Methimazole

Methimazole

Methimazole is an antithyroid medication used primarily to treat hyperthyroidism, a condition where the thyroid gland produces excessive amounts of thyroid hormones. It is particularly effective in managing Graves' disease, an autoimmune disorder that is the most common cause of hyperthyroidism.

Pharmacology

Methimazole belongs to the class of drugs known as thionamides. It works by inhibiting the enzyme thyroid peroxidase, which plays a crucial role in the synthesis of thyroid hormones thyroxine (T4) and triiodothyronine (T3). By blocking this enzyme, methimazole reduces the production of these hormones, thereby alleviating the symptoms of hyperthyroidism.

Indications

Methimazole is indicated for:

• Treatment of hyperthyroidism.
• Preparation for thyroidectomy or radioactive iodine therapy in patients with hyperthyroidism.
• Management of thyrotoxic crisis or thyroid storm, although propylthiouracil is often preferred in this acute setting.

Dosage and Administration

The dosage of methimazole varies depending on the severity of the hyperthyroidism and the patient's response to the medication. It is typically administered orally, with doses ranging from 5 mg to 30 mg per day, divided into multiple doses. The goal is to maintain normal thyroid hormone levels, which are monitored through regular blood tests.

Side Effects

Common side effects of methimazole include:

• Rash
• Itching
• Upset stomach
• Headache

Serious side effects, though rare, can occur and include:

• Agranulocytosis (a potentially life-threatening drop in white blood cells)
• Hepatotoxicity (liver damage)
• Vasculitis

Patients are advised to report any signs of infection, such as fever or sore throat, immediately, as these may indicate agranulocytosis.

Contraindications

Methimazole is contraindicated in patients with:

• Known hypersensitivity to methimazole or any of its components.
• Severe liver disease.

Pregnancy and Lactation

Methimazole is classified as a pregnancy category D drug. It can cross the placenta and may cause fetal harm, particularly during the first trimester. Therefore, it is generally avoided during early pregnancy, with propylthiouracil being preferred. However, methimazole may be used in the second and third trimesters if necessary.

Methimazole is excreted in breast milk, but in small amounts. It is considered relatively safe for use during lactation, with monitoring of the infant's thyroid function.

Mechanism of Action

Methimazole inhibits the thyroid peroxidase enzyme, which catalyzes the iodination of tyrosine residues in thyroglobulin and the coupling of iodotyrosines to form T3 and T4. By blocking this enzyme, methimazole effectively reduces the synthesis of thyroid hormones.

History

Methimazole was first introduced in the 1940s and has since become a mainstay in the treatment of hyperthyroidism. It is favored over propylthiouracil for long-term management due to its more favorable side effect profile and once-daily dosing.

Also see

• Hyperthyroidism
• Graves' disease
• Thyroid hormone
• Propylthiouracil