Antithyroid Agent (Oral, Rectal)

Antithyroid Agent is a medicine that makes it more difficult for the body to consume iodine in order to create the thyroid hormone,


What are Antithyroid Agents?

Antithyroid Agents are a group of drugs available only by a doctor's prescription that are used to treat conditions where the body is making too many thyroid hormones. This condition is also referred to hyperthyroidism or over-active thyroid.

What drugs are considered Antithyroid Agents?

Two antithyroid drugs commonly used are methimazole and propylthiouracil. These drugs are also known under the following brand names, which they are sold under:

  • Pima
  • SSKI
  • Tapazole
  • ThyroShield
  • Zemplar

Both drugs are available in tablet form, liquid filled capsules or as a solution or syrup and can be administered via an oral route or a rectal route.

How do these Antithyroid Agent drugs work?

These Antithyroid Agent drugs work by reducing the amount of thyroid hormone that the body has in circulation at any one time. They are used both in short-term therapy to prepare the patient for radioactive iodine treatment or surgery as well as long-term therapy, with the goal of curing patients with Graves' disease, hyperthyroidism or thyrotoxicosis.

These medicines make it more difficult for the body to use iodine in the making of thyroid hormones, which are essential to the body's metabolism.

What is hyperthyroidism?

Hyperthyroidism is when the thyroid gland is aggressively producing the thyroid hormone, leading to excessive amounts of the hormone in the body. The thyroid hormone controls the body's metabolism, which is the road map to how your body uses energy, your heart rate and breathing, nervous system, weight, body temperature and many other functions.

When the thyroid is overactive, the body's processes speed up which gives the sufferer a host of symptoms that range from:

  • Nervousness
  • Anxiety
  • Rapid heartbeat
  • Hand tremors
  • Sweating
  • Weight loss
  • Insomnia
  • Fatigue
  • Muscle weakness
  • Mood swings

Some patients may even develop an enlarged thyroid gland that feels like a swollen lump in the front of the neck. This is called a goiter and is a key symptom of hyperthyroidism.

What is Graves' disease?

The most common cause of hyperthyroidism is an autoimmune disorder known as Graves' disease. Graves' disease is defined as when the body makes an antibody that causes the thyroid gland to make too much thyroid hormone, leading to hyperthyroidism. Grave's disease is most commonly found in women and is typically hereditary.

What else causes hyperthyroidism?

Hyperthyroidism can also be brought on as the result of the virus or an immune system disease that temporarily causes symptoms of hyperthyroidism. Rarely, some people taking supplements or medications containing iodine or those who are eating too many iodine-rich foods may also exhibit signs of hyperthyroidism.

Conditions treated

Type of medicine

  • Hormone antagonist

Side effects

Some medications, in the course of their desired effects, also cause side effects during the course of treatment. These side effects are often mild and of short duration, typically stopping completely once the course of treatment is over. Antithyroid Agent medications sometimes exhibit the following side effects:

  • Cough
  • Fever
  • Chills
  • General discomfort
  • General ill feeling
  • Loss of taste
  • Nausea and vomiting
  • Dizziness
  • Stomach pain
  • Weakness
  • Hoarseness
  • Mouth sores
  • Pain, swelling or redness in joints
  • Throat infection
  • Skin rash or itching
  • Yellow skin or eyes

If these side effects occur and you consider them to be severe or prolonged, contact your health care professional immediately. The following side effects, if experienced, could possibly mean that there are underlying health issues that need to be addressed prior to continuing treatment. If you experience any of these side effects, contact your doctor immediately:

  • Backache
  • Black, tarry stools
  • Blood in urine or stools
  • Increased bruising or bleeding
  • Increase or decrease in urination
  • Numbness or tingling in fingers, toes or face
  • Pinpoint red spots on skin
  • Shortness of breath
  • Swelling of feet or lower legs
  • Swollen lymph nodes
  • Swollen salivary glands

Again, if these side effects are present, you should contact your healthcare provider immediately.

The following side effects are directly the result of an overdose of Antithyroid Agent medication and should be reported to your doctor right away:

  • Change in menstrual periods
  • Coldness
  • Constipation
  • Dry, puffy skin
  • Headache
  • Listlessness or sleepiness
  • Muscle aches
  • Swelling in the front of the neck
  • Unusual weakness or fatigue
  • Weight gain


Antithyroid Agent medication will be prescribed by your doctor with your body type, symptoms and personal diagnosis in mind. Do not increase or decrease the prescribed dose or use it more frequently or for a longer duration than your doctor has prescribed for you. Doing so will compromise the safety of the medication as well as decrease the effectiveness.

Antithyroid Agent medication works best when there is a constant amount of it in the body, so no doses should be missed when taking them. If your prescription is for multiple doses per day, it's best to always space the doses apart as your doctor has instructed.

