Methimazole

Methimazole ( Metirox 5 and 10 mg) is a member of the class of imidazoles that it imidazole-2-thione in which a methyl group replaces the hydrogen that is attached to nitrogen. It has a role as an antithyroid drug.

Methimazole is a thionamide antithyroid agent that inhibits the synthesis of thyroid hormones. It was first introduced as an antithyroid agent in 1949 and is now commonly used in the management of hyperthyroidism, particularly in those for whom more aggressive options such as surgery or radioactive iodine therapy are inappropriate. On a weight basis, methimazole is 10 times more potent than the other major antithyroid thionamide used in North America, [propylthiouracil], and is the active metabolite of the pro-drug [carbimazole], which is an antithyroid medication used in the United Kingdom and parts of the former British Commonwealth. Traditionally, methimazole has been preferentially used over propylthiouracil due to the risk of fulminant hepatotoxicity carried by the latter, with propylthiouracil being preferred in pregnancy due to a perceived lower risk of teratogenic effects. Despite documented teratogenic effects in its published labels, the true teratogenicity of methimazole appears to be unclear and its place in therapy may change in the future.

Methimazole is a Thyroid Hormone Synthesis Inhibitor. The mechanism of action of methimazole is as a Thyroid Hormone Synthesis Inhibitor.

Mechanism of Action

Methimazole’s primary mechanism of action appears to be interference in an early step in thyroid hormone synthesis involving thyroid peroxidase (TPO), however, the exact method through which methimazole inhibits this step is unclear. TPO, along with hydrogen peroxide, normally catalyzes the conversion of iodide to iodine and then further catalyzes the incorporation of this iodine onto the 3 and/or 5 positions of the phenol rings of tyrosine residues in thyroglobulin. These thyroglobulin molecules then degrade within thyroid follicular cells to form either thyroxine (T4) or tri-iodothyronine (T3), which are the main hormones produced by the thyroid gland. Methimazole may directly inhibit TPO but has been shown in vivo to instead act as a competitive substrate for TPO, thus becoming iodinated itself and interfering with the iodination of thyroglobulin. Another proposed theory is that methimazole’s sulfur moiety may interact directly with the iron atom at the center of TPO’s heme molecule, thus inhibiting its ability to iodinate tyrosine residues. Other proposed mechanisms with weaker evidence include methimazole binding directly to thyroglobulin or direct inhibition of thyroglobulin itself.

or
Methimazole inhibits the synthesis of thyroid hormones by interfering with the incorporation of iodine into tyrosyl residues of thyroglobulin; the drug also inhibits the coupling of these iodotyrosyl residues to form iodothyronine. Although the exact mechanism(s) has not been fully elucidated, methimazole may interfere with the oxidation of iodide ion and iodotyrosyl groups. Based on limited evidence, it appears that the coupling reaction is more sensitive to antithyroid agents than the iodination reaction. Methimazole does not inhibit the action of thyroid hormones already formed and present in the thyroid gland or circulation nor does the drug interfere with the effectiveness of exogenously administered thyroid hormones.
Methimazole inhibits the synthesis of thyroid hormones resulting in an alleviation of hyperthyroidism. The onset of action occurs within 12 to 18 hours, and its duration of action is 36 to 72 hours, likely due to the concentration of methimazole and some metabolites within the thyroid gland after administration. The most serious potential side effect of methimazole therapy is agranulocytosis, and patients should be instructed to monitor for, and report, any signs or symptoms of agranulocytosis such as fever or sore throat. Other cytopenias may also occur during methimazole therapy. There also exists the potential for severe hepatic toxicity with the use of methimazole, and monitoring for signs and symptoms of hepatic dysfunction, such as jaundice, anorexia, pruritus, and elevation in liver transaminases, is prudent in patients using this therapy.

Indications

  • Antithyroid Agents especially hyperthyroidism.
  • In the United States, methimazole is indicated for the treatment of hyperthyroidism in patients with Graves’ disease or toxic multinodular goiter for whom thyroidectomy or radioactive iodine therapy are not appropriate treatment options. Methimazole is also indicated for the amelioration of hyperthyroid symptoms in preparation for thyroidectomy or radioactive iodine therapy. In Canada, methimazole carries the above indications and is also indicated for the medical treatment of hyperthyroidism regardless of other available treatment options.
  • Methimazole is an antithyroid medication that is now considered the first-line agent for medical therapy of hyperthyroidism and Graves disease. Methimazole has been linked to serum aminotransferase elevations during therapy as well as to a clinically apparent, idiosyncratic liver injury that is typically cholestatic and self-limited in course.
  • A prospective randomized control study of combination therapy with low-dose methimazole and a cholesterol absorption inhibitor as for the initial treatment of childhood-onset Graves disease
  • Methimazole /is/ indicated in the treatment of hyperthyroidism, including prior to surgery or radiotherapy, and as adjunct in the treatment of thyrotoxicosis or thyroid storm. Propylthiouracil may be preferred over methimazole for use in thyroid storm, since propylthiouracil inhibits peripheral conversion of thyroxine (T4) to triiodothyronine (T3)
  • Graves’ Disease
  • Hyperthyroidism
  • Toxic multinodular goiter
  • Thiamazole, also known as methimazole, is a medication used to treat hyperthyroidism.[rx] This includes Graves disease, toxic multinodular goiter, and thyrotoxic crisis.[rx] It is taken by mouth.[rx] Full effects may take a few weeks to occur.[rx]

