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Antithyroid Drugs

Antithyroid drugs, or thyreostatics, fall into a group of medications that are prescribed with the aim to inhibit the synthesis of thyroid hormones in endogenous thyrotoxicosis syndrome (in most cases, drugs are used to treat diffuse toxic goiter). Find out what these drugs are and how antithyroid drugs work. Learn what you should know about the possible adverse body reactions before you start taking drugs from this group.

What Are Antithyroid Drugs?

Antithyroid drugs are effective for the treatment of patients whose bodies produce thyroid hormones in a large amount (for example, diffuse toxic goiter). This group includes such medicines as thiamazole (Mercazolil, Metizol, Tyrozol), propylthiouracil (Propicil), potassium perchlorate.

Antithyroid drugs (thyrostatics) can inhibit peroxidase, a key enzyme in the synthesis of thyroid hormones, and prevent the accumulation of iodine in the thyroid gland. As a result, the synthesis of thyroid hormones is disrupted (thyrostatic action).

Antithyroid Drugs Mechanism of Action

Check the mechanism of the antithyroid drug’s action before you begin the treatment course. The synthesis of thyroid hormones is regulated by the thyroid-stimulating hormone of the pituitary gland and depends on factors such as:

  • The intake of adequate amounts of iodine into the thyroid gland, which is part of the thyroid hormones (T3 – triiodothyronine and T4 – thyroxine), which are iodized derivatives of the amino acid tyrosine.
  • Intact pathways of iodine metabolism in the gland.
  • Production of thyroglobulin protein by the follicular cells of the thyroid gland, in which the synthesis of thyroid hormones is carried out.

The processes of synthesis and secretion of thyroid hormones are carried out in four stages:

  • Active transport of iodine from blood plasma to the thyroid gland.
  • Oxidation of iodine with the participation of the thyroid gland’s enzyme peroxidase together with the formation of inactive precursors of thyroid hormones – monoiodotyrosine and diiodotyrosine.
  • Formation of thyroid hormones from monoiodotyrosine and diiodotyrosine in the thyroglobulin molecule with the participation of the peroxidase enzyme.
  • Secretion of thyroid hormones into the blood.

Antithyroid drugs (thiamazole and propylthiouracil) interfere with the synthesis of thyroid hormones by inhibiting peroxidase (a key enzyme in the synthesis of thyroid hormones). As a result, the concentration of thyroid hormones in the blood decreases (thyrostatic effect).

Synthesis and secretion of thyroid hormones and mechanisms of action of antithyroid drugs

The thyrostatic effect of potassium perchlorate is associated with the ability of this drug to prevent the accumulation of iodine by the thyroid gland, which leads to inhibition of the formation of thyroxine.

By the mechanism of action, four groups of thyreostatics drugs are distinguished:

  • Disturbing the transport of iodine into the follicles (potassium perchlorate);
  • Violating the synthesis of thyroid hormones (thionamides: thiamazole, carbimazole, propylthiouracil);
  • Inhibiting the release of thyroid hormones (iodides in pharmacological doses, lithium carbonate);
  • Destroying the follicles of the thyroid gland (radioactive iodine).

Thionamides (thiourea derivatives) contain a thionamide group. Currently, only two drugs based on the imidazole ring are used – methimazole and propylthiouracil, which has a pyrimidine ring.

The main function of the drugs that fall into the antithyroid group of medications is to make the level of hormones T3 and T4 (hormones that the thyroid gland produces) lower. Antithyroid drugs can be prescribed with different aims.

Function of Antithyroid Drugs

The main function of the drugs that fall into the antithyroid group of medications is to make the level of hormones T3 and T4 (hormones that the thyroid gland produces) lower. Antithyroid drugs can be prescribed with different aims.

Preparing for thyroid surgery or radioiodine. 

Clinicians ask patients who are preparing for thyroid surgery or radioiodine to take antithyroid drugs for some time. It is done with the aim to stabilize the patient’s condition.

Initial treatment of Graves’ disease

Patients diagnosed with Graves’ disease should take thyreostatics for 1-2 years. Rarely, they help fight the disease to the full extent but in 30% of cases, patients experience the remission of the Graves’ disease. Antithyroid drugs are the only solution for Graves’ disease.

Treating of hyperthyroidism associated with toxic goiter or adenoma

In the above-mentioned cases, antithyroid drugs are used to prepare patients for the surgery. However, in some cases, drugs turn out to be helpful, and patients manage to avoid unwanted surgery or radioiodine therapy.

Treating women with hyperthyroidism during pregnancy

Pregnant women can be treated with thyreostatics only if the doctor prescribes to take them. These medications should be taken when the expected health benefits for the future mother exceed possible dangers to an unborn baby.