Taking antithyroid agent medications with food may affect the amount of the medication that is able to enter your bloodstream. Try to take antithyroid agent medication either with meals or on an empty stomach, whichever your schedule allows; but be consistent.

Methimazole dose

For the treatment of hyperthyroidism in adults and teenagers who have been prescribed the oral tablets, at first the dose ranges from 15 to 60 milligrams per day, depending on the patient, for six to eight weeks. The dosage after this initial period of time may be lowered to anywhere from 5 to 30 milligrams per day. This dose may be divided into two doses per day or taken all at once.

Children are prescribed doses based on their body weight and determined by their doctor. In general, the dose for children is 0.4 milligram per kilogram of body weight per day. The dose will typically be lowered after an initial period to 0.2 milligrams per kilogram of body weight on a daily basis. Again, the dose may be divided into two in a day or given all at once, depending on your doctor's prescription.

In a thyroid emergency, known as thyrotoxicosis, adults and teenagers will be given the oral dose of medication in the amount of 15 to 20 milligrams every four hours until the condition improves. Children will have their dose determined by their health care professional based on their body weight. Typically this emergency dose will be 0.4 milligrams per kilogram of body weight per day.

For rectal dosage used in the treatment of a thyroid emergency (thyrotoxicosis), adults will be given 15 to 20 milligrams inserted into the rectum every four hours. Children will again have the dose of antithyroid agent determined by their body weight, but the dose will typically be 0.4 milligrams per kilogram of body weight.

Propylthiouracil dose

In the oral dosage form, for treatment of hyperthyroidism or overactive thyroid, adults and teenagers will be prescribed 300 to 900 milligrams per day for the initial treatment period with some patients requiring up to 1200 milligrams per day. The dose may be lowered to a range of 50 to 600 milligrams per day by your doctor, depending how your condition is responding to your treatment and how well you are tolerating the drug.

Children who are 10 years of age and older will at first take an average daily dose of 50 to 300 milligrams per day, with adjustment made after the initial treatment period. Children 6 to 10 years of age will be prescribed anywhere from 50 to 150 milligrams per day during the initial phase of treatment, which may be adjusted by their health care professional.

All ages groups may take their total dose once per day or divide it into two or four doses per day, which depends on how your doctor has prescribed the medication for your particular situation.

In the case of treatment for thyrotoxicosis, a thyroid emergency, adults and teenagers will require 200 to 400 milligrams every four hours as prescribed by your health care professional. Newborn infants will have their dose based on body weight, which is typically 10 milligrams per kilogram of body weight per day, divided into more than one dose per day.

In rectal dosage forms, available in enemas or suppositories, used during treatment of thyrotoxicosis thyroid emergencies, adults and teenagers will have 200 to 400 milligrams inserted into the rectum every four hours. Children 10 and over have an average dose of 50 to 300 milligrams of propylthiouracil while children 6 to 10 average a dose of 50 to 150 milligrams per day, possibly divided into two to four doses per day, depending on the doctor's recommendation. Newborn infants will have a dose of this medication that is based on their body weight and is typically 10 milligrams per kilogram of body weight, inserted into the rectum and divided into more than one dose per day.

Patients who have been prescribed antithyroid agent medications should do their best to take their prescribed dose at the time and frequency recommended by their physician. Missing a dose should not lead the patient to double dose as this could cause dangerous overdose side effects. If you miss a dose, take it as soon as you remember unless you are close to the next dosing time. In that case, skip the missing dose and resume your regular schedule.


It is of critical importance that you disclose your full medical history as well as details of any drugs you are currently taking or have taken in the recent past before you begin a course of drug therapy treatment with antithyroid agent medications or any other drug. This will ensure the safety of the drug treatment therapy as well as increase the effectiveness of the benefits that the therapy will provide to your symptoms.

Be certain to include all prescription as well as non-prescription medications you may be taking and details of any vitamin, herbal or holistic supplements or treatments you are currently taking or have taken in the recent past.

Methimazole interactions

Methimazole has been known to have interactions following common medications:

  • Ambien (zolpidem)
  • Aspirin Low Strength (aspirin)
  • Ativan (lorazepam)
  • Bupropion
  • Calcium 600 D (calcium / vitamin d)
  • Cymbalta (duloxetine)
  • Fish Oil (omega-3 polyunsaturated fatty acids)
  • Flexeril (cyclobenzaprine)
  • Lasix (furosemide)
  • Lyrica (pregabalin)
  • Metoprolol Succinate ER (metoprolol)
  • Metoprolol Tartrate (metoprolol)
  • Nexium (esomeprazole)
  • Plavix (clopidogrel)
  • Tegafur
  • Tylenol (acetaminophen)
  • Vitamin B12 (cyanocobalamin)
  • Vitamin C (ascorbic acid)
  • Vitamin D2 (ergocalciferol)
  • Vitamin D3 (cholecalciferol)
  • Xanax (alprazolam)
  • Zyrtec (cetirizine)

