Methimazole Contraindications
Absolute Contraindication
The only absolute contraindication to methimazole is hypersensitivity to the drug or any of its components 1.
Critical Clinical Situations Requiring Alternative Therapy
Pregnancy - First Trimester
Methimazole should be avoided during the first trimester of pregnancy due to increased risk of congenital abnormalities 2, 3.
- Propylthiouracil (PTU) is the preferred antithyroid medication during the first trimester because it carries a lower risk of birth defects compared to methimazole 2, 4.
- Methimazole-associated birth defects include a specific pattern of rare anomalies: choanal atresia, esophageal atresia, aplasia cutis, and facial abnormalities 4, 5.
- Switch to methimazole in the second and third trimesters to reduce the risk of PTU-induced hepatotoxicity with prolonged use 2, 3, 4.
- Women must wait four months after radioactive iodine (I-131) treatment before attempting pregnancy, as I-131 is absolutely contraindicated during pregnancy due to fetal thyroid ablation 2.
Severe Hepatotoxicity or Agranulocytosis
If a patient develops agranulocytosis or severe hepatotoxicity on methimazole, the drug must be permanently discontinued 6, 7.
- Agranulocytosis occurs in approximately 3 per 10,000 patients and presents with sore throat and fever 6.
- Both methimazole and PTU can cause hepatitis, vasculitis, and thrombocytopenia in less than 5% of cases 6.
- Desensitization to methimazole is NOT an option for patients who experienced agranulocytosis or hepatotoxicity 7.
- For patients with these serious adverse effects, definitive therapy (radioactive iodine or thyroidectomy) should be considered 2, 6.
Relative Contraindications and Special Considerations
Minor Allergic Reactions
- For patients experiencing rash or pruritus (without agranulocytosis or hepatotoxicity), switching to PTU is one option 7.
- Alternatively, desensitization to methimazole under allergist supervision is feasible for continued medical therapy or as a bridge to definitive treatment 7.
Breastfeeding
- While not an absolute contraindication, women must wait four months after I-131 treatment before breastfeeding 2.
- Methimazole can be used during breastfeeding with appropriate monitoring, though this requires careful consideration 2.
Common Pitfalls to Avoid
- Do not continue methimazole in the first trimester if pregnancy is confirmed—switch immediately to PTU 2, 3.
- Do not attempt desensitization in patients with prior agranulocytosis or hepatotoxicity—these are permanent contraindications requiring alternative therapy 7.
- Do not use radioactive iodine during pregnancy or within four months of planned conception—this causes irreversible fetal thyroid damage 2.
- Inadequately treated hyperthyroidism increases risks of preeclampsia, preterm delivery, heart failure, and miscarriage, so maintaining thyroid control is essential even when switching medications 2.