Types of Antithyroid Drugs

Today, two types of antithyroid drugs are used all over the world. They are known as Methimazole and Propylthiouracil. Learn more about them to be aware of all the mechanisms of action and distinctive features of each one. The antithyroid list of drugs includes two medications that are used all over the world today as they have been approved as the most effective.


Methimazole (another name is Tapazole) is the drug that is prescribed with the aim to lower the level of T4. The medication doesn’t provide an instant result. You should take it for at least one month and two weeks more to achieve the desired result. The drug is taken once a day when a patient is preparing for radioiodine therapy. The drug differs from other thyreostatics with fewer health risks. But keep in mind that there are certain contraindications. You shouldn’t start the treatment course if you are sensitive to one of the ingredients, suffer from leukopenia or agranulocytosis, have been diagnosed with hepatic failure or serious liver diseases. If you have had the treatment with Methimazole already and experienced  severe side effects, don’t take it any longer and ask your healthcare specialist to prescribe another drug or a lower dosage of Methimazole. Inform the doctor about the history of severe adverse or allergic reactions to propylthiouracil or methimazole before you start the course of treatment.


Compared to Methimazole, this antithyroid drug can cause adverse body reactions more often. It can provide you with the desired effect to decrease the level of the hormone T4 within a longer period. Doctors warn about the possible side effects the drug causes to the liver. You need to take it 2-3 times a day, which may be damaging to the liver. That’s why most healthcare specialists recommend taking the drug only when you have had the course of Methimazole that was ineffective.

You should take this medication inside, without chewing, drinking plenty of liquid. Adults and children over 10 years old – 75-100 mg/day, in severe cases and after a previous load with iodine – up to 300-600 mg/day in several doses. The maintenance dose is 25-150 mg per day. For children 6-10 years old, the initial dose is 50-150 mg per day while the maintenance dose is 25-50 mg per day. Newborns – 5-10 mg/kg/day in 3 divided doses (in the absence of effect, the dose is increased by 1.5-2 times) while the recommended maintenance dose is 3-4 mg/kg /day. The interval between doses should be not less than 6-8 hours. The duration of the course of treatment is determined individually, depending on the severity of the patient’s symptoms.

The average single dose is 100-300 mg; the average daily dose for children from 6 to 10 years old is 50-150 mg per day, over 10 years old – 150-300 mg per day. The duration of therapy can be maximum of 1-1.5 years. During the treatment period, you should monitor whether there are any changes in the condition of your thyroid gland. Patients who are treated with this type of drugs should control the blood composition on a regular basis. Why should you do this? The results of these tests will show what level of hormones you have. In case, the test determines the unwanted change in blood cells or hormones, you’ll have to go to the doctor and he/she will think how to deal with this situation, whether to decrease the dosage or switch to another drug.

The main danger is the appearance of the signs of a severe side effect known as agranulocytosis. It can develop with the speed of the light. That’s why you should be aware of all the possible agranulocytosis signs, which may develop within a short time. You should tell the doctor about any strange side effects, among which are stomatitis, fever, sore throat. Take into account that the treatment must be stopped instantly in case of the goiter development. If you don’t have any contraindications and severe side effects, you can continue leading your usual everyday life, including driving a car and working with mechanisms requiring concentration as the drug doesn’t cause negative effects on the ability to control machines.

Methimazole (Tapazole)


Intake period: no less than 1 month.

Intake period: It’s determined individually. The maximum period is up to 1-1.5 years.

Fewer risks

Higher risks to experience severe side effects caused to liver due to the longer period of treatment

Side Effects of Antithyroid Drugs

Like any other drug, antithyroid drugs may cause side effects. It isn’t obligatory that each patient will suffer from all the possible adverse reactions. However, it’s necessary to be aware of all the possible health risks to know when you should seek urgent medical help. Check what side effects of antithyroid drugs you may experience when taking one of the most commonly prescribed medications from the same group.

Minor side effects

Among minor side effects are such as swelling, itching, pain in joints. Some patients say that they have had nausea but this side effect may appear if you take large doses of the drug. If you start taking a lower dosage of the drug, the unpleasant side effect may disappear.

Major side effects

Few patients who have the course of treatment with thyreostatics complain that they have had severe side effects. However, you should be aware of all the possible serious side effects to be able to take urgent medical measures in case of a need. One of the possible major side effects is the increased production of white blood cells. This condition takes place very rarely and is more dangerous for elderly people. In this case, the human body can’t fight any bacteria and infections. Among other serious side effects are such as liver damage, vasculitis, and anemia. Fortunately, serious side effects don’t appear often and are reported by a few people taking antithyroid drugs in the right doses prescribed by healthcare specialists.

Remember that you should inform your doctor about any side effects during the course of treatment. Your healthcare specialist may prescribe you to try another medication from the group of thyreostatics. In some cases, this way to avoid negative reactions is helpful. But there is no 100% guarantee that you won’t face unwanted body reactions when you switch to some other drug as half of the patients who have complained about the side effects and switched to another medication had the same or similar side effects.