Contraindications

  • Methimazole is contraindicated if there is hypersensitivity to the drug or any of its components.
  • It is relatively contraindicated during pregnancy.
  • Hypersensitivity to the active substance or any product excipients

Dosages

Strengths: 5 mg; 10 mg; 15 mg; 20 mg

Hyperthyroidism

Initial dose:

  • Mild hyperthyroidism: 15 mg orally per day
  • Moderately severe hyperthyroidism: 30 to 40 mg orally per day
  • Severe hyperthyroidism: 60 mg orally per day

Maintenance dose:

  • 5 to 15 mg orally per day. Daily doses are usually given in 3 divided doses at approximately 8 hour intervals
  • For the treatment of Graves’ disease with hyperthyroidism or toxic multinodular goiter in whom surgery or radioactive iodine therapy is not an appropriate treatment option.
  • To ameliorate symptoms of hyperthyroidism in preparation for thyroidectomy or radioactive iodine therapy.

Pediatric Dose for Hyperthyroidism

  • Initial dose: 0.4 mg/kg orally per day
  • Maintenance dose: 0.2 mg/kg orally per day (approximately half the initial dose)
  • Daily doses are usually given in 3 divided doses at approximately 8 hour intervals

or

Methimazole is available as oral tablets in 5 mg and 10 mg strengths. The starting dose is between 20 to 40 mg per day, depending upon the severity of the disease.

  • The daily dose gets divided into three doses every 8 hours.
  • As per the “titration regimen,” the high starting dose is then tapered after 4 to 8 weeks. A maintenance dose of 5 to 20 mg follows after almost 4 to 6 months of therapy, which continues for an extra 12 to 18 months.
  • As per the “block–replace regimen,” a high dose of antithyroid drugs is maintained, but with levothyroxine therapy to maintain a euthyroid state. It has the added benefit of needing fewer thyroid function tests (TFTs) for monitoring but with a slightly increased side effect frequency.

The treatment of thyroid storm includes a starting dose of 60 to 80 mg/day orally until achieving control, also given at 8-hour intervals. Adjust the subsequent doses and duration of treatment as per patient response.

Methimazole has a narrow therapeutic window. Therefore it is essential to note the maximum allowed dosage.

  • Adults:40 mg/day orally; up to 60 mg/day in severe disease.
  • Geriatric:40 mg/day orally; up to 60 mg/day in severe disease.
  • Adolescents: Maintenance doses rarely exceed 30 mg/day orally; 1 mg/kg/day orally in severe hyperthyroidism. Patients who have attained full growth doses may approach adult dosing.
  • Children: Maintenance doses rarely exceed 30 mg/day orally or 1 mg/kg/day if severe hyperthyroidism.
  • Infants:1 mg/kg/day if severe hyperthyroidism.
  • Pregnant Women: It is classified as Pregnancy Category D medicine. Fetal harm is reported when methimazole is administered to a pregnant woman. If the woman becomes pregnant while taking methimazole or when methimazole is used during pregnancy, the patient should be counseled on the potential risk to the fetus. In addition, prescribe alternative treatments for hyperthyr

Administration advice:

  • Take orally in equally divided doses at approximately 8-hour intervals

General:

  • Patients treated with this drug should be under close surveillance due to the potential for serious adverse effects.
  • Avoid use during the first trimester of pregnancy due to reports of fetal abnormalities; after the first trimester it may be preferable to switch to methimazole due to hepatotoxicity of propylthiouracil.

Monitoring:

  • Routine monitoring of TSH and free T4 levels is necessary to avoid under or over-treatment
  • Consider monitoring prothrombin time if there are concerns of bleeding; prothrombin time should be monitored before surgical procedures
  • Obtain complete blood counts if there are signs and symptoms of agranulocytosis
  • Perform liver function testing if there are any symptoms of hepatic dysfunction

Patient dosage advice:

  • Patients should understand the importance of contacting their healthcare provider promptly if they experience any signs or symptoms of liver dysfunction, low blood counts, bleeding, or vasculitis.
  • Women should be instructed to speak to their healthcare provider if they are pregnant or plan to become pregnant.