Additionally, those with the following medical conditions should be aware of the risk of increased side effects when taking methimazole:

  • Blood dyscrasias
  • Hepatotoxicity
  • Hypoprothrombinemia

Propylthiouracil interactions

Propylthiouracil has been known to interact with the following common medications:

  • Ambien (zolpidem)
  • Aspirin Low Strength (aspirin)
  • Ativan (lorazepam)
  • Bupropion
  • Fish Oil (omega-3 polyunsaturated fatty acids)
  • Inderal (propranolol)
  • Lasix (furosemide)
  • Lexapro (escitalopram)
  • Lipitor (atorvastatin)
  • Metoprolol Succinate ER (metoprolol)
  • Metoprolol Tartrate (metoprolol)
  • Plavix (clopidogrel)
  • Prozac (fluoxetine)
  • Singulair (montelukast)
  • Synthroid (levothyroxine)
  • Tegafur
  • Vitamin A, D (multivitamin)
  • Vitamin B12 (cyanocobalamin)
  • Vitamin C (ascorbic acid)
  • Vitamin D3 (cholecalciferol)
  • Xanax (alprazolam)
  • Zocor (simvastatin)

Additionally, those with the following medical conditions should be aware of the risk of increased side effects when taking propylthiouracil:

Patients are urged to fully disclose all prescription and non-prescription drugs they are currently taking or have taken in the recent past as well as any vitamin, herbal or holistic supplements and remedies they are on during this course of treatment.

Close attention should be paid to your prescription for direction on taking these antithyroid agent medications with or without food, as interactions with certain foods or with food in the stomach in general can occur. Consult with your healthcare professional if you should be consuming alcohol or using tobacco products with these medications.

If you have been diagnosed with liver disease, be aware that you are at risk for certain side effects due to the body being unable to get these antithyroid agents out of the bloodstream at a normal rate.

These antithyroid agent medications have been successfully used in children and teenagers without an exhibited increase of side effects. Elderly patients may have an increase of certain side effects, however, and should be informed of these risks by their doctor prior to starting therapy with these medications.

Pregnant women on too high of a dose of these medications may harm their unborn child, so it is of utmost importance to follow your doctor's prescription if you are taking these medications while pregnant. Breastfeeding women on a low dose of these medications may be allowed to continue breastfeeding with a low risk to their infants; however, women on larger doses will be advised to stop breastfeeding in order to protect their child from being exposed to these drugs.


These medications should be stored in its original closed container at room temperature, away from excessive heat, moisture and direct light. Store this and other medications out of sight and reach of children. Do not allow these antithyroid agent medications to freeze. If you find yourself with unused or expired antithyroid medications, consult with your healthcare provider on the proper method of disposal. Do not keep these medications past the expiration date or past your treatment time.


Antithyroid agent medication is used in the treatment of the condition of over-active thyroid known as hyperthyroidism which includes Grave's disease as well as thyrotoxicosis thyroid emergencies. Prescribed only by a physician, methimazole and propylthiouracil are used to treat these conditions which are the result of the thyroid gland producing too much of the thyroid hormone. By making it more difficult for the body to use the mineral iodine, found in common foods and supplements, to make the thyroid hormone, these drugs regulate the thyroid hormone, which is used by the body's metabolism for many life-sustaining processes.

Available in tablet, liquid filled capsule, solution and syrup form, the dose of these drugs is determined by a healthcare professional and is based on the patient's condition, age, weight, symptoms and overall health. These drug treatments are safe for use in infants, children, teenagers, and adults but are warned to affect elderly patients with more side effect risks. Low doses are typically safe for pregnant women and women who are breastfeeding.

This drug treatment may be prescribed in a once daily, twice daily or multiple times daily format. It may or may not be recommended that you take your doses with food. Following your doctor's prescription details is of utmost importance with this and any drug treatment program. Additionally, any prescription, non-prescription, vitamin, herbal or holistic remedies you are currently taking or have taken in the recent past should be disclosed in detail to your health care provider.

Methimazole and propylthiouracil have been known to interact with hundreds of drugs, many of which are very common. Patients who have been diagnosed with liver disease may be at a higher risk for certain side effects of taking antithyroid agent medications. For these reasons, it is very important to make sure your doctor is aware of your medical history including drug history and current medications.

These medications should be stored in their original containers, kept closed for safety and out of sight and reach of children. Do now allow antithyroid agent medications to freeze or be exposed to excessive heat or moisture or direct sunlight. Disposal of unused or out-of-date medications should be done under the advice of medical professionals to observe safe practices.

Last Reviewed:
December 22, 2017
Last Updated:
April 04, 2018