Dose of Using

Take drugs after meals up to 4 times per day. The frequency of intake depends on the severity of the symptoms the patient has. In most cases, these drugs are taken for 6 weeks and longer. After the remission, the doctor will tell you how to decrease the dosage gradually. However, you shouldn’t stop taking the drug after the remission. It’s necessary to achieve a stable therapeutic effect continuing the intake of the drug at least once in three days. Adults and kids should take different doses of drugs. Check what they are as the violation of the dosage may lead to dangerous health conditions or the drug won’t show any positive effects.

For Children

The dosage of the medication for children is calculated depending on the weight of the kid. Usually, doctors prescribe to take 0.3-0.5 mg per 1 kg of weight. After the remission, the dosage should be decreased up to one dosage per day. It’s better to take the drug in the morning as long as the doctor has prescribed until there is a noticeable stable therapeutic effect.

For Adults

Adults can take up to 60 mg per day while the amount of one dosage shouldn’t exceed 10 mg. The doctor may either increase or decrease the dosage of the drug in each individual case after you take some tests, including the antibodies test. Pregnant women shouldn’t exceed the dosage of 10 mg a day.

Warnings and Precautions

There are certain warnings you need to keep in mind when taking antithyroid drugs. It’s recommended to do a blood test to control the level of hormones regularly during the course of treatment. Certain groups of patients should take antithyroid drugs with special care. Check whether you refer to one of the following categories.


Make sure you aren’t allergic to the main ingredients of thyreostatics before you start the course of treatment. You are highly recommended to inform the doctor about all the medications you are taking at the moment to prevent the negative consequences of interactions. Also, inform the doctor if you feel swelling, difficulties with breathing, and other manifestations of allergic reactions.


Breastfeeding moms aren’t recommended to take any of the antithyroid drugs as the substances will be transferred to a baby with the mother’s milk. That’s why it’s better to wait until the breastfeeding period is over and start the treatment course with thyreostatics.

Pregnancy and Lactation

What about the intake of antithyroid drugs during pregnancy? Take into account that the drug should be chosen carefully depending on the trimester of pregnancy. Doctors prescribe Propylthiouracil in the first 3 months of pregnancy as it’s safer for the unborn baby than Methimazole. The opinions differ but most healthcare specialists recommend changing the drug in 3 months and start the treatment course with Methimazole as it doesn’t cause severe liver damage as Propylthiouracil does. However, some doctors think that it is better to continue taking one drug and not switch to another one.

Pregnant women are recommended to discuss all the health conditions and side effects with the doctor to avoid unwanted side effects and risks for the unborn baby. The best way to avoid possible side effects during pregnancy is to start the treatment course at least half a year before you plan to become pregnant. Women aren’t recommended to take thyreostatics during the lactation period as they are transferred with the breast milk. Propylthiouracil crosses the placenta and causes hypothyroidism and goiter in the fetus, however, the potential risk of side effects of antithyroid drugs and the negative effects of prolonged thyrotoxicosis during pregnancy must be weighed. Propylthiouracil is considered by some clinicians as the drug of choice for antithyroid therapy during pregnancy.

Short-Term Effects

In several weeks, patients start feeling better, their symptoms disappear or become not so disturbing. However, such an effect doesn’t last for long after patients stop taking antithyroid drugs.
The short-term intake of antithyroid drugs (not longer than 6 weeks) will lead to the alleviation of severe symptoms only but you are unlikely not to face the hyperthyroidism problem again with time.

Long-Term Effects

The long-term effect can be achieved only if you take antithyroid drugs for up to 18 months.
During the treatment course, your doctor should control all the changes that take place in your body using special tests. The period of intake of antithyroid drugs shouldn’t be shorter than 12 weeks. Most patients admit that they felt better after 6 weeks. But the alleviation of symptoms doesn’t mean you need to stop taking thyreostatics. You should continue the treatment course as the doctor has prescribed and decreased the dosage gradually under the constant control of a competent healthcare specialist. You will achieve a strong therapeutic effect if you take antithyroid drugs the right way. The long-term intake of antithyroid drugs with the correctly chosen dosage will help you forget about hyperthyroidism forever. So, you need to take the drug regularly for the prevention of the disease recurrence.

Storage Сonditions of Antithyroid Drugs

You shouldn’t take thyreostatics after the expiration date. Store them in a dry place where the temperature doesn’t exceed 25 degrees. You should make sure that there is no exposure to the sunlight in the place where you keep antithyroid drugs. It’s necessary to store the medication in a place where children can’t find it. Remember that the correct storage of the drug is the guarantee of its effectiveness. If you store antithyroid drugs not the way it’s written in the instructions, there is a risk of taking the drug that won’t show the desired effect.

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