Methimazole is a medication used to manage hyperthyroidism, a condition of excessive thyroid hormone production. It is classified as an antithyroid drug, specifically a thioamide. Methimazole is administered orally, typically in tablet form.
Effectiveness of Methimazole
Methimazole works by interfering with the thyroid gland’s ability to synthesize new thyroid hormones. It blocks the incorporation of iodine into thyroglobulin, which is a step necessary for the production of thyroxine (T4) and triiodothyronine (T3). This action reduces the overall levels of these hormones in the body, helping to normalize thyroid function.
Patients begin to see improvement in their thyroid hormone levels within 2 to 4 weeks of starting methimazole treatment. While initial symptom relief may be noticed within 14 days, reaching a stable and normal thyroid state often takes about 6 to 12 weeks. In a 2021 meta-analysis, 72% of adults achieved normal free T4 levels within 10 weeks on standard doses.
Successful treatment with methimazole involves the normalization of thyroid hormone levels and a significant improvement in hyperthyroid symptoms. This medication is often used as a long-term treatment option for conditions like Graves’ disease, or as preparation before thyroid surgery or radioactive iodine therapy.
Potential Side Effects
While generally effective, methimazole can cause various side effects. Common side effects may include skin rash, itching, upset stomach, nausea, vomiting, joint pain, muscle pain, and abnormal hair loss. Some individuals might also experience swelling, drowsiness, or dizziness.
Serious side effects include agranulocytosis, which is a significant reduction in white blood cell count. This condition increases the risk of serious infections, and symptoms like fever, chills, or a sore throat warrant immediate medical attention. Agranulocytosis is a rare occurrence, affecting less than 1% of patients.
Liver damage is a rare but serious side effect. Symptoms of liver problems can include pain or tenderness in the upper stomach, pale stools, dark urine, loss of appetite, nausea, vomiting, or yellowing of the skin or eyes. If these symptoms appear, it is important to contact a doctor right away. Severe liver injury, though rare, occurs in approximately 0.03% to 0.5% of patients.
Vasculitis has also been reported as a rare severe side effect. Additionally, methimazole can lead to hypothyroidism, where thyroid hormone levels become too low. Symptoms of hypothyroidism include fatigue, increased sensitivity to cold, constipation, dry skin, and weight gain.
Important Treatment Considerations
Methimazole is administered orally. For adults, initial doses may range from 15 to 60 milligrams per day, often divided into three doses. Once thyroid hormone levels stabilize, a maintenance dose of 5 to 15 milligrams per day is common.
Consistent daily dosing is important to maintain stable thyroid hormone levels and achieve the best treatment outcomes. Patients should avoid stopping the medication abruptly without consulting their healthcare provider. Stopping treatment without medical guidance can lead to a return of hyperthyroid symptoms.
Regular blood tests are necessary to monitor thyroid hormone levels to ensure the medication is working effectively and to adjust the dose as needed. These tests also help monitor for potential side effects, such as low white blood cell counts (agranulocytosis) or liver issues. If blood counts become too low, precautions may be needed to reduce infection risk.
Precautions are important for pregnant or breastfeeding individuals. Methimazole can cross the placental barrier and has been linked to congenital malformations, especially if used during the first trimester. While it may be used in the second and third trimesters, propylthiouracil is often preferred in early pregnancy. Methimazole is excreted in breast milk, but studies have shown that doses up to 20 mg per day do not affect the thyroid function or intellectual development of breastfed infants. Patients with pre-existing liver conditions should discuss these with their doctor, as methimazole has been associated with liver injury.