Side Effects

The Most Common

  • skin rash
  • itching
  • abnormal hair loss
  • upset stomach
  • vomiting
  • loss of taste
  • abnormal sensations (tingling, prickling, burning, tightness, and pulling)
  • swelling
  • joint and muscle pain
  • drowsiness
  • dizziness
  • decreased white blood cells
  • decreased platelets
  • sore throat
  • fever
  • headache
  • chills
  • unusual bleeding or bruising
  • right-sided abdominal pain with decreased appetite
  • yellowing of the skin or eyes
  • skin eruptions

More Common

Other known side effects include:

  • skin rash
  • itching
  • abnormal hair loss
  • upset stomach
  • vomiting
  • loss of taste
  • abnormal sensations (tingling, prickling, burning, tightness, and pulling)
  • swelling
  • joint and muscle pain
  • drowsiness
  • dizziness
  • decreased platelet count (thrombocytopenia)
  • aplasia cutis congenital (prenatal exposure)
  • thyroid gland enlargement (prenatal exposure)
  • choanal atresia (prenatal exposure during the first trimester of pregnancy)
  • acute pancreatitis

Rare

Serious adverse effects:

Agranulocytosis

  • The cut-off criterion for it is an absolute granulocyte count of less than 500 per mL.
  • It most frequently occurs in the first three months of starting therapy but can occur even after a year or more of exposure or during repeated exposures when treating a relapse.
  • Regular monitoring of granulocyte count is considered useless by most experts.
  • Fever and sore throat are the most common presenting features of agranulocytosis. Therefore, all patients should get verbal and written instruction regarding the importance of getting an urgent white cell count if these symptoms arise to confirm the absence of this complication for continued antithyroid drug therapy.
  • Stop methimazole if the count is less than 1000 per ml. Treat fever or any apparent infections with intravenous antibiotics.

    • IV granulocyte colony-stimulating factor is known to reduce the length of hospitalization and recovery time.
  • Propylthiouracil (PTU) and methimazole have cross-reactivity for agranulocytosis, so avoid using the former in such patients.

Hepatotoxicity 

  • The hepatic toxicity of methimazole is more of a cholestatic process than allergic hepatitis seen in propylthiouracil and recovers slowly after discontinuing the drug.

Teratogenicity 

  • Methimazole can cross the placental membrane readily due to its insignificant protein binding. During the organogenesis phase, it causes immense fetal adverse effects, especially when administered in the first trimester. Possible congenital disabilities seen in infants born to mothers who received methimazole during pregnancy include goiter, cretinism, aplasia cutis, umbilical abnormalities, facial dysmorphism, esophageal atresia, craniofacial defects, and choanal atresia.
  • Propylthiouracil is the preferred antithyroid drug during pregnancy, especially for the first trimester, since the incidence of congenital anomalies is much less than methimazole. Clinicians should attempt to use the lowest effective dose, and if continuous monitoring shows the need for increased drug dosage, surgery is a consideration.

Hypothyroidism 

  • Methimazole can cause hypothyroidism. Therefore it is crucial to monitor T3 T4 levels in the serum, to adjust the dose to maintain a euthyroid state. In addition, since it crosses the placenta readily, it is capable of causing hypothyroidism and cretinism in newborns.

If you experience a serious side effect, you or your doctor may send a report to the Food and Drug Administration’s (FDA) MedWatch Adverse Event Reporting program online (http://www.fda.gov/Safety/MedWatch) or by phone (1-800-332-1088).

Drug Interactions

Pregnancy and Lactation

Pregnancy and Lactation

Use during Lactation

  • Maternal methimazole therapy does not affect thyroid function or intellectual development in breastfed infants with doses up to 20 mg daily. Taking methimazole right after nursing and waiting for 3 to 4 hours before nursing again should minimize the infant dosage. No cases of thyroid function alteration have been reported among infants exposed to methimazole via breastmilk. Some experts now recommend that methimazole should be considered the antithyroid drug of choice in nursing mothers.
  • The American Thyroid Association recommends only monitoring infants for appropriate growth and development during routine pediatric health and wellness evaluations and routine assessment of serum thyroid function in the child is not recommended. Rare idiosyncratic reactions (e.g., agranulocytosis) might occur, and the infant should be watched for signs of infection. Monitoring of the infant’s complete blood count and differential is advisable if there is a suspicion of a drug-induced blood dyscrasia.

