Do not use more or less of it and do not use it more often or for a longer time than your doctor ordered. To do so may increase the chance of side effects.

This medicine works best when there is a constant amount in the blood. To help keep the amount constant, do not miss any doses. Also, if you are taking more than one dose a day, it is best to take the doses at evenly spaced times day and night. For example, if you are to take 3 doses a day, the doses should be spaced about 8 hours apart. If this interferes with your sleep or other daily activities, or if you need help in planning the best times to take your medicine, check with your health care professional.

Food in your stomach may change the amount of methimazole that is able to enter the bloodstream. To make sure that you always get the same effects, try to take methimazole at the same time in relation to meals every day. That is, always take it with meals or always take it on an empty stomach.

Dosing—

The dose of these medicines will be different for different patients. Follow your doctor's orders or the directions on the label. The following information includes only the average doses of these medicines. If your dose is different, do not change it unless your doctor tells you to do so.

The number of tablets that you take or the number of suppositories that you use depends on the strength of the medicine. Also, the number of doses you take each day, the time allowed between doses, and the length of time you take the medicine depend on the medical problem for which you are taking antithyroid agents.

For methimazole
• For oral dosage form (tablets):
  • For treatment of hyperthyroidism (overactive thyroid):
    • Adults and teenagers—At first, 15 to 60 milligrams (mg) a day for up to six to eight weeks. Later, your doctor may want to lower your dose to 5 to 30 mg a day. This may be taken once a day or it may be divided into two doses a day.
    • Children—Dose is based on body weight and must be determined by your doctor. The usual dose is 0.4 mg per kilogram (kg) (0.18 mg per pound) of body weight a day. Later, your doctor may want to lower the dose to 0.2 mg per kg (0.09 mg per pound) of body weight a day. The dose may be taken once a day or it may be divided into two doses a day.
  • For treatment of thyrotoxicosis (a thyroid emergency):
    • Adults and teenagers—15 to 20 mg every four hours.
• For rectal dosage form (suppositories):
  • For treatment of thyrotoxicosis (a thyroid emergency):
    • Adults and teenagers—15 to 20 mg inserted into the rectum every four hours. Your doctor may change your dose as needed.
    • Children—The dose is based on body weight and must be determined by your doctor. The usual dose is 0.4 mg per kg (0.18 mg per pound) of body weight inserted into the rectum a day. This may be used as a single dose or it may be divided into two doses a day.

For propylthiouracil
• For oral dosage form (tablets):
  • For treatment of hyperthyroidism (overactive thyroid):
    • Adults and teenagers—At first, 300 to 900 milligrams (mg) a day. Some people may need up to 1200 mg a day. This may be taken as a single dose or it may be divided into two to four doses in a day. Later, your doctor may lower your dose to 50 to 600 mg a day.
    • Children 6 to 10 years of age—At first, 50 to 150 mg a day. This may be taken as a single dose or it may be divided into two to four doses in a day. Later, your doctor may change your dose as needed.
    • Children 10 years of age and older—At first, 50 to 300 mg a day. This may be taken as a single dose or it may be divided into two to four doses in a day. Then, your doctor may change your dose as needed.
  • For treatment of thyrotoxicosis (a thyroid emergency):
    • Adults and teenagers—200 to 400 mg every four hours. Your doctor will lower your dose as needed.
    • Newborn infants—Dose is based on body weight and must be determined by your doctor. The usual dose is 10 mg per kilogram (kg) (4.5 mg per pound) of body weight a day. This is usually divided into more than one dose a day.
• For rectal dosage forms (enemas or suppositories):
  • For treatment of thyrotoxicosis (a thyroid emergency):
    • Adults and teenagers—200 to 400 mg inserted into the rectum every four hours. Your doctor may change your dose as needed.
    • Children 6 to 10 years of age—50 to 150 mg inserted into the rectum a day. This dose may be used as a single dose or it may be divided into two to four doses in a day. Your doctor may change your dose as needed.
    • Children 10 years of age and older—50 to 300 mg inserted into the rectum a day. This dose may be used as a single dose or it may be divided into two to four doses in a day. Your doctor may change your dose as needed.
    • Newborn infants—Dose is based on body weight and must be determined by your doctor. The usual dose is 10 mg per kg (4.5 mg per pound) of body weight inserted into the rectum. This is usually divided into more than one dose a day. Your doctor may change your dose as needed.

Missed dose—

If you miss a dose of this medicine, take it as soon as possible. If it is almost time for your next dose, take both doses together. Then go back to your regular dosing schedule. If you miss more than one dose or if you have any questions about this, check with your doctor.

Storage—

To store this medicine:

• Keep out of the reach of children.
• Store away from heat and direct light.
• Do not store in the bathroom, near the kitchen sink, or in other high-moisture areas. Heat or moisture may cause the medicine to break down.
• Do not keep outdated medicine or medicine no longer needed. Be sure that any discarded medicine is out of the reach of children.

It is very important that your doctor check your progress at regular visits to make sure that this medicine is working properly and to check for unwanted effects.

It may take several days or weeks for this medicine to work. However, do not stop taking this medicine without first checking with your doctor. Some medical problems may require several years of continuous treatment.

Before having any kind of surgery (including dental surgery) or emergency treatment, tell the medical doctor or dentist in charge that you are taking this medicine.

Check with your doctor right away if you get an injury, infection, or illness of any kind. Your doctor may want you to stop taking this medicine or change the amount you are taking.

While you are being treated with antithyroid agents, and after you stop treatment with it, do not have any immunizations (vaccinations) without your doctor's approval. Antithyroid agents may lower your body's resistance and there is a chance you might get the infection the immunization is meant to prevent. In addition, other persons living in your household should not take or have recently taken oral polio vaccine since there is a chance they could pass the polio virus on to you. Also, avoid other persons who have taken oral polio vaccine. Do not get close to them, and do not stay in the same room with them for very long. If you cannot take these precautions, you should consider wearing a protective face mask that covers the nose and mouth.

Before you have any medical tests, tell the doctor in charge that you are taking this medicine. The results of some tests may be affected by this medicine.

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:

• Less common
  • Cough;  fever or chills (continuing or severe);  general feeling of discomfort, illness or weakness;  hoarseness;  mouth sores;  pain, swelling, or redness in joints;  throat infection 
  
  

• Rare
  • Yellow eyes or skin  
  
  

Check with your doctor as soon as possible if any of the following side effects occur:

• More common
  • Fever (mild and temporary);  skin rash or itching 
  
  

• Rare
  • Backache;  black, tarry stools;  blood in urine or stools;  shortness of breath;  increase in bleeding or bruising;  increase or decrease in urination;  numbness or tingling of fingers, toes, or face;  pinpoint red spots on skin;  swelling of feet or lower legs;  swollen lymph nodes;  swollen salivary glands 
  
  

• Symptoms of overdose
  • Changes in menstrual periods;  coldness ;  constipation;  dry, puffy skin;  headache;  listlessness or sleepiness;  muscle aches;  swelling in the front of the neck;  unusual tiredness or weakness;  weight gain (unusual) 
  
  

Other 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. However, check with your doctor if any of the following side effects continue or are bothersome:

• Less common
  • Dizziness;  loss of taste (for methimazole) ;  nausea;  stomach pain;  vomiting 
  
  

Other side effects not listed above may also occur in some patients. If you notice any other effects, check with your doctor.