Action and Spectrum

Actions and Spectrum: 

The actions of methimazole include: 

• Inhibiting thyroid hormone synthesis: methimazole inhibits the production of thyroid hormones by blocking the enzyme thyroperoxidase, which is required to produce thyroid hormones. 
• Reducing thyroid hormone release: methimazole also reduces the release of thyroid hormones from the thyroid gland. 

The spectrum of methimazole includes: 

• Treatment of hyperthyroidism: methimazole is primarily used to treat hyperthyroidism, a condition in which the thyroid gland produces too much thyroid hormone. 
• Preparation for thyroid surgery: methimazole may also be used to prepare patients for thyroid surgery by reducing the amount of thyroid hormone in the body. 
• Thyroid storm: methimazole may be used in the treatment of thyroid storm, a life-threatening condition in which there is a sudden and severe increase in thyroid hormone levels. 
• Graves’ ophthalmopathy: methimazole may also be used in the treatment of Graves’ ophthalmopathy, an autoimmune disorder that affects the eyes in people with hyperthyroidism. 

Drug Interaction

Adverse Reaction

Frequency not defined 

Agranulocytosis 

Alopecia 

Aplastic anemia 

Dermatologic reactions  

Hepatotoxicity 

Leukocytoclastic vasculitis 

Neuritis 

Polyarthritis 

Pruritus 

Swelling of salivary gland  

Skin pigmentation 

Thrombocytopenia 

Urticaria 

Vertigo 

Black Box Warning

Contraindication / Caution

Contraindication/Caution: 

• Hypersensitivity: methimazole should not be used in patients who have a known hypersensitivity or allergy to the medication. 
• Pregnancy and breastfeeding: methimazole can cross the placenta and enter breast milk, potentially causing harm to the developing fetus or newborn. It is generally not recommended for use during pregnancy or breastfeeding unless the benefits outweigh the risks. 
• Liver disease: methimazole can cause liver damage, especially in patients with pre-existing liver disease. Liver function tests should be monitored regularly during treatment. 
• Blood disorders: methimazole can cause blood disorders such as agranulocytosis, a condition in which there is a severe decrease in white blood cell count. Regular blood tests should be performed during treatment to monitor for any potential blood disorders. 
• Surgery: methimazole should be discontinued several days before any surgery, as it can increase the risk of bleeding. 
• Children and elderly: methimazole should be used with caution in children and elderly patients, as they may be more susceptible to side effects. 
• Other medications: methimazole may interact with other medications, including blood thinners and diabetes medications. It is important to inform the healthcare provider of all medications being taken before starting treatment with methimazole. 

Pregnancy / Lactation

Pregnancy consideration:  

Category D 

Breastfeeding warnings:  

methimazole is distributed in breastmilk and hence contraindicated during breastfeeding  

Pregnancy category: 

• Category A: Satisfactory and well-controlled studies show no risk to the fetus in the first or later trimester. 
• Category B: No evidence shown of risk to the fetus found in animal reproduction studies, and there are not enough studies on pregnant women    
• Category C: Adverse effects on the fetus found with evidence in animal reproduction studies and no adequate evidence for a product in humans; Pregnant women must take care of the potential risks. 
• Category D: There is adequate data available with sufficient evidence of human fetal risk from various platforms. However, despite potential dangers may be used only in emergency cases for potential benefits. 
• Category X: Drugs listed in this category outweigh risks over benefits. The drug is not for pregnant women. 
• Category N: No data is available for the drug under this category. 

Pharmacology

Pharmacology: 

• Mechanism of action: methimazole works by inhibiting the enzyme thyroperoxidase, which is required for the synthesis of thyroid hormones. By blocking this enzyme, methimazole reduces the production and release of thyroid hormones, leading to a decrease in the hyperactivity of the thyroid gland. 
• Absorption and distribution: methimazole is well absorbed from the gastrointestinal tract and is distributed throughout the body. It is highly protein-bound and has a half-life of approximately 6 to 13 hours. 
• Metabolism and elimination: methimazole are metabolized in the liver and excreted primarily in the urine. Its elimination half-life is approximately 6 to 13 hours. 
• Dosage and administration: methimazole is usually administered orally in tablet form. The dose and duration of treatment depend on the severity of the hyperthyroidism and the patient’s response to the medication. 
• Adverse effects: methimazole can cause several adverse effects, including skin rash, fever, joint pain, gastrointestinal upset, and liver dysfunction. In rare cases, it can cause a serious blood disorder known as agranulocytosis, which can be fatal. 
• Therapeutic monitoring: During the treatment with methimazole, the healthcare provider may monitor the patient’s thyroid hormone levels and liver function tests to ensure that the medication is effective and not causing any harm. 
• Drug interactions: methimazole can interact with other medications, such as blood thinners, diabetes medications, and other antithyroid medications. It is important to inform the healthcare provider of all medications being taken before starting treatment with methimazole. 

Pharmacodynamics: 

methimazole’s primary pharmacodynamic effects include: 

• Inhibition of thyroperoxidase: methimazole inhibits the activity of the enzyme thyroperoxidase, which is involved in the synthesis of thyroid hormones. This leads to a reduction in the production and release of thyroid hormones from the thyroid gland. 
• Decreased thyroid hormone levels: As a result of inhibiting thyroperoxidase, methimazole decreases the levels of thyroid hormones, such as thyroxine (T4) and triiodothyronine (T3), in the bloodstream. This helps to reduce the symptoms of hyperthyroidism. 
• Restoration of normal thyroid function: By reducing the production and release of thyroid hormones, methimazole helps to restore normal thyroid function in patients with hyperthyroidism. 

Pharmacokinetics: 

Absorption 

The bioavailability is 80-95% 

Onset of action for oral absorption every 12-18 hours 

The duration of action is 36-72 hours 

Peak plasma concentration is achieved in 1-2 hours 

Distribution 

No protein is bound 

Metabolism 

The drug is metabolized by liver 

Elimination and Excretion 

The half-life is 4-6 hours 

The drug is excreted in urine 

Adminstartion

Administration: 

methimazole is usually administered orally in tablet form, and the dosage and administration schedule depend on the severity of the hyperthyroidism and the patient’s response to the medication. 

The usual starting dose for methimazole is 10 to 15 milligrams per day, divided into two or three doses. The healthcare provider may adjust the dose based on the patient’s response to treatment, but the maximum daily dose is usually no more than 60 milligrams per day. 

methimazole is best taken with food to minimize gastrointestinal upset. Patients should take the medication at the same time each day to maintain a consistent level of the medication in the bloodstream. 

Patient Information Leaflet

Patient information leaflet 

Generic Name: methimazole 

Pronounced: Meth-ee-ma-zol 

Why do we use methimazole? 

methimazole is used to treat hyperthyroidism, a condition in which the thyroid gland produces too much thyroid hormone. Hyperthyroidism can cause a variety of symptoms, including weight loss, anxiety, tremors, increased heart rate, and heat intolerance. 

methimazole works by inhibiting the production of thyroid hormones in the thyroid gland. By reducing the levels of thyroid hormones in the bloodstream, methimazole helps to relieve the symptoms of hyperthyroidism and restore normal thyroid function. 

methimazole is a first-line treatment for hyperthyroidism and is often used in patients with Graves’ disease, an autoimmune disorder that causes hyperthyroidism. Methimazole can also be used to prepare patients for surgery to remove the thyroid gland or to control hyperthyroidism that has not responded to other treatments.