Methimazole

Hyperthyroidism Treatment

Methimazole is primarily used to treat hyperthyroidism, which occurs when the thyroid gland is overactive and produces an excess of thyroid hormones. By reducing the amount of hormone the gland makes, the medication helps bring thyroid levels back into a healthy range and relieves symptoms associated with the condition.

Pre-Surgical and Procedural Use

In addition to long-term management, healthcare providers may prescribe methimazole for short-term use in specific situations, including:

• Thyroid Surgery: To stabilize thyroid hormone levels before a thyroidectomy (surgical removal of the thyroid gland).
• Radioactive Iodine Therapy: To control thyroid activity before or after treatment with radioactive iodine.

It may also be used when other treatments, such as surgery or radioactive iodine, are not appropriate or must be delayed.

How it works in the body

Methimazole belongs to a class of medications called antithyroid agents. Its primary role is to stop the thyroid gland from overproducing thyroid hormones, specifically thyroxine (T4) and triiodothyronine (T3).

The medication works by interfering with the chemical process the thyroid gland uses to turn iodine into hormones. By blocking this production step, methimazole helps lower the high levels of thyroid hormone in the body.

Timing of effects

• Existing hormones: Methimazole only stops the production of new hormones. It does not affect the hormones that the thyroid gland has already made and stored, nor does it affect hormones already moving through the bloodstream.
• Delayed response: Because it does not deactivate existing hormones, it often takes several days or weeks of treatment before a person begins to feel the full effects of the medication. The body must first use up the supply of hormones it had already created.
• Hormone therapy: This medication does not interfere with the action of thyroid hormones taken as a supplement or injection.

Common Side Effects

Methimazole can cause several mild side effects as the body adjusts to the medication. These may include:

• Nausea or vomiting
• Stomach upset
• Headache
• Dizziness or drowsiness
• Muscle or joint pain
• Numbness or tingling
• Skin rash, itching, or changes in skin color
• Hair loss
• Loss of the sense of taste

Serious Side Effects

While less common, some side effects can be severe. Seek medical attention immediately if you notice:

• Infection symptoms: Fever, chills, sore throat, or mouth sores, which may indicate a dangerous decrease in white blood cells (agranulocytosis).
• Liver issues: Yellowing of the skin or eyes (jaundice), dark urine, pale stools, or severe pain in the upper right abdomen.
• Blood disorders: Unusual bleeding or bruising.
• Swelling: Swollen lymph nodes in the neck or under the arms.
• Lupus-like syndrome: Joint pain accompanied by a fever or a butterfly-shaped rash across the cheeks.

Serious Blood Disorders

Agranulocytosis is a rare but potentially life-threatening condition where the body stops producing enough white blood cells. This increases the risk of severe infections. Symptoms that require immediate medical attention include:

• Fever
• Sore throat
• Chills
• General feeling of illness

Liver Toxicity

Methimazole has been linked to serious liver injury, including liver failure. Healthcare providers typically monitor liver function tests during treatment. Signs of liver problems include:

• Yellowing of the skin or the whites of the eyes (jaundice)
• Dark-colored urine
• Pain in the upper right side of the abdomen
• Severe nausea or vomiting

Pregnancy and Breastfeeding

Methimazole can cause fetal harm when administered to a pregnant woman, especially during the first trimester. Birth defects have been reported in infants born to mothers who took this medication early in pregnancy. If pregnancy occurs or is planned, a healthcare provider may suggest switching to an alternative medication or adjusting the treatment plan. Methimazole also passes into breast milk, and its use during breastfeeding should be discussed with a doctor.

Other Precautions

Methimazole may affect blood clotting by decreasing prothrombin levels, which can increase the risk of bleeding. This is particularly important for individuals scheduled for surgery or those taking blood-thinning medications. Additionally, over-treatment can lead to hypothyroidism (underactive thyroid), which may cause symptoms like fatigue, weight gain, and cold intolerance.

Pregnancy

Methimazole crosses the placenta and can cause harm to a developing fetus. Research indicates that exposure during the first trimester is associated with rare birth defects, including skin defects on the scalp (aplasia cutis) and malformations of the esophagus or face. Due to these risks, healthcare providers often prefer using alternative medications during the first trimester of pregnancy.

It is also important to manage hyperthyroidism during pregnancy, as untreated overactive thyroid can lead to serious complications such as preeclampsia, miscarriage, or preterm birth. If you are planning to become pregnant or discover you are pregnant while taking methimazole, contact your doctor immediately.

Breastfeeding

Methimazole is present in human breast milk. While research suggests that doses up to 20 mg per day may not significantly affect the thyroid function of a nursing infant, caution is still advised. Healthcare providers typically monitor the thyroid status of infants who are breastfed by individuals taking methimazole.

Available Forms

Methimazole is available for administration in the following form:

• Oral Tablet: This medication is taken by mouth.

The tablets are typically taken at regular intervals throughout the day as directed by a healthcare provider. The specific appearance of the tablets may vary depending on the manufacturer.

FDA Approval and Indications

Methimazole is approved by the U.S. Food and Drug Administration (FDA) for the medical treatment of hyperthyroidism (overactive thyroid). Its approved uses include:

• Managing hyperthyroidism in patients with Graves' disease or toxic multinodular goiter when surgery or radioactive iodine therapy is not an appropriate option.
• Ameliorating symptoms of hyperthyroidism to prepare patients for thyroid surgery (subtotal thyroidectomy) or radioactive iodine therapy.
• Treating thyroid storm, a life-threatening condition caused by extremely high levels of thyroid hormones.

Regulatory Warnings and Safety

The FDA-approved labeling for methimazole includes significant warnings regarding potential serious adverse effects. These include:

• Agranulocytosis: A severe and potentially fatal reduction in white blood cells that can increase the risk of serious infections.
• Liver Toxicity: Rare but serious cases of liver injury, including liver failure and hepatitis, have been reported.
• Fetal Harm: Methimazole can cause birth defects when administered during the first trimester of pregnancy. Regulatory guidance generally suggests avoiding its use during the first trimester if alternative treatments are available.

There are currently no Risk Evaluation and Mitigation Strategies (REMS) required for methimazole, and it does not carry a boxed warning, which distinguishes it from some other antithyroid medications that have boxed warnings for severe liver injury.

Risks During Pregnancy

Methimazole can cause harm to an unborn baby. It crosses the placenta and has been linked to rare but serious birth defects, particularly when taken during the first trimester. These defects may include:

• Aplasia cutis (a condition where skin is missing from the scalp)
• Esophageal atresia (a blockage of the esophagus)
• Choanal atresia (a blockage of the nasal passages)
• Facial abnormalities

Treatment Timing

Because of the risk of birth defects, healthcare providers typically recommend using a different medication, such as propylthiouracil, during the first three months of pregnancy. Methimazole may be considered for use during the second and third trimesters if hyperthyroidism must be managed.

Fetal Thyroid Health

Methimazole can also affect the development of the baby's thyroid gland. In some cases, it may cause the baby to develop a goiter or hypothyroidism. If you are pregnant or planning to become pregnant, it is important to discuss the risks and benefits of treatment with your doctor. Frequent monitoring of thyroid levels is usually required to ensure the lowest possible dose is used to maintain the health of both the parent and the baby.