Effects in Breastfed Infants

  • Five mothers taking methimazole 5 to 15 mg daily at 6 pm breastfed their infants during the day using expressed milk or formula at night. Thyroid Five mothers taking methimazole 5 to 15 mg daily at 6 pm breastfed their infants during the day using expressed milk or formula at night. Thyroid function test remained normal during 90 days of study and none of the infants showed any clinical signs of hypothyroidism and methimazole was undetectable (<10 mcg/L) in the infants’ serum.
  • Twelve mothers taking methimazole 5 mg daily breastfed their infants from the time of delivery. Another 17 women were given methimazole 5 mg twice daily beginning 2 to 8 months postpartum and allowed to breastfeed their infants exclusively with supplementation given to those over 6 months of age. A third group of 6 mothers took methimazole 10 mg twice daily. All infants were normal clinically and had normal thyroid function tests when measured 2 to 4 weeks after the start of therapy. (Note: this paper is apparently the full publication of a previous report in abstract form.)
  • The breastfed infants of 16 mothers who became hypothyroid during methimazole therapy were studied 5 times between birth and 12 months of age in comparison to a control group of 18 breastfed infants whose mothers did not become hypothyroid during methimazole therapy and a group of 24 infants of normal mothers. There were no differences in thyroid function tests of the infants from the 3 groups.
  • Fifty-one infants of mothers who took methimazole during pregnancy and continued during lactation with a dose of 5 mg daily and 88 infants of mothers who took the drug starting 2 to 8 months postpartum were studied. All infants had normal thyroid function tests for up to a year of maternal therapy with doses of 5 to 20 mg daily of methimazole. Fourteen children who had been breastfed as infants were compared to a control group of children whose mothers did not take methimazole. Performance on IQ tests did not differ between the two groups. Forty-two of these children were restudied after 4 years of age and found to have IQ scores equal to children under the age of 6 in a matched control group.

Effects on Lactation and Breastmilk

  • Relevant published information was not found as of the revision date.

How should this medicine be used?

Methimazole comes as a tablet and usually is taken three times a day, approximately every 8 hours, with food. Follow the directions on your prescription label carefully, and ask your doctor or pharmacist to explain any part you do not understand.

What special precautions should I follow?

Before taking methimazole,

  • tell your doctor and pharmacist if you are allergic to methimazol, lactose, or any other drugs.
  • tell your doctor and pharmacist what prescription and nonprescription medications you are taking, especially anticoagulants (‘blood thinners’) such as warfarin (Coumadin), beta blockers such as propranolol (Inderal), diabetes medications, digoxin (Lanoxin), theophylline (Theobid, Theo-Dur), and vitamins.
  • tell your doctor if you have or have ever had any blood disease, such as decreased white blood cells (leukopenia), decreased platelets (thrombocytopenia), or aplastic anemia, or liver disease (hepatitis, jaundice).
  • tell your doctor if you are pregnant, plan to become pregnant, or are breast-feeding. Methimazole should not be used during pregnancy or breast-feeding. If you become pregnant while taking methimazole, call your doctor immediately. Methimazole may harm the fetus.
  • if you are having surgery, including dental surgery, tell the doctor or dentist that you are taking methimazole.

What should I know about storage and disposal of this medication?

Keep this medication in the container it came in, tightly closed, and out of reach of children. Store it at room temperature and away from excess heat and moisture (not in the bathroom).

Unneeded medications should be disposed of in special ways to ensure that pets, children, and other people cannot consume them. However, you should not flush this medication down the toilet. Instead, the best way to dispose of your medication is through a medicine take-back program. Talk to your pharmacist or contact your local garbage/recycling department to learn about take-back programs in your community. See the FDA’s Safe Disposal of Medicines website (http://goo.gl/c4Rm4p) for more information if you do not have access to a take-back program.

It is important to keep all medication out of sight and reach of children as many containers (such as weekly pill minders and those for eye drops, creams, patches, and inhalers) are not child-resistant and young children can open them easily. To protect young children from poisoning, always lock safety caps and immediately place the medication in a safe location – one that is up and away and out of their sight and reach. http://www.upandaway.org

In case of an emergency/overdose

In case of overdose, call the poison control helpline at 1-800-222-1222. Information is also available online at https://www.poisonhelp.org/help. If the victim has collapsed, had a seizure, has trouble breathing, or can’t be awakened, immediately call emergency services at 911.

What other information should I know?

Keep all appointments with your doctor and the laboratory.

Do not let anyone else take your medication. Ask your pharmacist any questions you have about refilling your prescription.

It is important for you to keep a written list of all of the prescription and nonprescription (over-the-counter) medicines you are taking, as well as any products such as vitamins, minerals, or other dietary supplements. You should bring this list with you each time you visit a doctor or if you are admitted to a hospital. It is also important information to carry with you in case of emergencies.

 

References